ISOLATION AND BIOLOGICAL CHARACTERIZATION OF GUIGNARDIA SPECIES FROM CITRUS IN FLORIDA
H.L. Er, K. Hendricks, E.M. Goss, M. Smith, T.S. Schubert, P.D. Roberts, A.H.C. van Bruggen
Citrus black spot (CBS) is an emerging citrus disease in Florida. The causal agent is Guignardia citricarpa, but non-pathogenic Guignardia mangiferae is often isolated from the same lesions. A hypothesis on CBS emergence in Florida was formulated: a change in environment, such as increased copper applications could have favored growth and infection by G. citricarpa, causing it to outcompete G. mangiferae, facilitating the emergence of CBS. Moreover, differential temperature optima could favor one of the two species. Four agar media were compared for isolation of both species from citrus tissues. The obtained isolates were subjected to sequence comparison in the ITS region with that of Guignardia species worldwide, to pathogenicity tests, growth tests at different temperatures and competition assays at different copper concentrations. There were no significant differences between the media for isolation of both Guignardia species. Comparison of the ITS region of the isolates to those in international databases confirmed the identity of both species, and revealed higher diversity among the G. mangiferae than G. citricarpa isolates. The G. citricarpa isolates had similar temperature response curves (optima at 26°C), whereas the G. mangiferae isolates were more diverse in their response to temperatures (optima at 25-30°C). Growth of G. citricarpa was increasingly suppressed at higher copper concentrations, while a slight increase or no change in growth rate was observed for G. mangiferae. Therefore, our initial hypothesis was rejected. The differential response to copper and temperature by G. citricarpa and G. mangiferae will serve as a basis for modeling and predicting the spread of CBS in Florida.