Can lignin degrading organisms reduce Verticillium wilt in pumpkin cropping systems?

Raised beds in pumpkin cropping systems are frequently mulched with polyethylene (PE), providing numerous benefits. However, PE mulch has negative environmental consequences since it can remain in the soil for years. Biodegradable plastic mulch films (BDM) that are soil incorporated and biodegraded by microorganisms may present a sustainable alternative. Verticillium dahliae causes Verticillium wilt on cucurbits and is difficult to manage because the pathogen produces highly durable survival structures in the form of melanized microsclerotia. Recently, grafting of a susceptible scion onto a resistant rootstock has been shown to be an effective practice for cucurbits. Crop rotation is another effective management strategy to reduce Verticillium wilt especially when rotations include high lignolytic crops. Organisms that degrade lignin produce laccases and peroxidases that may degrade BDMs and fungal melanin. This study will determine if: (1) microsclerotia survival decreases when soils are amended with BDM in the presence of Streptomyces viridisporus, a lignin degrading organism, and (2) if Verticillium wilt severity and microsclerotia survival are reduced when a high-lignin, non-host crop (sweet corn) is followed by a susceptible host (either grafted or non-grafted), and coupled with soil incorporation of certain BDM and S. viridisporus.

Two trials have been completed to evaluate the effect of BDM on microsclerotia survival. Sachets containing microsclerotia were buried within Deepot conetainers amended with one of four mulch treatments (polyethylene, Organix, experimental PLA/PHA, or WeedGuard Plus) or non-amended, and amended with S. viridisporus or non-amended. Microsclerotia viability was evaluated on NP-10 medium for germination and percent non-germinated microsclerotia (%NGM) was calculated. No significant differences were observed for %NGM harvested from the conetainers regardless of the amended BDM (P =0.675) and whether S. viridisporus was amended into the field soil or non-amended (P = 0.885).

Two trials, each with two crop rotations, have been completed to evaluate the effect of crop rotation on V. dahliae microsclerotia. Sachets containing microsclerotia were buried within nursery pots filled with field soil and amended with S. viridisporus and V. dahliae or non-amended, and planted with  ‘Kickstarter’ corn, ‘Secretariat’ watermelon, ‘Secretariat’ watermelon grafted onto ‘Tetsukabuto’ squash, or fallowed. Dried plant debris was incorporated into the soil, and the experiment was repeated for the second crop rotation. Only fallow-fallow rotations resulted in significantly reduced %NGM. All other treatments resulted in high %NGM. The observed decrease in microsclerotia viability (or high %NGM) in all treatments except those that were fallow was most likely due to a direct interaction with the crop, such as root exudates.

The results of this study provide foundational data for future studies where lignin degrading organisms in the soil are identified and their breakdown of BDM is quantified. Results will also expand information on the role of lignin-degrading organisms on BDM degradation and effects of crop rotation.