Strawberry Disease Management Improvement for Macrophomina Root Rot and Botrytis Fruit Rot

Strawberry production in California is limited by plant diseases such as Macrophomina root rot (caused by Macrophomina phaseolina) and Botrytis fruit rot (BFR) (caused by Botrytis cinerea). Current disease management strategies are compromised due to fumigant regulations or ineffective disease management practices. This thesis investigated methods to potentially improve the management of these two diseases. Host plant resistance evaluations for Macrophomina root rot were conducted for the 2020-2021 and 2021-2022 growing seasons. Fifty-one strawberry genotypes were screened in two field experiments where plants were inoculated artificially with Macrophomina phaseolina in both seasons. A wide range of plant resistance to Macrophomina root rot was observed. The three most resistant genotypes based on final plant mortality were ‘17C721P606’, ‘Yunuen’, and ‘Xareni’ in 2020-2021; ‘UCD Mojo’, ‘Mariposa’, and ‘Dayana’ in 2021-2022. A summary of similar experiments done in the previous four years showed ‘Osceola’ as highly resistant. Disease severity varied among years for specific genotypes as well as the average final mortality for all genotypes in the experiments. Strong positive associations were found for soil temperature during the first month after planting (R2= 0.79, P2= 0.79, P A survey of BFR levels in commercial strawberry fields with and without fungicide applications was conducted in Santa Maria, CA in 2021 and 2022. Weather stations were installed at each field to collect leaf wetness duration and temperature data and calculate the BFR risk factor based on the Strawberry Advisory System (StAS) developed at the University of Florida. There were no statistically significant differences between fungicide and no-fungicide treatments for both in-field and postharvest BFR incidence in 2021 and in-field BFR incidence in 2022, while no-fungicide treatment showed higher postharvest BFR incidence in 2022. BFR levels were low in both years. In 2021, average in-field BFR incidence for fungicide and no-fungicide treatments were 2.6 ± 0.3% and 2.5 ± 0.4%, respectively. Average postharvest BFR incidence for fungicide and no-fungicide treatments were 1.8 ± 0.2% and 2.0 ± 0.3%, respectively. In 2022, average in-field BFR incidence for fungicide and no-fungicide treatments were 3.0 ± 0.4% and 3.7 ± 0.4%, respectively. Average postharvest BFR incidence for fungicide and no-fungicide treatments were 0.6 ± 0.1% and 1.5 ± 0.2%, respectively. Risk factor from StAS was significantly associated with BFR incidence in 2021, but not in 2022. Screening new strawberry genotypes against Macrophomina root rot should be ongoing as part of a standard process for determining the susceptibility of currently grown and potentially new cultivars. Additional research under more diverse weather conditions is necessary to verify the impacts of reducing fungicide use in BFR management and to validate the use of StAS in making fungicide use decisions in California fields