|Mediterranean Garden Society|
The most popular article in the MGS journal, The Mediterranean Garden, has proved to be one about pruning olive trees. It seems that gardening in a Mediterranean country goes hand in hand with growing olives – from planting a new grove to tackling one or two neglected trees. So here in their entirety are the three articles about olive cultivation which have appeared in TMG in the past followed by lecture notes from an olive expert who gave a workshop at the 2007 AGM Symposium.
Photographs by Davina Michaelides unless otherwise stated
Pruning Olive Trees by Brian Chatterton
Olive Oil Production by Brian Chatterton
Organic Olives by Chevrel Traher
Training and pruning olives by Dr. Peter A. Roussos, Laboratory of Pomology, Agricultural University of Athens. (Dr Roussos presented a workshop on olive pruning at the MGS Symposium: The Dry Garden – Practice and Philosophy, Athens, 2007.)
In the Olive Grove - A diary by Brian Chatterton
Glyphosate – too much of a good thing?
Glyphosate is a very effective herbicide. Many people know it under its original trade name of Roundup which is made by Monsanto. The patents have now expired and it is available in much cheaper generic forms with a variety of names. As a herbicide it combines many useful characteristics. Firstly it is a systemic herbicide. That means it enters the sap of the plant and kills it completely not just the leaves that have been sprayed. Unlike some other herbicides it is not residual. In the soil or when dead plants break down the soil bacteria break down the glyphosate so it does not linger in the soil and kill other crop plants. That story is now changing as we begin to understand the long-term effects of high application rates over a long period.
Glyphosate applied to weeds in reasonable quantities is probably quite safe but unfortunately it is often applied to food crops and this has led to residues in food and moves to ban it altogether. At the moment there is a tussle between the European Parliament, the European Commission and the European Council of Ministers over the renewal of the licence for glyphosate which expired this year. It has been renewed for 18 months as a temporary measure but its future is on notice.
Glyphosate can enter the food chain through Roundup Ready crops. These are genetically modified crosses between crops and the soil bacteria that destroy the glyphosate. The GM crop becomes resistant to the glyphosate because the crop has acquired the gene that breaks down the glyphosate. This turns the glyphosate into a selective herbicide. It kills the weeds but not the GM crop. That is the theory but the reality is different as the weeds are becoming resistant too.
For example in Australia annual ryegrass is a serious weed in crops. It is easily killed with glyphosate but there are tens of millions of plants per hectare and as one or two seem to be resistant to glyphosate the resistant gene spreads and the whole population of annual ryegrass becomes resistant in six or seven years.
The GM crop picks up the herbicide (not all of it is broken down) and glyphosate then enters the food chain. There is not a great deal of GM in Europe at present but imported grain and oil seeds have glyphosate residues.
In Europe the use of glyphosate has been extended from killing weeds to drying off cereal crops before harvest. From the perspective of Umbria this seems strange but in northern Europe summers are much wetter and cereal crops often have a large amount of green material in them at harvest time. This makes harvesting difficult as the machines are designed to handle dry material. Obviously such applications are an effective route for glyphosate contamination of cereals.
The third possible source of contamination that may be relevant to olive growers are the glyphosate residues in the soil. Not all the glyphosate is broken down by the bacteria and some is attached to the clay particles in a similar manner to other phosphate compounds. This can then be released and taken up by the crop. If the crop is not a GM version the growth will be affected and the glyphosate will enter the food chain.
The debate on glyphosate is a classic example of the conflict between a market economy and sustainability. The market economy encourages makers to increase sales of the herbicide by searching for new uses,COMMA while a sustainable approach would restrict its use to applications where it is extremely difficult to find an alternative.
A ban on glyphosate is becoming more and more likely. For the olive grower it will be an irritation as glyphosate kills stubborn perennial weeds and its use in limited quantities for this application is not a hazard to food quality.
Here in Umbria we are surrounded by forests and the invasion of the olive groves with perennial plants from the forest environment is a constant problem. The four important ones are the spina bianca, so called because of its white flowers in early spring. In English it is called blackthorn because of its black fruit. It has an invasive root system and the thorn bushes sprout from the roots. If left unchecked it will take over the grove and even the small shoots are a hazard for the nets during harvest. Fortunately it can be controlled without herbicides. Mowing a couple of time each year will keep it down to manageable levels. If the grove has been abandoned for some years the stems will be too big for the normal mower and you will need a trinciatore (mulcher) to chop through the spina the first time.
Wild roses and blackberries (also called brambles) thrive under frequent mowing. The mowing is a severe pruning and it seems to reinvigorate them. Control is important again due to their nuisance value at harvest when they snag the nets. Glyphosate is most effective if applied in mid-season. Early in spring there are still root reserves of starch and the plants will shoot again. Late in summer the plants have stopped growing and the herbicide is ineffective as it works through a disruption of the growth process. One only needs a very small amount as the individual plants are spot sprayed. When I restored a grove of 400 trees for my neighbour we used 20 litres of glyphosate the first year but now five years later we use about one litre. We will probably continue at this rate for some years until all the bank of seeds in the soil have germinated. After that we will probable spot spray every second year as there will be a few new plants brought in through seeds dropped by birds.
The fourth perennial weed is Old Man's Beard. It gets its name from the white fluffy flowers in late summer. It is a nuisance but not as serious as the others as it has no thorns but it climbs through the trees and needs to be removed at pruning time. Controlling it is difficult with herbicides as one does not want to spray any herbicide on the olive trees.
