Diversity and Pathogenicity of Pythium Species Associated with Reduced Yields of Processing Tomatoes ( Solanum lycopersicum ) in Victoria, Australia

Yield decline associated with poor crop establishment, stunting, wilting, and diminished root systems was reported in processing tomato crops in Victoria, Australia. During surveys between 2016 and 2018 species were isolated by soil baiting and by culturing from the diseased roots and collars of plants exhibiting these symptoms. Eleven species of were identified based on cultural characteristics and phylogenetic analysis with ITS, Cox-1, and Cox-2 gene sequences. None of the 11 species had been reported previously from processing or fresh tomatoes in Australia. was the most abundant and widespread species isolated during surveys in each of two growing seasons. In pathogenicity tests, these species ranged from nonpathogenic to highly aggressive. , , and were consistently the most aggressive species, causing serious damage or death at the pregermination, postgermination, and later stages of plant growth. Five processing tomato cultivars varied significantly in their susceptibility to Pythium disease. These results suggest that species could be contributing to yield loss in processing tomatoes in Victoria both in the crop establishment phase and through the season.