First Report of Rhizoctonia solani AG-4 HGI Causing Corky Cracks on Potato Tubers in Mauritius

Potato (Solanum tuberosum L.) is of great economic importance for Mauritius. During a disease survey carried out in a field in one of the major potato-growing regions of Mauritius, corky cracked lesions with halo and mycelial growth were observed on the skin surfaces of around 45% of tubers. The suspected fungus from the affected areas was isolated on amended potato dextrose agar and after 7 days, several fast-growing, pale brown Rhizoctonia-like colonies with a few sclerotia were observed. Additionally, microscopic analysis revealed some cells being multinucleate, forming hyphal branching at right angles with slight constriction together with septum near the branch base. Pathogenicity tests confirmed development of the same symptoms on potato tubers and the causal agent was reisolated successfully. Sequence analysis of the ITS region of representative isolate P114 amplified using ITS1F/ITS4 primers showed that it was identical to several isolates of multinucleate Rhizoctonia solani AG-4 HGI, and neighbour-joining phylogenetic analyses confirmed the identification. To our knowledge, this is the first report of R. solani AG-4 HGI-infecting potato tubers and causing corky cracks in Mauritius.