Olive cultivation: mechanical work to prevent Xylella

Numerous machines and types of equipment are at work in olive cultivations in a series of crop operations for improving the dynamics between the tree and soil to activate the immunological defenses of the plants themselves. Pruning, weeding, removing infestations and grass cover with compatible vegetable species are some of the operations necessary for olive cultivations to fend off Xylella made possible with appropriate mechanical technologies

The bacterium Xylella fastidiosa, subspecies pauca, is attacking olive cultivations in the Italian Salento, a district of the southern Puglia Region, and to stop the spread of the pathology to other olive growing areas scientists and technicians are not only researching systems capable of controlling the population of the vector insects, and their endemic effects, but also by improving the immunological systems of the olive trees. Doing so would make it possible to curb the Xylella fastidiosa present and allow the bacterium to live in cohabitation with the plant as has been the case with other fruit trees for which phytopathological defense, the management of the soil and pruning have been the crop techniques used for increasing the capability to counter biotic and abiotic stress caused by the bacterium.

These good agricultural practices, however, require caution in dealing with disinfecting pruning equipment, the removal of infestations after mowing and burning the pruning residues. Unfortunately, some of these essential crop operations are not carried out in the perimeters of olive cultivation areas because due to the rustic characteristic of olives mechanized operations, those of land management, work, weeding, fertilization, irrigation and hygienic defense, are not considered necessary. This view is wrong in that the crop operations which mark the vegetative and productive cycle of olives optimize the dynamics between the tree and soil which, other than enhancing the potential of the tree itself, activates its immunological system.

The management of olive cultivation areas can be performed on worksites for different types of machinery and equipment in accordance with the agronomic techniques intended for operations. These boil down essentially to two: keeping the areas always free from vegetation in the spring with summer operations with machines for the task or the use of chemical products for total or partial weeding;  working on the permanent vegetable cover or the temporary grass cover with the selection of the vegetation in place and eliminating infesting species and seeding species of grass or legumes. One solution can be integrated or combined with the other with weeding along the olive tree rows and the permanent or temporary removal of the grass cover in the rows by plowing under grass seed species in the autumn-winter period to increase organic substances content and nitrogen in the soil. The worksites differ according to the type of overall management of the land as well as the biological structure of the soil. As is known, conservation management does not involve overturning the terrain but only mixing it to renew capillary action of the soil and the fixation of carbon, the Calvin cycle. These operations are usually carried out after the olives are harvested, in the autumn-winter period, to facilitate burying mineral fertilizers and post-harvest residues from mowing and chipping and cut grass infestations. This work is usually performed with weed plows and harrows combined with subsoilers with a plowshare to loosen and break up soil beyond the effective soil depth to achieve chemical-biological processes at depths below the level of traditional plowing. According to the geopedological nature of the soil surface of the cultivation area and degree of compaction, these operations are carried out at intervals by work for subsoiling down to a depth of 80-90 cm with single vertical cuts and plowing to 40-50 cm for reworking the effective soil depth, the fertile soil layer. Plowing is done with a disc harrow plow which ensures faster work and greater fragmentation and mixing of the soil compared to operations with a mouldboard plow. Machinery with rotary tools, a cultivator powered by a tractor PTO equipped with spading, drilling, tilling tools and the like, produce excessive loosening of the soil, called dust-mulch, a fine loose layer of surface soil, especially on soil with a high clay content. This procedure favors the formation of a work sole in the soil layer under the one being worked which changes the soil-root apparatus ratio with negative results for taking up mineral or organic fertilizers and the circulation of rainwater. This means that it is advisable to perform operations at intervals with machinery with rotary tools with equipment for mixing the soil with the use of subsoilers periodically, every3-4 years, working at depths of 40-50 cm in the center of the rows. The equipment used for spring-summer operations are weeders and harrows, though the tendency is to deploy these machines also for the post- olive harvest period. Traditional weeders are equipped at their extremities with various types of tools, such as small shares, which penetrate the soil down to an average depth of 10 cm and perform a horizontal cut of the roots of infestations. The work of weeders does not bring subsoil to the surface but restores the soil structure to favor aeration and rainwater penetration. Weeders perform a partial mix of the soil and weed control for infestations with the most superficial roots. The most common type of machine is the disc weeder equipped with discs and oscillating teeth used for spring-summer operations in olive cultivations. Some weeders are e­qui­p­ped with scallop discs ideal for working clods and compacted soil. Permanent or temporary weeding of the cultivation surface improves the structure of the soil to facilitate work of machinery and equipment, even following weather events, and normalizes the humidity levels of the terrain which undoubtedly favors the takeup of such elements as iron, magnesium and nitrogen of great importance for the trees’ physiological processes. It is a good idea to take into account that olive trees and surface weeds compete for the takeup of water resources, especially in the first years of the olive cultivation, which can have a negative vegetative-productive result for the trees. This means it would be advisable to carry out weeding between the rows periodically by using mowing equipment usually used for harvesting forage whereas along the rows infestations can be controlled by inter-row mowers, mulch and chemical weedkillers. To limit water-nutrition competition between olive trees and grassy terrain weeding is performed 2-3 times a year with a mower bar, a grass mulcher, shredder roundbaler and a shredder of prunings for mulching these residues. The first mowing is generally done in April-May, after the winter rains, to avoid competition at the onset of the plants’ growing cycle. Successive cuttings are performed when the grass cover reaches 15-20 cm in height with mowing at 5-6 cm off the ground to not compromise new growth. Mowing-mulching is normally done two years after seeding so that the grass cover takes on density to act as a mass of absorbent material between sublayers of soil and the surface grass. Then is a good time to to work with harrows equipped with teeth to aerate the soil and bury fertilizers. Pruning is another operation fundamental for maintaining vegetative activities of the olive trees with targeted fertilization capable of preventing the spread of Xylella fastidiosa. Pruning should be ordinary and not drastic and the residues should be removed and destroyed and pruning should be done before the appearance of the Xylella fastidiosa by starting on the dry parts of the plant. Pruning shears are used below the vegetative axis and the removal of suckers, branches growing directly from the lower trunk, should be removed to avoid infestation at this level.