New insights on Xylella fastidiosa subsp. pauca vector transmission to olive plants

The spittlebug Philaenus spumarius L. is known to be the predominant vector of Xylella fastidiosa Wells et al. (Xf) to olive trees in the Apulia Region of Italy. However, factors influencing Xf transmission by P. spumarius have not been studied and, consequently, the epidemiology of Xf-associated emerging diseases cannot be properly understood. Here we investigated 1) the kinetics of the bacterial persistence and multiplication, along with transmission efficiency by the vector, and 2) the spread rate of Xf among olive plants in summer and autumn. For the first objective, olive-to-olive transmission efficiency, Xf retention and multiplication in P. spumarius adults were tested in four separate assays carried out in 2017-2018 in different seasons. For the second objective, experimental assays in microcosms were carried out in the same periods to assess influence of i) IAP duration, ii) climatic conditions (semi-field vs controlled), and iii) season on the spread of Xf infection within an olive seedling population. The results show that i) P. spumarius is a competent Xf vector to olive throughout the season and adult life ii) bacterial load in the vector foregut reaches a plateau after 2-3 weeks from acquisition iii) different climatic conditions and period of the year may result in significant differences in transmission rates and iv) differential survival of vectors — influenced by insect age, season and climatic conditions — may affect the spread of Xf in olive plants. In summary, this work describes the influence of seasonal and environmental factors on the transmission characteristics of Xf by P. spumarius. Such transmission dynamics may explain epidemic patterns in Apulian olive agroecosystem. The transmission parameters obtained in this study can be used in modelling the pathogen spread, by explicitly incorporating the effect of insect vectors, with the aim of designing effective control and prevention measures.