Phyllosticta citriasiana sp. nov., the cause of Citrus tan spot of Citrus maxima in Asia

Abstract

Guignardia citricarpa, the causal agent of Citrus Black Spot, is subject to phytosanitary legislation in the European Union and the U.S.A. This species is frequently confused with G. mangiferae, which is a non-pathogenic, and is commonly isolated as an endophyte from citrus fruits and a wide range of other hosts. Recently, necrotic spots similar to those caused by G. citricarpa were observed on fruit of Citrus maxima intercepted in consignments exported from Asia. In these spots, pycnidia and conidia of a Guignardia species closely resembling G. citricarpa were observed, and therefore measures were taken for the consignments in line with the European Union legislation for G. citricarpa. To determine the identity of the causal organism on this new host, fungal isolates were subjected to DNA sequence analysis of the internal transcribed spacer region (ITS1, 5.8S, ITS2), translation elongation factor 1-alpha (TEF1) and actin genes. A combined phylogenetic tree resolved three species correlating to G. citricarpa, G. mangiferae and a previously undescribed species, Phyllosticta citriasiana sp. nov., closely related to G. citricarpa. Morphologically P. citriasiana can be distinguished from G. citricarpa by having larger conidia, longer conidial appendages, and in not producing any diffuse yellow pigment when cultivated on oatmeal agar (OA). Furthermore, it is distinguishable from G. mangiferae by having smaller conidia, with a narrower mucoid sheath. In culture, colonies of P. citriasiana can also be distinguished from G. citricarpa and G. mangiferae by being darker shades of grey and black on OA, malt extract agar (MEA), potato-dextrose agar, and cornmeal agar. Furthermore, cultures of P. citriasiana achieved optimal growth after 2 weeks at 21-27°C, and ceased to grow at 30-33°C. In contrast, colonies of G. citricarpa and G. mangiferae achieved optimal growth at 27-30°C, and ceased to grow at 30-36°C Colonies of P. citriasiana also grew faster than those of G. citricarpa and G. mangiferae on OA and MEA. Phyllosticta citriasiana appears to be a harmful pathogen of Citrus maxima, causing a tan spot on fruit, underlining the need for further surveys and research to determine its distribution and host range.