Abstract
Yellow leaf spot disease was observed on Rhynchostylis gigantea orchids on an orchid farm in the Nonthaburi province of Thailand. Symptoms started with yellowish patches lacking definitive borders on the leaves of the orchids. The patches slowly expanded and merged, creating larger patches; a brown spot or black necrotic area then emerged at the center of some of the patches. R. gigantea leaves exhibiting these symptoms were used as the source for isolation studies. Based on the morphological characteristics of the fungal isolate (colony appearance, macroconidia, microconidia, conidiophore and chlamydospore structures), phylogenetic analysis of translation elongation factor-1α sequence and fulfillment of Koch’s postulates, the causative pathogen was identified as Fusarium sacchari. To our knowledge, this study is the first to report F. sacchari as a pathogen of R. gigantea.
Highlights
Orchids are the most exported tropical ornamental crop of Thailand
On an orchid farm in Nonthaburi province, Thailand, we found many Rhynchostylis gigantea plants damaged by yellow leaf spot disease, leading to progressive death from the infected area
Symptoms of the yellow spot disease described by Ichikawa and Aoki (2000) in Cymbidium orchids infected with Fusarium proliferatum and Fusarium subglutinans began with small, water-soaked patches on the leaves
Summary
Orchids are the most exported tropical ornamental crop of Thailand. Among the ornamental orchids, the genus Rhynchostylis is one of the most popular cultivations due to its unique fragrance and long tapering inflorescences (Thammasiri, 2016). Fungal infestation is a major problem in cultivated Rhynchostylis species. This led us to survey fungal diseases on orchid farms in the central region of Thailand. On an orchid farm in Nonthaburi province, Thailand, we found many Rhynchostylis gigantea plants damaged by yellow leaf spot disease, leading to progressive death from the infected area. The yellow leaf spot disease caused by Fusarium species can only be found in orchids (Han et al, 2015; Ichikawa and Aoki, 2000; Leslie and Summerell, 2006). Symptoms of the yellow spot disease described by Ichikawa and Aoki (2000) in Cymbidium orchids infected with Fusarium proliferatum and Fusarium subglutinans began with small, water-soaked patches on the leaves.
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