Stem and root rot in hydrangea caused by Pythium myriotylum and Globisporangium spp. and susceptibility of hydrangea cultivars

Severe stem and root rot was found on potted common hydrangeas (Hydrangea macrophylla) in commercial greenhouses in Shimane Prefecture, Japan. Pythium and Globisporangium spp. that were consistently isolated from the plants were identified as Pythium myriotylum, Globisporangium splendens, G. spinosum, and an undescribed species of Globisporangium based on morphological analysis and rDNA-internal transcribed spacer and cox1 gene sequences. All four species reproduced stem and root rot on inoculated potted hydrangea. Plant cultivars significantly differed in their susceptibility to the pathogens. The cultivar Mangekyo was the most susceptible to all the pathogens.