Improvement in the Excised Citrus Leaf Assay to Investigate Inoculation of Candidatus Liberibacter asiaticus' by the Asian Citrus Psyllid Diaphorina citri

Abstract

Huanglongbing (FMB) is a difficult-to-control and highly destructive citrus disease that, in Brazil, is associated mainly with the bacterium 'Can-didatus Liberibacter asiaticus' transmitted by the psyllid Diaphorina citri. The aim of this study was to improve our understanding of the 'Ca. L. asiaticus' infection process by exposing excised, fully expanded, immature citrus leaves in 50-m1 Falcon tubes to one, four, or eight adults from a 'Ca. L. asiaticus'-exposed colony for 1-, 3-, 7-, or 15-day periods for access to inoculation (IAP). The leaves were incubated at 26 degrees C for 1, 3, 7, 15, and 21 days (incubation period [in. Infection frequencies and 'Ca. L. asiaticus' titers were assessed by quantitative polymerase chain reaction (qPCR). 'Ca. L. asiaticus' infection was a function of leaf age, number of insects, IAP, and IP. In general, higher infection rates were ob-served on younger leaves inoculated with higher numbers of insects and after longer IAP and IP. The immature excised leaf method allowed de-termination of 3 to 7 days as the range of time required by 'Ca. L. asiaticus' to reach qPCR detectable levels. Even though leaf survival could be prolonged by the maintenance of a branch segment at the base of the leaf petiole, leaf degradation, visible after about 15 days IP, did not allow ob-servation of the entire infection process which, in the intact plant, culmi-nates with the appearance of the blotch mottling symptom on leaf blades.