This rather benign picture of herbicide use is not true of all olive growing. In Puglia the trees are very tall and picking with beaters and ladders is only possible on the bottom third or half of the tree. The top part is out of reach and they let the olives fall on the ground. Many growers now clear all the vegetation under the trees using herbicides – mostly glyphosate. This is not spot spraying of a few perennial weeds but boom spraying all the grasses and clovers that form a natural meadow. The olives can be picked up off the bare ground using a sweeping machine.
This technique has caused considerable controversy in the southern part of Puglia where the Xylella fastidiosa (Xf) is present. There is considerable debate about the cause of the dead trees. Tests show that only a small proportion of the dead trees are infected with Xf and the alternative view is that the constant use of herbicides has created a sterile soil which is having an adverse effect on the health of the trees. I have not been able to find any research on the effect of glyphosate residues on olive trees but if the situation is similar to other crops the glyphosate could be accumulating in the soil due to frequent applications and low biological activity in the soil. The two are linked. The control of weeds reduces the soil organic matter and that in turn reduces biological activity. It is the activity of soil bacteria that breaks down the glyphosate. Whether this is killing the trees has not yet been proved but it is quite possible as it has been found to kill or reduce the growth of other crop plants. The other concern is the possibility of glyphosate residues in the olive oil.
Of course it is perfectly possible that both herbicides and Xf are having an impact.
Glyphosate is a case study in the future of farming. It is an example of the constant conflict between the market which is trying to expand and sell more and more products and the people's concern with sustainability and human health. It is not just the pesticides used in agriculture but also antibiotics that fall into this area of conflict. The challenge is to use these valuable products sensibly and not turn them into failures or hazards by gross overuse.
Yvonne Barton comments: In our area of Umbria there are several olive growers who spray herbicide all around their trees to keep down weeds (a cosmetic effect rather than practical I fear) and hence we risk a build-up of the type Brian describes in Puglia. What a pointless and destructive approach!
It is time to think about your olive fly control programme for this season. We all remember the disastrous year of 2014 when the olive crop was devastated by olive fly throughout most of Italy. That was due to a strange combination of weather that was extremely favourable to the olive fly.
The olive flies hatch out from their pupae during mid to late June. They are then on the wing and start to build to serious numbers during June and July. By the last week of July they are laying eggs on the green olives. Early control in April or May is therefore useless.
The olive fly numbers in 2014 were unusually high because July that year was cool and wet. We had 140 mm of rain that year which is extraordinary as July on average is our driest month with an average of less than 10 mm. Some years we have recorded no rain at all in July. The rain came with cool weather whereas heat kills a lot of olive flies. August was not as wet but it stayed cool so it was not surprising that the olive flies multiplied and caused such severe damage. The olive fly can have three to five generations between August and harvest in October or November. The flies lay their eggs on the olive. They hatch into grubs which eat a large portion of the olive; however in 2014 it was mould that caused just as much damage as the grubs. The mould entered the olive through the breathing hole of the grub and turned the uneaten flesh of the olive brown and smelly. Those who crushed their badly infected olives found that the oil was inedible.
The grubs then turn into pupae which fall to the ground and overwinter until the following year. The pupae are tough and resistant to cold but severe frost can reduce numbers. Some growers put lime on the soil after the 2014 severe infestation but the general opinion is that this does little to control the pupae.
The automatic reaction to such a pest is to reach for a bottle of deadly insecticide. Fortunately, this is not very easy for the professional grower of olives, and for the small grower it is very difficult indeed. There are insecticides but they have to be applied very precisely to be effective. One needs to examine a sample of olives every two or three days for eggs and small grubs. If 10 to 15% are infected one needs to spray. If the grubs grow and penetrate further into the olive they are out of range of the insecticide. For the small grower and even the professional this is too much work. The European Union is banning Dimethoate,the most commonly used insecticide for olive fly control.
Most professional growers use another method based on repelling the olive fly. The trees are sprayed with a copper spray similar to that used on vines or with Caolin (Kaolin). Kaolin is a clay. The copper-based compound or the clay coats the olives and makes it unattractive for the flies to lay eggs on them. The olive flies are on the wing by the end of June but they lay their eggs in July and August, so this is the critical time to apply the copper or Kaolin. It is not a job that can be done by the average small grower. You cannot take your backpack sprayer and dose the trees. To get a good coverage of the olives you need an expensive sprayer with an air blast to ensure that the droplets penetrate the foliage and cover each olive. Of course the copper or kaolin can be washed off by summer rain. Another application will then be needed. In fact three or more applications are common.
There is one spraying option that can be used by the small grower. It is based on an “attract and kill” system. Spintor fly is the product used. It is not applied to the whole tree but each tree receives a measured dose. The flies are attracted to the chemical and then killed. There is a backpack sprayer with a battery operated measure that controls the dose for each tree.
The story so far is all doom and gloom for the small grower. The insecticide Dimethoate is being banned and it is too difficult apply at the right time. The repellent sounds better but application of it requires expensive machinery. Fortunately traps provide a low-cost alternative for the small grower.
