Identification, Phylogeny, and Fungicide Sensitivity of Oomycota Species Causing Stem and Root Rots on Chrysanthemums in Korea.

Root and stem rot, caused by Pythiales (Oomycota), poses a significant threat to chrysanthemum (Chrysanthemum spp.) cultivation worldwide. In Korea, previously undocumented rot and blight symptoms were observed on stems, roots, and leaves of Chrysanthemum morifolium (=Dendranthema morifolium), a chrysanthemum species with high global production. This study identified the causal agents as Globisporangium ultimum and Phytopythium helicoides based on morphological features and molecular phylogenetic analysis of the internal transcribed spacer rDNA (ITS) region and the cytochrome c oxidase subunit mtDNA (cox1 and cox2) genes. Pathogenicity assay demonstrated the high aggressiveness of both species toward chrysanthemums. Fungicide sensitivity testing revealed high sensitivity to picarbutrazox, highlighting its potential as an effective control measure. These findings enhance our knowledge of identifying and managing G. ultimum and P. helicoides in chrysanthemum cultivation.