Phyllosticta species on citrus: Risk estimation of resistance to QoI fungicides and identification of species with cytochrome b gene sequences

Isolates of three fungal species associated with citrus, Phyllosticta citricarpa, Phyllosticta citriasiana and Phyllosticta capitalensis, collected from different citrus growing countries of the world, were investigated for their sensitivities to the QoI fungicides pyraclostrobin and azoxystrobin. Isolates were highly sensitive in microtiter tests and EC50 values were in narrow ranges, which indicate no acquired adaptation to QoIs. The resistance risk of P. citricarpa to QoIs is considered low since an intron was found immediately after codon 143 in the cytochrome b gene. The presence of an intron is known to reduce the risk of the G143A mutation, the mutation which causes QoI resistance with high resistance factors. The other two species had no intron and therefore are considered having a higher resistance risk. Impact of these two species is rather low, since P. citriasiana is restricted in its regional and host distribution and P. capitalensis is non-pathogenic. Furthermore, the development of a rapid and reliable assay for species detection and identification was made possible based on an analysis of the cytochrome b gene.

► Citrus Black Spot (Phyllosticta citricarpa) is classified as an A1 quarantine disease by EPPO.
► QoIs are effective fungicides for control of Citrus Black Spot.
► P. citricarpa has low risk for G143A in cyt b and therefore for QoI resistance.
► Low risk of G143A is caused by an intron after codon 143 in cyt b gene.
► PCR assay based on cyt b gene was developed for specific detection of P. citricarpa