There are two main types of traps. There is the measuring trap and the control trap. The measuring trap is a tent like structure with a sticky underside. In the centre there is a tube of pheromone which attracts the olive flies. As they circle the tube they are caught on the sticky underside of the tent. One then counts them every week to determine the severity of the infestation. When it reaches a certain level one puts out the control traps. The pheromone traps are much too expensive to use for control. In my case I put out two in the whole grove and carefully counted the olive flies each week. By the end of July I had a chart of the figures and tried to find out how many flies were too many. The local advisers could not give me any numbers to work with. In August we had a spell of really hot weather and the olive fly population crashed to a zero count on the sticky tent so I was not concerned.
After that I read in The Mediterranean Garden a letter from a grower in Greece describing a home-made trap that he used. These are so simple and cheap that I put them out as a precaution every year and do not worry about the population count.
After the disaster harvest of 2014 there were meetings of growers throughout Italy to talk about olive fly control. We had a meeting of 300 or more growers in the cooperative frantoio (olive press) at Cetona which was addressed by an entomologist from Pisa University. He explained that traps were the only practical means of control for the small grower without access to expensive machinery. The trap he recommended was the Eco-trap.
I find that the bait for the traps is generally felt to be unbelievable. By this I mean that people find it extremely difficult to believe that the olive fly is attracted by ammonia. Ammonia is a constituent of smelling salts. Smelling salts are ammonia compounds that release ammonia gas. They were a great Victorian remedy for genteel ladies who had fainted from an attack of the vapours: ammonia is not a pleasant smell and the shock was supposed to revive them. I find that most people cannot accept that it is the favourite flavour of the olive fly and bait their traps with honey and other materials that would be something they would like themselves.
The Eco-traps are baited with smelling salts. The ammonia evaporates and the trap is exhausted after 60 days. This means that the grove will need to have the traps replaced during the season as protection is needed for more than 60 days.
The home-made traps are much more ingenious. One starts with a plastic bottle – a one and a half litre water bottle is ideal. I paint the bottles with yellow paint. The theory is that yellow attracts insects. Even if this is not true the paint protects the bottle against sunlight and means it will last for many years. I then drill two holes 6 mm in diameter near the top. In the bottle I put two or three large spoonfuls of soil. This is ordinary garden soil – nothing special. I put in about three spoonfuls of urea. Urea is a very common fertiliser used by cereal farmers and can be obtained from any farm supply shop. I then fill the bottle with water to about the two-thirds level. The easiest way to do this is to put the urea and the soil in a funnel and wash it in with the water.
I have called the home-made trap the Ureaka trap. No, it is not a misspelling of Eureka but based on the active ingredient – urea. The Ureaka traps needs about a week to become effective. The bacteria in the soil break down the urea and release ammonia. They do this slowly so the trap is effective for at least four or five months and does not need replacing halfway through the season. It can be used again in following years by adding more water and urea. There is no need to add more soil.
The bottle is attached to the tree with wire and then plastic string. If you look at the photograph carefully you will see the 6 mm hole where the flies enter. They stay in the trap and eventually drown.
At the Cetona meeting a grower asked the visiting expert if the Ureaka traps were effective. The entomologist replied that they were “all right.” The farmer was not happy with this lukewarm response and asked for a more definitive opinion. The entomologist responded by saying they were just as good as the Eco trap and cost a small fraction of the price.
I put out a trap for every two or three trees but one needs to accept the limitations of the traps if one is surrounded by groves where no control is undertaken. I have friends who put a “wall” of traps against the invading flies and then one trap for two or three trees in the rest of the grove. Traps will not give you total control but they should be effective in keeping the infestation level low and the damage insignificant.
One needs to revisit the life cycle of the olive fly for the timing. The flies are on the wing in June so I put the traps out then, whereas other control measures are normally taken in late July when the flies begin to lay eggs on the olives.
In 2014, the year of the great olive fly plague, I had a crop that was slightly below average. I had olive fly but it was only a low level of infection and I picked the olives early before the mould established itself in the olives so the quality of the oil was good.
During a Mediterranean Garden Society visit to Puglia I spoke to an olive grower who told me about a traditional trap used against olive flies. It was baited with anchovies, which of course release ammonia when they rot.
In Spain they are experimenting with controlling the olive fly using the sterile male technique. Huge numbers of sterile males are released to outnumber the fertile males and so reduce and perhaps eradicate the population of olive flies. It is a well-recognised technique and could be a cheap and effective means of control.
Figures released by the Italian farmers' organisation Coldiretti show some interesting trends in the olive oil market over the last 25 years. The most spectacular increase has been Japan where sales have increased by 1400% The only other country that comes anywhere near this rate of increase is the UK where sales have jumped by 763%. One should not get too excited by these figures, however, as they demonstrate a very low base level of sales rather than huge increases. These big percentage increases amount to 107,000 additional tonnes of olive oil whereas the USA market has increased by “only” 250% yet this amounts to an extra 320,000 tonnes of olive oil sold while Spain's increase of 24% is 96,000 tonnes or almost as much as Japan and the UK put together. The catastrophic fall in the standard of living in Greece has reduced consumption by 26% and as it is a natural olive oil-consuming country that amounts to 54,000 tonnes which is more than the UK has expanded.
On the production side there is now more olive oil being produced outside the Mediterranean Basin but it has yet to have a significant impact on world output. Australia for example has increased olive oil production from virtually zero 25 years ago to about 8,000 tonnes on average but it is a drop in the ocean compared to Italy's 600,000 tonnes and Spain's production of about double that.
