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.

Genome Analysis of Melon Necrotic Spot Virus Incursions and Seed Interceptions in Australia.

Melon necrotic spot virus (MNSV) was detected in field-grown Cucumis melo (rockmelon) and Citrullus lanatus (watermelon) plants in the Sunraysia district of New South Wales and Victoria, Australia, in 2012, 2013, and 2016, and in two watermelon seed lots tested at the Australian border in 2016. High-throughput sequencing was used to generate near full-length genomes of six isolates detected during the incursions and seed testing. Phylogenetic analysis of the genomes suggests that there have been at least two incursions of MNSV into Australia and none of the field isolates were the same as the isolates detected in seeds. The analysis indicated that one watermelon field sample (L10), the Victorian rockmelon field sample, and two seed interception samples may have European origins. The results showed that two isolates (L8 and L9) from watermelon were divergent from the type MNSV strain (MNSV-GA, D12536.2) and had 99% nucleotide identity to two MNSV isolates from human stool collected in the United States (KY124135.1, KY124136.1). These isolates also had high nucleotide pairwise identity (96%) to a partial sequence from a Spanish MNSV isolate (KT962848.1). The analysis supported the identification of three previously described MNSV genotype groups: EU-LA, Japan melon, and Japan watermelon. To account for the greater diversity of hosts and geographic regions of the MNSV isolates used in this study, it is suggested that the genotype groups EU-LA, Japan melon, and Japan watermelon be renamed to groups I, II, and III, respectively. The divergent isolates L8 and L9 from this study and the stool isolates from the United States formed a fourth genotype group, group IV. Soil collected from the site of the Victorian rockmelon MNSV outbreak was found to contain viable MNSV and the virus vector, a chytrid fungus, Olpidium bornovanus (Sahtiyanci) Karling, 18 months after the initial MNSV detection. This is a first report of O. bornovanus from soil sampled from an MNSV-contaminated site in Australia.