Temporal dynamics of the survival of Verticillium dahliae microsclerotia with or without melanin in soils amended with biocontrol agents

Verticillium dahliae is a soilborne pathogen that causes wilt in many economically important crops. It produces melanized microsclerotia for its long-term survival in soil. Accurate quantification of viable microsclerotia in soil prior to planting is essential for predicting the risk of wilt. Melanin is believed to help microsclerotia resist to or tolerate abiotic stresses. We studied the temporal dynamics of both melanized and melanin-deficient microsclerotia of V. dahliae in four types of soils with or without addition of two biocontrol products (one based on Trichoderma viride ; and the other based on two Bacillus strains: B. subtilis and B. amyloliquefaciens ). Of the four soil types, two were from wheat and maize (non-host to V. dahliae ) fields and the other two from sites with history of continuous cropping of cotton and pepper (both susceptible to V. dahliae ). Results showed that the survival of microsclerotia in soils over time can be satisfactorily described by a negative exponential decline model. Microsclerotium mortality was much greater in the soils from wheat and maize field than from cotton and pepper, irrespective of biocontrol agents. Similarly, mortality of melanin-deficient microsclerotia was much greater than melanized microsclerotia. Both biocontrol products resulted in additional mortality of microsclerotia, especially Bacillus spp. There were significant interactions between soil origin, microsclerotium type and biocontrol treatment in affecting microsclerotium mortality. These results demonstrated that melanin contributes to the long-time survival of microsclerotia in soil and suggested that combination of biocontrol with rotation with non-host crops can be effective in reducing the number of viable microsclerotia of V. dahliae in soil.