Evaluation of Grafting for Management of Southern Blight in Processing Tomatoes in California.

Options for managing southern blight of processing tomato (caused by Athelia rolfsii) in California are limited. The objectives of this study were to: (i) evaluate grafting with the resistant rootstock Maxifort for southern blight management in processing tomato and (ii) evaluate increasing the height of the graft union to further reduce incidence of southern blight in grafted plants. We evaluated two cultivars (Heinz 5608 or Heinz 8504) and a grafting factor with three levels (grafted to Maxifort rootstock with standard scion height, grafted to Maxifort rootstock at a tall height, and nongrafted) in a field study with natural inoculum or in inoculated greenhouse experiments. Southern blight severity was low in both greenhouse experiments in 2018 and 2019, and no consistent trends were observed. In field experiments in 2018 and 2019, mean incidence in nongrafted plots was 6.2 to 17.0 times higher when compared with either the standard or tall grafted treatments. Southern blight was numerically lower in tall grafted plots compared with standard, but the magnitude was small and not statistically significant. Based on our studies, grafting can reduce losses of processing tomato in California to southern blight, but increasing the height of the graft union does not offer a tangible benefit.

Soil Microbiomes Associated with Verticillium Wilt-Suppressive Broccoli and Chitin Amendments are Enriched with Potential Biocontrol Agents.

Two naturally infested Verticillium wilt-conducive soils from the Salinas Valley of coastal California were amended with disease-suppressive broccoli residue or crab meal amendments, and changes to the soil prokaryote community were monitored using Illumina sequencing of a 16S ribosomal RNA gene library generated from 160 bulk soil samples. The experiment was run in a greenhouse, twice, with eggplant as the Verticillium wilt-susceptible host. Disease suppression, plant height, soil microsclerotia density, and soil chitinase activity were assessed at the conclusion of each experiment. In soil with high microsclerotia density, all amendments significantly reduced Verticillium wilt severity and microsclerotia density, and increased soil chitinase activity. Plant height was increased only in the broccoli-containing treatments. In total, 8,790 error-corrected sequence variants representing 1,917,893 different sequences were included in the analyses. The treatments had a significant impact on the soil microbiome community structure but measures of α diversity did not vary between treatments. Community structure correlated with disease score, plant height, microsclerotia density, and soil chitinase activity, suggesting that the prokaryote community may affect the disease-related response variables or vice versa. Similarly, the abundance of 107 sequence variants correlated with disease-related response variables, which included variants from genera with known antagonists of filamentous fungal plant pathogens, such as Pseudomonas and Streptomyces. Overall, genera with antifungal antagonists were more abundant in amended soils than unamended soils, and constituted up to 8.9% of all sequences in broccoli+crabmeal-amended soil. This study demonstrates that substrate-mediated shifts in soil prokaryote communities are associated with the transition of Verticillium wilt-conducive soils to Verticillium wilt-suppressive soils, and suggests that soils likely harbor numerous additional antagonists of fungal plant pathogens that contribute to the biological suppression of plant disease.