Occurrence of Phyllosticta citricarpa and Phyllosticta citriasiana in EU imported Citrus spp. and its impact on diagnostics of citrus black spot on fruits

Abstract

Citrus black spot (CBS) is a fungal disease on Citrus spp. caused by Phyllosticta citricarpa, which is listed as a regulated pest in the European Union (EU). There are several closely related Phyllosticta species that also occur on citrus including Phyllosticta citriasiana which causes citrus tan spot and is not regulated by the European Union. A real-time PCR assay was developed to aid the detection of P. citriasiana. Subsequently an extensive sampling on the presence of P. citricarpa and P. citriasiana was performed on 273 citrus fruit consignments that were put on hold by the Dutch national plant protection organization (NPPO-NL) because of suspected presence of CBS. P. citricarpa was found on several Citrus spp. but never on Citrus maxima whilst P. citriasiana was only found on C. maxima and never on other Citrus spp. The unique fungus host associations observed after testing almost 1100 individual lesions, are somewhat obscured by previously reported (some unverified) observations of C. maxima fruits harbouring black spot. Nevertheless, it was shown that a quick and simple direct morphological identification of P. citricarpa, based on the presence of CBS lesions harbouring pycnidia producing characteristic Phyllosticta conidia, is feasible on non-Citrus maxima fruits. It is proposed to adapt the European and Mediterranean Plant Protection Organisation (EPPO) and International Plant Protection Convention (IPPC) diagnostic protocols accordingly.