The new olive oil countries have shaken up the industry in spite of their tiny contribution to world production. In Australia the local olive growers have successfully lobbied the government to achieve a reform of the labelling laws to remove the ambiguities and misrepresentations that are still used internationally. Local growers in the USA and Australia have been testing imported oils on sale and have found that many do not meet the standards for Extravergine. Instead of trying to improve the enforcement of the standard, the big producing countries have branded the Americans and Australians as “unhelpful.” Here in Italy the Carabinieri have found more examples of olive oil fraud. Initially the government proposed to reduce the penalties for fraud but fortunately the farmer organisations seem to have successfully stopped that proposal.
The European Union seems to be unaware of the image problems that olive oil is having in markets such as the USA and has proposed a relaxation of the labelling laws. Once again the farmers are protesting and so is the Italian Minister of Agriculture. The EU proposal is to relax the 18-month rule on Extravergine olive oil. After 18 months the oil may have changed so much that it should not be labelled as Extravergine. The EU is suggesting a “Best before” date instead but that would be determined by the manufacturer. The manufacturers' lobby seems to have been hard at work because they have persuaded the EU, in the draft law at least, to water down the indication of country of origin. So far these protests seem to have had little effect.
Olive oil suffers from the fact that most of the big processors and marketers are international food companies and they see olive oil as a global commodity while olive growers want to develop the market is terms of the flavour characteristics of each country and region.
Selling sansa oil to Asian countries may be good business in the short term but will do nothing to develop an appreciation of the qualities of olive oil. I was delighted some years ago to see that Taiwan had banned some sansa oil but unfortunately it was not due to the sansa oil itself but because the green colouring was not registered. Not only is this an additional level of deception (Sansa oil is never green in its natural state. All the natural colour has been destroyed during the processing) but exports resumed once the artificial colouring agent had been officially registered. To give the Taiwan government its due, it has successfully prosecuted a businessman for mixing olive oil with low-grade palm oil and selling it as Extravergine. He received a four-year sentence but the expectations seem to be that it will be reduced to a fine on appeal.
In India the major olive oil brand is owned by Cargill, the world's largest commodities trader. Commodity trading is about making a quick trade not developing a market for the long term. The selling of sansa oil has been justified on the grounds that Indian food is so strongly flavoured that the flavour of a good Extravergine oil would be swamped by the spices. The argument sounds plausible enough except that Indians are curious about exotic cuisines and Italian cooking is high on their list of foreign cuisines to try. They are disappointed because of the poor quality of the olive oil.
World trade and international markets may seem to be remote from small farms in Italy. While most of us produce olive oil for our own use we often have a small surplus to sell. Hopefully this may cover some of our costs but the prices paid by the frantoio are low. It is extremely difficult to make a profit from a small olive grove unless you have a considerable amount of free labour. Here in Umbria one can see the consequences – hundreds of thousands of abandoned olive trees. These abandoned groves are a constant source of the olive fly.
The European Union makes funds available for regional development of agricultural products. Some years ago we had one of these development projects for the Alta Orvietana region. The managers of the project promoted our local olive oil throughout Italy. All the evidence shows that this is a futile strategy in such a well developed olive oil market. Certainly sales increased while the promotion was in place but the increase was reversed when other regions began to do similar promotions. The European Union should insist that promotions take place in emerging markets such as India, China or Japan where a more permanent increase in sales is likely. If this was done by regional group these new markets would begin to understand the range of flavours in Extravergine olive oil that they will never find in the sansa oil currently available.
By March the pruning of the olive trees should be nearly finished.
In January I visited our family in New Zealand and gave a pruning demonstration on a couple of their olive trees. Of course it was completely the wrong time of year to do the pruning as it was late summer in New Zealand but I only pruned a few trees as a demonstration. Winter is a much better time to prune as the olives are dormant but pruning them in summer makes it easier to demonstrate the fruiting wood and non-fruiting wood. One can actually see the olives on the tree. For the novice pruner, olive picking is a good time to observe where the olives are on the tree. You can see the vigorous vertical shoots with hardly an olive on them while the hanging shoots carry a heavy crop. You can also see how some of the hanging shoots on the inside are becoming exhausted and have fewer olives.
Our family in New Zealand have only a few dozen trees as their real interest is vines but some of the trees had not been pruned for years and they reminded me of the neglected olive trees we acquired when we purchased our farm 25 years ago. The Marlborough region of New Zealand has seen the almost total disappearance of olives over the last 25 years as vineyards have proved to be much more profitable. The olives that are left are in groups of ten or twenty where for some reason planting vines was not practical.
The trees I was working on had had some rudimentary pruning at an early age but years of neglect meant that the centre of the vase was now full of thick growth. The trees were growing taller and taller to obtain light. They were also growing laterally for the same reason. In the meantime many of the leaves and branches in the centre of the tree had died due to a lack of light.
The first thing to do was to clear the centres out. This allowed light into the tree and meant that the lateral shoots received light from the inside as well as from the outside. This has the effect of reducing the growth outwards to find more light.
It was now possible to see more clearly the shape of the tree; unfortunately the original three or four main branches which formed the basic shape of the vase had been allowed to divide, with the result that many of the trees had seven or more main branches. I was reluctant to thin these back to the classic three or four in one season since it would have meant removing well over half the tree but I would certainly do this over a period of a couple of years.
Mechanical picking with beaters has reduced the need for a strict control over the height of the trees. Nevertheless it is still worth keeping them from growing too tall since the beaters are easier to work if they are not fully extended. It is most important to pick without ladders as they slow down the picking and are the cause of most of the accidents in the grove.
