Water stress and elevated salinity exacerbate Macrophomina root rot and reduce marketable yield in California...

Macrophomina root rot, caused by the soilborne fungus Macrophomina phaseolina, is a disease responsible for devastating economic losses in California strawberry production. Disease incidence and severity are influenced by plant stress, emphasizing the importance of identifying which abiotic stressors contribute most to disease development. Two field experiments (in 2024 and 2025) were conducted to evaluate the impact of water and salt stress on plant mortality of cultivars Fronteras (moderately susceptible) and Sweet Ann (highly susceptible). Strawberry transplants were inoculated with M. phaseolina and subjected to one of five treatments: grower standard (i.e., low/no stress), water stress, two elevated chloride levels, and high electrical conductivity (EC_W). Plant mortality was assessed weekly, and fruit yield was recorded. Salt-related foliar injuries were evaluated with visual ratings and drone-based aerial imagery. Across both seasons, treatment (P = 0.008 and P = 0.024, respectively) and cultivar (P < 0.0001 and P < 0.0001, respectively) significantly affected plant mortality, with no significant treatment × cultivar interaction (P > 0.05). Water stress in Year 1 and high EC_W stress in Year 2 resulted in significantly greater plant mortality and reduced total fruit yield compared to the grower standard. When comparing cultivars, Sweet Ann had significantly higher average plant mortality than Fronteras across both seasons. Under the study conditions, visual ratings and aerial imagery did not reliably distinguish differences in salt-related injuries among treatments. These results highlight the importance of utilizing soil moisture sensors, low-saline irrigation water, and disease-resistant cultivars to minimize plant mortality caused by M. phaseolina.