First report of Pythium aphanidermatum causing root rot on common ice plant (Mesembryanthemum crystallinum).
In June 2012, a severe root rot was found on commercially grown ice plants (Mesembryanthemum crystallinum) in a greenhouse in Osaka, Japan. Root rot appeared suddenly on five- to six-week-old plants, and approximately 300 plants in the greenhouse (i.e. a quarter of all the ice plants grown) were found to be affected by the disease. The plants were grown in a conventional hydroponic system that used 'Otsuka A' nutrient solution, amended with 0.1% sea salt. The greenhouse had a controlled temperature regime (23/18°, day/night), and 16 h of artificial light per day. The affected tissues were soft and discoloured, and wilting of the plants was observed. Abundant aplerotic oospores were found in diseased roots. Based on morphological characteristics, sequence analysis of the internal transcribed spacer region and pathogenicity test, the causal agent was identified as Pythium aphanidermatum. In Japan, this fungus is a devastating pathogen on many plants, especially on common bean and sugarbeet, and has been reported since 1935. However, it has never been reported on common ice plant. This is thought to be the first report of P. aphanidermatum causing root rot on common ice plant worldwide. Moreover, no Pythium spp. have been recorded previously from this plant.