I cut back the tall-growing shoots on the trees to control the height but one needs to go through the whole trees to take out all vertical shoots, not just those at the top. Vertical shoots produce few or no olives. Almost all the energy of the tree goes into these verticals rather than into the horizontal shoots that produce the olives. This is where the out of season pruning is a useful demonstration. One can say in winter that the olives will grow on the laterals but actually seeing them on the laterals is a more effective demonstration.
One of the most difficult things to learn is how much one should prune. The Italian saying is until a bird can fly through the tree. In Italy a more practical means is to look at the neighbouring trees. This gives you the confidence to prune quite hard – something that is essential for trees that have been abandoned for years. In New Zealand, or in the Marlborough regions at least, I did not find a single well-pruned tree to act as a reference point.
If you are restoring old trees, clearing the centre and taking out the verticals is probably all you need to do but after some years your pruning will have shifted the balance in the trees towards more horizontal and downward-hanging shoots. This is where the fruit is formed. Over the years this fruiting wood will become tired and will need to be thinned. The weak and exhausted fruiting wood should be removed.
I prune our trees every year but many people in central Italy prune every second year. It takes longer but not twice as long so for the commercial olive growers it is a saving. In the south, in Puglia, they prune less often and correspondingly much more drastically.
I simply use hand secateurs and carry a folding saw in my pockets for the bigger branches. Unfortunately pruning involves ladder work although I did go to a demonstration by Dr Panelli from Spoleto Olive Research Centre where he used extension secateurs all the time. I was most impressed but could not get the hang of it myself.
We now burn the prunings. For many years we chopped them with a trinciatore – a mulching machine - which worked very well. Now we use a conventional rotary mower to cut the grass in the grove as it can pass under the trees. The rotary mower cannot chop the prunings as the trinciatore did, so we burn them. Although in principle I would like to see the organic matter returned to the soil, in fact our grass and legume pasture returns so much that I do not think the prunings would make a significant difference.
During the Mussolini era it was compulsory to burn olive prunings because it was believed that they spread disease. I do not think there is any strong evidence that they do. Chop or burn – whichever suits your land management plan.
March is the time to think about fertiliser for your trees. The conventional olive mixture is NPK + B with a lower level of nitrogen than the usual NPK mixtures. The olive mixes also include boron which is most important for olives. Unlike in southern Spain where boron deficiencies are acute, most soils in Italy have some boron but the application of small amounts will improve the health of the trees. Make sure the mixture you buy has NPK+B on the bag. It is difficult to apply boron without buying the complete mixture yet the other elements may not be needed.
Nitrogen should be applied only in small quantities, if at all. Nitrogen will stimulate the growth of the tree but not the fruiting. If you have a pasture under the trees with a good legume content the trees will receive all the nitrogen they need from the legumes.
Potassium (K) is important for fruiting but its availability depends on the pH of the soil. If the soil is alkaline, then potassium deficiency is unlikely.
Finally there is the phosphate. I apply this because it stimulates strong legume growth. This means there is a vigorous pasture under the trees. When this is cut and mulched it increases the organic matter in the soil and provides nitrogen for the trees in a slow-release form. I believe a healthy soil is fundamental for healthy trees.
Some months ago I wrote about land management and advocated the use of pasture in the olive grove rather than ploughing or cultivation. I have had a considerable response to the piece. I wrote it from general principles and my own experience as a farmer but I have since caught up with some newly reported research from Tuscany which fleshes out some of my argument with figures. Nadia Vignozzi, Sergio Pellegrini and Stefania Simoncini write in the 8/14 October issue of L'informatore agrario about their trial that compared pasture with cultivation under olives. They make the same points about cultivation causing the mineralisation of organic matter and the poor soil structure that develops as cultivation is used year after year. The soil absorbs less rainfall, it caps over with heavy rain and compaction layers form below the depth to which the soil is cultivated. What is so interesting about the article is that it puts figures on these general comments and they even startled me in their stark differences.
First, the comparison was between a spontaneous pasture (there was no attempt to improve its botanical composition and push it towards more legumes) and what they called a minimum cultivation programme of 3 to 4 cultivations to a depth of about 10 cm with tined implements (cultivators with tines or teeth).
With the pasture the rainfall could infiltrate the soil at the rate of 35 mm per hour on average. There was a range between 25 mm and nearly 50 mm per hour. This is remarkable as rainfall is rated as abundant if it is above 5 mm per hour. Above 10 mm per hour it is really heavy. Rainfall at a rate of 25 mm an hour or more is cloudburst stuff and is extremely rare. 25 mm is more likely to be the rainfall for a day not an hour. Of course the rain can still run off if the soil profile is full. Soil is like a sponge. It takes in a great deal of water but finally fills up. However the pasture will still hold the soil so erosion is minimal.
With the cultivated land the infiltration rate averaged slightly less than 5 mm per hour with a range of 8 mm to almost zero. One can see how run-off and erosion are such a common occurrences in cultivated groves.
The pasture improves the organic matter of the soil and its ability to hold moisture at a tension that is available to olive trees. “Tension” is the term used to describe the tightness with which water is held in the soil. Plants are only able to extract some of the water from the soil. The remainder is held too tightly to be of any use to plants. Organic matter holds water at low tensions thus making it available to plants. The researchers’ measurements showed that 170 mm (again an average with variations above and below) were available from the soil profile under pasture and 150 mm with the cultivated grove.
