Cultural characteristics and pathogenicity analysis of Phytophthora palmivora, causal pathogen of black rot in orchids

Abstract

Phytophthora palmivora is the causal pathogen of black rot, one of the major diseases affecting orchids. To evaluate its variability, P. palmivora was isolated from infected orchid leaves in Thailand by PARBPH selective medium, was purified again by the modified baiting method, and was cultured on Rose Bengal medium. Single-spore isolates were obtained and 5 representatives were cultured on potato dextrose agar (PDA), pea sucrose agar (PSA), cereal meal potato agar (CMPA; a newly developed medium) and V8 juice agar (V8) for cultural characterization. It was found that the different culture media affected growth, appearance, and sporulation. The best culture media for surface mycelium growth were PSA and V8, while CMPA induced sporulation earlier than other media. Pathogenicity analysis using 5 Dendrobium "Earsakul" lines and 5 P. palmivora isolates showed that NK-53-9 was the most virulent isolate, which is useful for future screening of black rot resistance. A Dendrobium mutant, "SUT13E18-A", was resistant to all isolates, suggesting its usefulness as a resistance source in future breeding program.