Xylella fastidiosa: patogenesi, danni economici e lotta al disseccamento rapido dell’olivo

fastidiosa is a xylem-limited plant pathogenic bacterium having a wide host range and many leafhopper vector species. It causes several serious plant diseases such as the Pierce’s Disease of grapevine (PD), Peach phony Disease (PPD) and Citrus variegated chlorosis (CVC). The bacterium was first detected in grapevine in the USA then spread to both central and southern Americas to appear in Europe in 2013 for the first time. The area involved was that of Salento (Apulia, Southern Italy) where it infected the plantations of olive trees and several other cultivated and wild plants. X. fastidiosa is a unique species including four subspecies, which differ each other for their biological and molecular properties, namely: X.fastidiosa ‘fastidiosa’, X.fastidiosa ‘multiplex’, X. fastidiosa ‘sandyi’, and X. fastidiosa ‘pauca’, this latter being the only subspecies detected so far in Salento. Symptoms on olive trees consist in marginal burning of leaves, which gradually extends to the whole leaflets, drying of branches rapidly expanding to the whole vegetation, and death of plants within 1-2 years after the early symptom appearance. Damage may be increased by two more parasites of the olive plants, the moth Zeuzera pyrina which carves deep galleries in both their trunk and branches, and the fungus Phaeoacrimonium sp. that can penetrate and cause diffuse rot of the plant wood. The disease due to Xylella in association with the two above parasites has been named “Olive quick die-back complex†(CoDiRO†). Three spittlebug species have been identified as local vectors of the disease, i.e. Phylaenus spumarius, Ph. italosignus and Neophylaenus campestris. The hoppers acquire Xylella by feeding on its many cultivated or wild host plants and retain the bacterial cells attached to the lining of their foregut: adult leafhoppers can transmit immediately after acquisition and retain infectivity for their whole life span while their immature forms, when infected, loose infectivity after molting. About 60 mlns olive trees are grown in Apulia, 22 mlns of which occur in the area affected by the disease. Five mlns of these are one-hundred or more years old while three-thousands are the so called “monumental plants†, i.e. those older than one-thousand years. It has been estimated that economic losses since the early detection of the disease are about 1,200 mlns Euro. One project to eradicate the disease started soon after detection but in a couple of years it was realized that it was not efficient as CoDiRO continued to spread causing more and more severe damage. It was then attempted to contain at least the disease by using control strategies most suitable in the local situation. They are briefly reported and commented in this paper, and include: insect vector control by insecticides and innovative products, removal of infection sources by using various cultural practices, eradication and destruction of infected olive trees and surrounding plants or just their branches with symptoms, spraying with various chemical products, looking for sources of resistance to Xylella in olive cultivar. It is finally pointed out that, on the basis of its pathogenetic characteristics, X. fastidiosa can be included in the new taxonomic group of “Vascular (insect-borne) phytopathogenic bacteria†.