Cover Cropping Attenuates Population Growth of <i>Macrophomina phaseolina</i> by Limiting Weed Biomass, Despite Asymptomatic Colonization of Cover Crops

<i>Macrophomina phaseolina</i> is a fungus that causes charcoal rot in strawberry and a wide variety of crop species. Little is known about its potential to asymptomatically colonize crop plants or grow saprophytically on their tissues, both of which would create a potential for alternate, asymptomatic hosts to lead to increases in inoculum. To test the impact of cover cropping on <i>M. phaseolina</i> abundance, we conducted randomized-block field experiments in soils infested by <i>M. phaseolina</i>. None of the 15 cover crop varieties showed symptoms of charcoal rot. All Fabaceae and Brassicaceae varieties were asymptomatically colonized at varying rates, but among Poaceae <i>M. phaseolina</i> was recovered from only one individual oat plant. Soil samples collected at the time of planting, tillage, and 8 weeks after tillage showed that cover cropping attenuated the growth of <i>M. phaseolina</i> relative to fallow plots harboring the weedy legume <i>Medicago polymorpha</i>. This weed species was abundantly colonized by this pathogen in both living root samples and plant residue collected 8 weeks after tillage. Cover cropping also influenced the diversity and composition of bulk soil bacterial and fungal communities, but these effects were not associated with <i>M. phaseolina</i> population density. Although <i>M. phaseolina</i> was not detected in living wheat tissues, it was recovered from wheat residue, suggesting that it may be facultatively saprophytic. These results suggest that cover cropping does not pose a risk for increasing disease caused by <i>M. phaseolina</i> and could be beneficial as conducive weed species, such as <i>M. polymorpha</i>, are suppressed.[Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.