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 Pythium species were isolated by soil baiting and by culturing from the diseased roots and collars of plants exhibiting these symptoms. Eleven species of Pythium were identified based on cultural characteristics and phylogenetic analysis with ITS, Cox-1, and Cox-2 gene sequences. None of the 11 Pythium species had been reported previously from processing or fresh tomatoes in Australia. Pythium dissotocum was the most abundant and widespread species isolated during surveys in each of two growing seasons. In pathogenicity tests, these Pythium species ranged from nonpathogenic to highly aggressive. P. aphanidermatum, P. ultimum, and P. irregulare 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 Pythium species could be contributing to yield loss in processing tomatoes in Victoria both in the crop establishment phase and through the season.

Genetic transformation of Colletotrichum truncatum associated with anthracnose disease of chili by random insertional mutagenesis.

An Agrobacterium tumefaciens -mediated transformation (ATMT) system was successfully developed for Colletotrichum truncatum, the causal agent of chili anthracnose. A. tumefaciens carrying a hygromycin phosphotransferase gene (hph) and a green fluorescent protein (gfp) gene was used to transform the conidiospores of two C. truncatum pathotypes F8-3B and BRIP26974. Optimum transformation efficiency was obtained when equal volumes of A. tumefaciens strain AGL1 carrying either pJF1 or pPK2 binary vector was used to transform C. truncatum conidiospores at 10(6) /ml and co-cultivated at 24 °C for three days. Southern blot analysis indicated that 87.5% of the transformants contained randomly inserted, single copies of the T-DNA. Infection and colonisation of chili fruit at the mature red stage with F8-3B-GFP and BRIP26974-GFP confirmed the maintenance of virulence within these transformed pathotypes. In situ studies of infection and colonisation of the susceptible genotype fruit using fluorescent microscopy and transformed isolates of C. truncatum expressing GFP revealed that the pathogen was able to colonise healthy fruit tissue intercellularly in an endophytic manner without producing secondary biotrophic infection structures. The developed transformation system will be used to study the function of pathogenicity genes in C. truncatum using both forward and reverse genetics approaches.