Stylet penetration activities linked to the acquisition and inoculation of Candidatus Liberibacter solanacearum by its vector tomato potato psyllid

The tomato potato psyllid (TPP), Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), is the main vector of the bacterium Candidatus Liberibacter solanacearum (Lso), a major disease of solanaceous crops. Feeding of TPP is associated with Lso transmission. However, very little is known about the stylet penetration activities linked to acquisition and inoculation of Lso. The electrical penetration graph (EPG)-DC system was used to monitor stylet penetration activities during acquisition and inoculation of Lso by individual TPP on tomato [Solanum lycopersicum L. (Solanaceae)]. Female TPP from Lso-free and Lso-infected colonies were used in acquisition and inoculation tests, respectively. In the acquisition tests, TPP were tested for Lso after EPG recording of their stylet penetration activities on Lso-infected tomato shoots. In the inoculation tests, samples from the tomato plants on which the stylet penetration of Lso-infected TPP had been recorded were tested for Lso infection. The relationships between qPCR results and the EPG waveforms (C, G, D, E1, and E2) representing the main stylet penetration activities performed by individual insects in inoculation and acquisition tests were investigated. Results confirmed that a single adult TPP is capable of infecting a plant with Lso. Our data suggest that acquisition of the bacteria occurs during phloem ingestion (E2), and inoculation is likely associated with salivation into the phloem sieve elements (E1). The durations of EPG parameters were not significantly different between Lso-infected and Lso-free TPP (later shown by qPCR) in acquisition tests. In inoculation tests, the durations of E1 or E2 recorded from TPP on Lso-infected and Lso-free plants that were later shown by qPCR were not significantly different. However, C was shorter on Lso-infected plants than on Lso-free plants, where TPP performed phloem activities. The minimum plant access period required for Lso transmission by a single TPP was estimated to be ca. 2h, with an acquisition threshold of about 36min.