Olive resistant cultivars significantly reduce transmission efficiency of Xylella fastidiosa ST53 by the main European vector, Philaenus spumarius (Hemiptera: Aphrophoridae)

The epidemic of Olive Quick Decline Syndrome (OQDS) caused by Xylella fastidiosa subsp. pauca ST53 in South Italy olive groves has a dramatic impact at the economic and social levels. The combination of a landscape dominated by susceptible olive cultivars and high density of the insect vector Philaenus spumarius (Hemiptera: Aphrophoridae) led to the loss of several millions of trees in about ten years .
Given that the bacterium is now endemic and control of vector populations at a landscape level appears difficult, replantings of resistant/tolerant olive cultivars, Leccino and FS17 being the main ones, is envisaged as the best option.
This study investigated the transmission efficiency of X. fastidiosa ST53 by P. spumarius from resistant infection sources and to resistant recipient plants.
Several acquisition and inoculation trials were performed in different seasons during three years in both field and controlled conditions in the infected area in Apulia region of Italy, comparing resistant olive cultivars with the most common susceptible ones grown in the infected area .
The experiments demonstrated that i) both resistant olive varieties were inefficient sources of the bacterium for the vector (and subsequent inoculation rates were very low), ii) when used as recipient plants, both resistant and susceptible olive cultivars were infected at a similar rate, iii) the inoculation rate on both susceptible and resistant plants was positively related to the number of infectious insects that fed on test plants.
These results highlight the potential positive impact of the cultivation of resistant olive cultivars in X. fastidiosa ST53 epidemiology in South Italy. Indeed, the main epidemiological cycle is olive-to-olive, and thus replacement of susceptible cultivars with resistant ones has the potential to suppress the bacterium inoculum and spread. Although resistant varieties only show mild symptoms of infection, the fact that they are still susceptible to infection – together with the incomplete comprehension of the epidemiology – make control of the vector populations and reduction of the sources of inoculum at the crop and landscape levels still of paramount importance. Our results provide useful data for modeling the future evolution of epidemic in Italy and implementing the use of resistant olive varieties, in order to cope with OQDS in infected areas and neighboring regions at risk of X. fastidiosa introduction.