On the crucial question of output and quality of the oil they could not come to a conclusion except to confirm the results of earlier experiments which showed that pastures compete for moisture with young trees and that cultivation is a good idea for the first couple of years with a new grove. Even then one does not need to cultivate the whole grove as the root zone of young trees is only a few metres in diameter. The question of yield is extremely difficult to determine in olive experiments. As we all know, the yield of olive trees is extremely capricious and this makes it difficult to determine whether an increased yield is due to some treatment or merely to chance. Researchers must subject their results to rigorous statistical analysis if they are going to maintain their scientific credibility but for us growers a lower level of proof would be helpful.
The other difficulty with the experiment is that they used the term “spontaneous plant cover” yet this can include a multitude of different pastures. I have advocated a pasture with a large legume content (clovers, vetches and medic). The legumes will restore the soil organic matter more rapidly and provide nitrogen for the trees. To obtain a good pasture you will need some phosphate fertiliser and you will need to cut the pasture at least twice a year to prevent the grasses shading out all the other species.
An olive pest, Xylella fastidiosa (Xf), has recently been identified in the Puglia region of Italy but there is considerable confusion over its impact. Bloomberg Business News reported the infestation as a major cause of the low level of olive oil production in Italy in 2014. In fact this was caused by the olive fly which has been with us for as long as olives have been grown in Italy. The Xf outbreak is extremely serious in the long term but the immediate impact on olive oil production for the whole of Italy is slight. Thousands of hectares of infected trees sound a great deal but in the context of the millions of hectares of olives in Italy it is insignificant. The disease is extremely serious in the longer term because there are currently no known control measures. If it spreads throughout Italy it would be a disaster.
The outbreak is in Puglia which is the most historic part of the Mediterranean for olive production. UNESCO has declared ancient olive trees a world heritage. Trees that are many hundreds of years old have been individually identified and recorded throughout the Mediterranean zone. Tens of thousands have been identified in Greece and other Mediterranean countries but Puglia alone has more than a million of these heritage trees. The disease is currently confined to the Salentino peninsular but all of Puglia and the rest of Italy is under threat.
Xf is a bacterial disease that is transmitted from tree to tree by sap-sucking insects. The primary carrier of the bacteria in Puglia is the spittlebug (Philaenus spumarius). Infected trees die rapidly because the xylem (sap tubes) become blocked and the disease is sometimes called Olive Quick Decline Syndrome (OQDS). The Xf group of bacteria will infect a wide range of commercial crop plants including vines, citrus, stone fruits as well as olives but it appears that they are different strains of Xf. Xf has not attacked these crops in Italy. However the disease in Italy has appeared in other species besides olives, including almond and oleander. Most surprising is its appearance in oak trees.
The appearance in oak trees is of great concern. There are millions of hectares of oak forest in Italy and if the disease becomes entrenched in these forests it will not only cause damage to the oaks but become a permanent reservoir of infection for the commercial tree crops.
The outbreak was first identified in the Province of Lecce in 2013 and spread over many thousands of hectares. Controlling the advance of the disease with insecticides does not seem to be a practical method although it could have some short-term benefits. The main insecticide for use against the carrier insect is the neonicotinoid group which is being banned by the EU because of its adverse effect on bees. Of course olives are pollinated by wind not bees so the Italian government could probably obtain an exemption for a short period but other methods of control will need to be found.
The Corpo Forestale is destroying infected trees but for this to be effective farmers need to obtain generous compensation so they will report outbreaks rapidly. Healthy trees need to be destroyed around the infected ones as a local buffer zone. The Corpo Forestale does not have the resources to monitor the vast area under olives in Puglia and needs to have the cooperation of farmers. They in turn must have a strong incentive to report their concerns immediately and not wait in the hope that the trees will recover. As well as generous compensation, the administration of the fund must be rapid. Thirty years ago the compensation for olive growers affected by severe frost in central Italy took years and years to be paid. The financial authorities must realise that the faster and better the payments, the less they will need to disburse in the long term. Short-term savings will translate into long-term costs.
The EU has adopted a ban on the movement of plant material outside the infected zone and has protected that area with a further buffer zone and security zone. It can be seen from this map that the infected area (orange) is in the Salentino peninsula but there has been one outbreak already that has jumped the main buffer zone (dark green) into the Province of Brindisi. The security zone is purple.
We were in the Salentino area during April 2015 on a tour organised by the Mediterranean Garden Society’s Italian Branch and saw many dead trees. I do not know whether they were infected with Xf but it is quite probable. We were certainly not aware of the various zones declared by the EU in February 2015. There were no signs on the roads or any other information indicating that we were entering the infected zone or buffer zone.
In Australia we have had considerable experience of quarantine and have managed to keep Phylloxera (the disease of vines) out of South Australia through strict controls for more than one hundred years. These measures include large warning signs on all roads indicating that the movement of plant material is prohibited and on the major highways we have road blocks where trucks and cars are stopped at random and inspected. If the quarantine is going to be effective in Puglia the authorities will certainly have to take serious action to make people aware of the bans and enforce them.
