During the summer and in July, in particular, the biggest phytosanitary problems for the olive grove and its precious fruit arise. The growers must be extremely alert and ready to react promptly to avoid losing their entire production.
An initial protective measure is to apply copper to the foliage after pruning. This treatment can be repeated in the autumn. Copper prevents one of the most common diseases of the olive, olive peacock spot, a fungus that appears on the leaves as small round black spots (hence its name). If left untreated, it can become chronic and cause various types of damage such as the yellowing and loss of leaves. In addition, it favours the onset of other diseases.
In any case, the principal threat to olive crops may appear already in the early summer. It is called the olive-fruit fly. Depending on the climatic conditions and the parasite population present in the area, the fly starts to fly and bite the olives.
The fly attacks the fruit, that is, the olive itself. When the olive starts to grow and develop flesh and, above all, when the stone starts to become hard, the female fly makes a puncture in the skin of the olive with its ovipositor and leaves an egg in the flesh below. A larva will hatch from the egg and will move inside the flesh, feeding on it until it reaches the stone. This larva will become an adult fly, will dig a hole to the outside and fly. If the climatic and regional conditions are favourable for the parasite, there may be three or four infesting fly generations. The olive undergoes severe damage: the fruit attacked deteriorates, drops early and even if collected has unpleasant organoleptic properties. It is therefore necessary to be on the look-out and take prompt action. The olive grove is to be monitored carefully, with pheromone traps or inspecting it several times and picking some olives to check for the presence of the characteristic bites on the skin of the olives. This year (2014) the first bites were found at the start of July. Very early, too early. This is because often, years of abundant production are followed by seasons in which the fly population increases. Olives that have dropped from the trees but have not been collected are one of the triggering factors: they are an ideal place for the olive fruit fly to spend the winter. In addition, this year olive production is low so damage is more severe due to its proportion.
When the percentage of infestation of the sampled olives exceeds 12% of those to be used for the production of oil and 4 % for those to be eaten, prompt action must be taken.
Several options are available: the use of products that stop the fly biting the fruit (kaolin, copper, etc.) or products that kill both the egg and the larvae of the fly and that are active on the fruit for a given number of days (preharvest interval). There are also systems that use traps based on pheromones to attract the adults and limit the populations. These, however, are expensive and difficult to apply on our uneven land.
In general, all these types of products are sprayed onto the foliage of the tree: the spraying machine is connected to a pipe leading to a tank on a tractor or other farming vehicle also equipped with a high-pressure pump. The pipe has to be unwound along the terraces so as to reach all the trees present. It is a long and tiring job so it is only done when there is a high risk of infestation. This is also because there are years in which this measure is not at all necessary or the need for it is limited.
Another widespread disease is called olive knot. It is caused by a bacterium transmitted via open wounds and spreads when pruning is performed with infected tools on sick trees. Olive knot is manifested with round tuberous excrescences on young branches, which are thus weakened and may dry up.
Olive knot is controlled by copper-based products and using sterilizing tools. Copper is also used to combat other diseases such as fumagine: a fungus that appears as a sooty layer of mould on the leaf surfaces. This disease is often a consequence of other infestations such as that caused by cochineals or poor growing conditions: for example, little air circulation in the foliage. A typical condition of unpruned trees in shaded areas.
There are also some less important parasites that may, however, if neglected, cause severe damage to the tree and its fruit. These include the olive moth, which is a moth whose larvae develop at the expense of the leaves but, above all, of the flowers and fruit. In this case, the tree is treated with pyrethrum products, using traps or natural moth parasites bred and selected for this purpose. This strategy is thus called integrated treatment with natural antagonists.
And unfortunately, that’s not all: other increasingly serious threats are posed by the olive gall midge, an insect that digs tunnels into the young wood, which then weakens and dries at the tips.
The olive fruit fly has always been the biggest scourge on the olive tree. There is a long history of infestations, damage and remedies for controlling it. Some of these appear to be empirical and unusual. Others, starting from the end of the 19th century, are already based on the use of chemicals. In those days, however, insufficient attention was paid to the environment and their times of effectiveness
Farmers gave great importance to requesting supernatural protection. On 25th April, for the Feast of Saint Mark, processions were held around the town where the cultivated fields began near sacred pillars, chapels or votive crosses. The people recited phrases for protection against the hazards of nature and war and prayed for a good harvest.
It should also be pointed out that unharvested olives represent a favourable environment for the incubation of fly larvae and thus the spread of the parasite. Today, abandoned olive groves (of which the number is fortunately decreasing in some areas) and the rapid mechanized harvesting of olives result in a number of olives being left on the ground however large the nets used are. In the past, when olive harvesting was done by hand (an extremely tiring task), even olives that had dropped between the stones in the walls and along the mule tracks were collected. This may have been detrimental to the quality of the product but no olives were left on the ground at all.