Impacts of organic amendments on insect herbivores and a soil-borne pathogen

Microbial communities can enhance plant resistance to insect herbivores and soil-borne pathogens. Farmers manage their soils to build soil fertility, shaping their soil microbial communities. However, we lack an understanding of how soil management practices, such as the addition of organic amendments, may enhance herbivore and disease suppression by altering soil microbial communities. Therefore, we conducted a fully factorial, randomized block field experiment in which we fertilized tomato (Solanum lycopersicum) plants of two heirloom (Cherokee Purple, Eva Purple Ball) and two hybrid (Mountain Magic, Defiant PhR) varieties with either chicken manure, worm castings, fish bone meal, a mix of all, or nothing. From May through October, we counted arthropods and infection by Southern Blight (Athelia rolfsii) biweekly. At the end of the season, we characterized bacterial and fungal rhizosphere communities via Illumina sequencing, and measured soil and foliar nutrient concentrations. Across tomato varieties, worm castings promoted rhizosphere bacterial diversity, while fish bone meal suppressed bacterial and fungal diversity. Plants fertilized with worm castings or nothing hosted the lowest abundance of insect herbivores. Hornworms (Manduca sexta) and aphids (Macrosiphum euphorbiae, Myzus persicae) were most abundant on plants fertilized with chicken manure and fish bone meal, respectively. Notably, infection by Southern Blight was suppressed by combinations of amendments and varieties that enhanced rhizosphere fungal diversity, such that heirloom varieties fertilized with worm castings had the lowest incidence of infection. Overall, our results suggest that organic amendments that promote diverse microbial communities may reduce herbivore pressure and infection by soil-borne pathogens.