While the outbreak of Xf was confirmed in 2013 it is fairly obvious that it started some years earlier. It seems to have come in through the importing of ornamental plants from South America. Oleanders and ornamental coffee plants from Costa Rica are thought to be the main culprits. There have been other outbreaks in Europe. One was in Paris where an infected ornamental coffee plant was discovered in a suburban market. Paris is hardly an olive-producing zone but one should not laugh as London and Paris have large numbers of ornamental olive trees. It would be quite easy for a Parisian or Londoner to take their potted olive tree to their summer house in the south of France and spread the disease. Another outbreak has been discovered on Corsica which has about 7,000 hectares of olive groves. The Xf was found in a myrtle-leafed milkwort plant, not olives, but it could easily spread to olives and other trees. So far research has shown that this strain of Xf does not infect citrus but we are still waiting for more comprehensive tests on other plants.
The EU is considering a ban on the importing of such plants. While this would be an excellent short-term measure it is time that the EU reconsidered its whole attitude to plant and animal quarantine. Many decades ago quarantine was used as an excuse for trade restrictions. The pendulum has now swung too far in the opposite direction and the free trade and globalisation advocates are in charge. Almost all quarantine has been swept away. It is time to rethink our policies and apply some more stringent needs-based tests. It is not enough to say that a particular plant may carry a certain disease. We should go further and ask whether the importing of a certain plant is really necessary. If it is not, then the assumption should be that it is not worth the risk. It may be a carrier of a disease not yet identified. In this particular instance Europe has plenty of oleander varieties. I cannot see any need to import more. Our standard of living will not be reduced if we cannot buy ornamental coffee plants. Merely testing the plants for disease is not enough. No one knew at the time that they carried Xf or that the strain of Xf they carried would infect olives. We have to assume there are other diseases out there that could be equally damaging.
It is time that farmers and gardeners make their voices heard against the free traders and obtain some protection against exotic diseases and quickly. We do not need more reports, studies and reviews but implementation before another series of exotic diseases invades Europe. To think that olives have been grown in Puglia for well over 2000 years and may now be destroyed as a by-product of free trade is a terrible indictment of current trade and economic policy.
At present the only method of control is the destruction of infected trees and those in the vicinity. There was however an interesting article in The Olive Oil Times in September 2015. It reported on research work being done at Texas A & M University on an Xf disease found in grape vines. The researchers made a mixture of four phages which they found significantly reduced the damage caused by Xf in grape vines.
Phages are viruses that attack bacteria. One is reminded of the nursery rhyme:
Big fleas have little fleas,
The ad infinitum may be a little exaggerated but the idea is sound. Most pests and diseases do have natural pests and diseases that control them in turn. We have found in Australia that new pests often arrive without their biological controls and we have to return to their country of origin to find their natural predators. The Xf that attacks vines does not appear to attack olives so other strains of phages will need to be found as well as effective means of introducing them into the infected olives. The researchers are only claiming a reduction in damage not a complete cure. All this will take some years so we will have to rely on the destruction of infected trees in the meantime. Hopefully this will be implemented rapidly.
An alternative view is put forward on the website xylellareport.it. This group claims that the measures taken so far are too extreme. They say that only 1.8% of the trees tested so far have proved positive for Xf. It is not clear whether the 1.8% refers to trees already showing signs of decline or a sample of all the olive trees selected at random. The Xylella report group claims the major problem is that the olive trees have been treated with too many chemicals – herbicides in particular. Certainly when we were in Puglia with the Mediterranean Garden Society we noticed how the frequent use of herbicides under the trees had created an ecological desert with an adverse effect on the biology of the soil. There may be some validity in the report group’s argument that the problem has been exacerbated by a dependence on chemicals and poor soil health but the fact that 1.8% of the trees have tested positive can be used as an argument in support of the eradication policy. If the incidence is so low there is a good chance it will work.
August and September will test your land management skills. During the last part of August or in September we usually have the heavy rain that marks the beginning of the growing season in the Mediterranean zone. This growing season has already started with some heavy falls that have caused flooding. These heavy rains can lead to a loss of water and soil from the olive grove. The loss of soil is the most visible effect and has the greatest long-term impact but the loss of water is also important. After a mostly dry summer (we recorded only 9 mm of rainfall between 20 June and 10 August) the olives need a drink. They are well able to stand this summer drought but need moisture in the two months leading to harvest in late October or early November.
The great land management debate goes back to the previous spring and hinges on whether to cultivate the land or not. Traditionally olive groves and vineyards have been cultivated in the spring throughout the Mediterranean. The practice has some scientific support as the weeds that grow in the spring take moisture from the soil that would otherwise be available for the olives or vines. Of course some moisture will evaporate from the bare cultivated soil but the roots of weeds will penetrate to a considerable depth and take much more water.
The opposing argument is that the cultivation of the soil year after year for decades and even centuries totally destroys the soil organic matter. Without organic matter clay soils have no structure. That means they pack down easily into something like the clay used for making ceramic pots. It is almost waterproof and the water runs off rather than soaking into the root zone. If the grove is on a slope the water will wash down hill taking some soil with it. In southern Spain there are large olive estates that have been cultivated for centuries and the olive trees are growing on small hillocks as much as a metre high. The roots of the olive trees have held the soil in the immediate vicinity of the trees but autumn and winter rains have washed enormous quantities of soil away between the trees.
The erosion crisis is not confined to the olive estates of Spain but is happening here in central Italy although cereal farming is probably a greater culprit that the olive groves. The British School in Rome conducted an archaeological dig near Gubbio in the late 1980s. They found that the soil deposited from the erosion of the surrounding hills during the Roman period amounted to 20 cm. This period of about 400 years included moderately intensive cereal growing. After the collapse of the Roman state the population fell and cereal production was less intensive. There were more animals grazing on pasture and over the next 1500 years or so another 20 cm were deposited in the erosion sediments. From 1950 a whole new range of artefacts appeared in the sediments. The archaeologists referred jokingly to them as the Coca Cola layers. From 1950 to the late 1980s two metres were deposited or five times the amount for the previous 2000 years. I don't know whether there has been any further work on the site but I imagine there has been another metre deposited as the farming system has not changed.
