Wilt and root rot of poinsettia caused by three high-temperature-tolerant Pythium species in ebb-and-flow irrigation systems

Poinsettia plants growing in ebb-and-flow irrigation systems developed wilting and root rot during the summer growing seasons of 2010 in Gifu Prefecture and 2011 in Aichi Prefecture. Pythium species were isolated from roots with rot symptoms. The isolates were identified as P. helicoides and P. myriotylum on the basis of morphological characteristics and sequence homologies in the rDNA internal transcribed spacer regions. In pathogenicity tests, these isolates caused severe wilting and root rot. This is the first report of poinsettia root rot disease caused by P. helicoides and P. myriotylum, although P. aphanidermatum was reported as a pathogen of poinsettia root rot. To better understand these diseases, we performed an epidemiological study of three high-temperature-tolerant Pythium species, P. aphanidermatum, P. helicoides and P. myriotylum. Disease incidence as a percentage of diseased plants was greatest at 35 A degrees C for all three species. Disease severity using the rating scale of root rot was also highest at 35 A degrees C, particularly with high zoospore inoculum densities (100.0 zoospores/mL). Although the disease incidence and severity were reduced at lower temperatures, the three Pythium species were able to cause disease at temperatures as low as 20 A degrees C.