Fusarium oxysporum f. sp. phalaenopsidis, a Novel Forma Specialis Causing Leaf Yellowing of Orchids

Fusarium wilt is a major agricultural disease causing significant crop losses. In Taiwan, several Fusarium species have been reported to cause yellow leaf disease symptoms in orchids. This study identified five isolates of Fusarium oxysporum (Fo) isolates (pfy-1 to pfy-5) based on phylogenetic analyses of the internal transcribed spacer of the nuclear rDNA, translation elongation factor 1-α, and sterol 14 α-demethylase genes. Multi-locus phylogenetic analysis using combined sequences of calmodulin, the second largest subunit of DNA-dependent RNA polymerase II, translation elongation factors, and β-tubulin II sequences grouped pfy-1 to pfy-4 within F. veterinarium (bootstrap value 87). However, morphological characteristics of the colonies and conidia of four Fo isolates were not match to F. veterinarium (ex-type culture CBS 109898). Mycelial growth of pfy-1 grows best at 28°C and pH 6.0, with high sporulation at 32°C, pH 5.5, and in darkness. Further phylogenetic analysis of the intergenic spacer (IGS) region and exopolygalacturonase gene 1 (pgx1) separated the five Fo isolates into two distinct clades. Pathogenicity tests on carnation, strawberry, common bean, and 47 cultivars of orchids was determined using the whole-plant inoculation method. The results showed that pfy-5 infected Vanilla and Phalaenopsis, while pfy-1 specifically infected Phalaenopsis. Based on genetic and phytogenetic characteristic, Fo isolate pfy-1 is proposed as a new forma specialis, named F. oxysporum f. sp. phalaenopsidis, which specific to Phalaenopsis orchids. The identification of this new forma specialis is important for both research and practical applications in orchid cultivation.