Gubbio is no exception unfortunately and I can see fields near Orvieto that will need to be abandoned soon as the erosion has been so severe that the bed rock is visible in places.
I have found that cutting the grass, wild flowers and weeds in the spring over the last 25 years rather than cultivating has produced a soil structure in my olive grove that is like a sponge; even when all the grass is dry at the end of summer the heaviest rains soak in. In winter the sponge is eventually full and run off occurs but it is crystal clear and carries no soil with it.
The arguments are not as simple as this, however – they rarely are in farming. Obviously the slope is important. In central Italy most olive groves are on a slope. Traditionally the valley floor has been used for cereals and in any case the floor is not sufficiently well drained for olives. The gentle slopes have been used for vines and the steeper and poorer ground for olives. The olives groves are therefore vulnerable to soil erosion. This is quite different from Puglia, for example, where the olives are grown on the plain or in areas with a gentle slope.
The soil type is also important. Clay soils will cap over and become resistant to the penetration of rainfall more than loam soils or sand. Soil organic matter and the good soil structure that goes with a meadow are therefore more important.
As well as the question of water, a well-managed meadow under the olive trees provides a healthy soil with more than enough nitrogen to satisfy the low requirements of the olives. The meadow will contain pasture legumes such as annual medicago, clover and vetch. When these are cut and mulched down in the spring they provide a fertile and healthy soil for the trees.
There are other non-scientific reasons not to cultivate. Years of cultivation in the grove destroy not only the annual plants but also their seed bank. When the rains come in autumn the plant cover is limited to a few species that are prolific and early producers of seed that have therefore evaded the spring cultivation. Many of the colourful wild flowers disappear altogether, together with the bulbs and ground orchids. The grove has become an ecological desert.
I cultivated my grove many years ago to level the ground. Before we purchased the grove it had been ploughed and many years later the ground was still uneven. I cross-cultivated to level out the furrows. Picking the olives that autumn was a battle against the mud. It was an experience I never want to repeat.
If you are convinced that a meadow is the better option you need to act soon. If the land was cultivated in the spring is may be quite rough. If you decide to turn it into a meadow it is better to try and smooth it as much as possible because that is the way it will stay. In my olive grove the land had been ploughed in one direction and even ten years after the grove was abandoned the furrows still existed and I had to cross-cultivate to smooth them out. If the grove has been ploughed for years, little will germinate except some spiky thistles. Over a long period a meadow will emerge but seeding in the autumn with annual medicago, clovers and vetches will give the meadow a good start.
While many plants in the garden are suffering from the heat and lack of rain the olive trees are fine. In California where they have had a severe drought for many years they are learning that olive trees do not require summer irrigation for a good crop of oil. Traditionally olives in California have been irrigated because the majority are used to produce olives for the table. Most of the olive oil comes from the olives that are rejected by the processors as too small for the table. The lack of water due to the drought has meant that the growers cannot irrigate and they have discovered the Italian tradition according to which irrigating olives for oil production needs to be carefully timed.
If you have the water it is beneficial (in the years when rainfall is inadequate) during the period of flowering and then in September and October when most of the oil is produced. Irrigation during the summer will increase the size of the olives but they will have a low oil content and you have achieved nothing except higher processing charges at the frantoio (oil press).
Earlier this year we went on the Italian Branch's tour of Puglia and were most surprised that so many olive trees were irrigated. The trees in Puglia are hundreds of years old and some even date back thousands of years. They have reached an enormous size without a single drop of extra water and we wondered why they needed it now.
A telephone call to Corrado Rodio, owner of Masseria Brancati which we visited just outside Ostuni where the ancient olive groves include some trees that are between 2500 and 3000 years old, clarified some issues but not others. He irrigates all young trees to encourage growth. He also irrigates all trees if there is no water for more than a month whatever the season. In Puglia usually this is not necessary during the flowering period nor in September/October as there tends to be natural rainfall. While he knows that watering will increase the risk of olive fly (they like humidity and plumped up fruit) he maintains that watering does increase oil production. And so opens our debate about best practices and cultivation techniques in the olive grove.
by Brian Chatterton
Art or science?
Cocktail glasses and dry martinis
Renovating old trees
Step by step guide
How can you tell?
Let there be light
© Brian Chatterton
Brian Chatterton has also written an E-book about olive cultivation.
This book was reviewed in The Mediterranean Garden No. 47 January 2007
by Brian Chatterton
The production of olive oil is a mystery. Unlike vines where teams of pickers or large machines bring in the vintage, the picking of olives is an inconspicuous operation. Pruning is the same. Vines are transformed from a tangled mass of twigs to neat pared rows of almost identical vines. One rarely sees the olive pruner at work. Perhaps there is a ladder propped against a tree and one sees some prunings on the ground before they are burnt or mulched. It would be a mistake to be fooled into believing that this is a nighttime operation carried out by elves. In fact the classic olive grove is the result of care and attention over many years. The olive trees in Umbria and Tuscany are not by any stretch of the imagination a natural landscape. The untended olive is an untidy bush and it is only through pruning that it has form.