Effects of cover cropping on soil microbial diversity and Macrophomina root rot suppression in strawberry cropping system

California ranks first in the nation in both conventional and organic strawberry production while using 0.5% of the state’s farmland. However, production has been challenged by Macrophomina root rot (MRR) in the post-methyl bromide era. The efficacy of using cereal cover crops to manage MRR via a pathogen suppressive microbiome has not been well documented. In both conventional and organic grower field studies conducted in California’s Santa Maria strawberry growing district, cover cropping with ‘Summit 515’ wheat and ‘Pacheco’ triticale significantly shaped the soil microbiome. Cover crop treated soil showed amplification of microbial groups such as Pseudomonads reported to induce disease suppression. However, one-time use of these practices did not yield significant MRR disease reduction in heavily infested grower fields. The combination of wheat and triticale cover cropping plus crop termination using metam potassium was also assessed alongside standard flat fumigation with chloropicrin. The conventional field had a significantly higher Macrophomina phaseolina colony forming units/g soil in the post-cover crop wheat than in the post-cover crop triticale treatment, while the organic field had no significant difference between the cover crops at the peak or post-growth stages. This outcome emphasizes the need for long-term cover crop studies to determine the effect of cover crops on pathogen inoculum level, saprophytic growth, and quantify potential co-benefits for soil health and fertility.