Compost effects on plectosporium blight

Abstract

Plectosporium blight, also known as Plecto, is an emerging pumpkin, zucchini, and squash disease persistent in the eastern United States since the late 1980s. Its pathogen, Plectosporium tabacinum, is a fungal saprophyte capable of surviving in the soil for several years with limited research available on its life cycle. Compost is widely used as a tool to improve soil health with the assumption of preventing or mitigating disease effects, but this is not always the case. Compost is most often characterized as having a suppressive effect; however, in rare instances, it has been found to increase disease. To better understand the mechanisms driving compost effects, this study examines the role of compost source versus microbial activity in P. tabacinum growth in vitro and in vivo. In a laboratory setting, five replicates of plant, animal, vermicompost, and mixed source compost treatments, with and without microbial sterilization, were added to 150 mm x 15 mm petri dishes centrally inoculated with P. tabacinum. After 7 days of incubation, the pathogen growth was measured radially using a spatial divider. Analysis revealed a significant interaction effect between the compost source and microbial activity (P = 0.01). The sterilized animal compost (4.6 ± 0.6, mean ± SE) and unsterilized mixed compost (5.0 ± 0.6, mean ± SE) resulted in significantly decreased growth of P. tabacinum (cm of radial growth) compared to the non-amended control (5.8 ± 0.2, mean ± SE). In contrast, the sterilized mixed compost (6.6 ± 0.6, mean ± SE) significantly increased its growth compared to the control. Each treatment underwent a nutrient and enzyme analysis to provide insight to their varied outcomes. There was a significant relationship between P. tabacinum growth and the enzymatic activity of β-glucosidase (BG) (P < 0.05). These findings suggest that the source material and microbial communities associated with composts both have varying effects of compost on P. tabacinum growth in vitro. Additionally, the enzymatic activity related to the breakdown of cellulose may play a role in the effects of compost on P. tabacinum growth. In an outdoor pot experiment, a compact variety of 'Black Beauty' zucchini (Cucurbita pepo) was transplanted into 24.3-liter pots that were amended with six replicates of sterilized and unsterilized treatments of plant, animal, and worm source composts. After establishing in the treated pots for two weeks, the plants were inoculated with P. tabacinum using a drench method applied to the stems and leaves. Plants were checked weekly for the following five weeks to record crop growth stage, disease severity, fruit production, and week of death. Plectosporium blight disease severity was only significantly affected by compost source in Week 5 (P < 0.001). The animal source compost presented significantly lower disease severity than the plant and worm composts during Week 5 with 30% less symptomatic area; this was most similar to the disease severity of the control. However, all plants regardless of treatment reached total plant death by Week 8. Plectosporium blight limited overall fruit production but did not significantly differ across treatment types. This in vivo study indicates that compost of varying sources and microbial activity had little to no overall effect on the disease plectosporium blight in zucchini.