Fungicides control black rot in Vanda: a strategy to avoid fungicide resistance

Vanda is a highly valued and important orchid in the horticultural industry; however, it is usually damaged by black rot disease, caused by Phytophthora species, leading to substantial economic losses. This study aims to evaluate multiple fungicides for black rot control. Oomycetes were isolated from diseased plants, and these isolates brought about a 100% disease incidence in Vanda with water-soaked lesions, becoming necrotic within 24 h. The isolates were identified as Phytophthora palmivora based on a phylogenetic analysis of internal transcribed spacers (ITSs) and cytochrome oxidase spacer regions. For controlling black rot disease, eight fungicides or mixtures with different target sites and modes of action were screened, including cyazofamid, dimethomorph, fosetyl-aluminum, metalaxyl, thiophanate-methyl, a mixture of fluopicolide and fosetyl-aluminum, a mixture of mancozeb and valifenalate, and a mixture of quintozene and etridiazole. All tested fungicides showed 33%–100% growth inhibition using the poisoned food technique. In greenhouse experiments, seven tested fungicide formulations, except for thiophanate-methyl, exhibited 20%–66% reduced lesion sizes compared with the no-chemical control. The information on fungicides with different filicide group codes could assist a fungicide spray program for black rot management in orchids.