Role of Soil Moisture in Disease Development of Charcoal Rot of Strawberries Caused by Macrophomina phaseolina

Charcoal rot, caused by the soilborne fungus Macrophomina phaseolina, is one of the most economically important diseases affecting strawberry (Fragaria ×ananassa) production in California. Previous studies on non-strawberry hosts have shown that proper soil moisture management can limit pathogen colonization of plants and decrease disease severity. We performed field and greenhouse studies for two seasons with the objective of investigating the role of soil moisture in disease development and management of charcoal rot of strawberries. Bare-root transplants of cultivars Monterey and Fronteras were inoculated or not inoculated and maintained at either a high, optimal, or low soil moisture level using tensiometers. Randomly selected plants from each treatment were sampled for pathogen colonization every 4 weeks after planting, and all plants were visually rated for disease severity every 2 weeks after symptom onset. In both seasons, low soil moisture significantly increased charcoal rot mortality among inoculated plants compared to optimal soil moisture by 16 and 24 percentage points, respectively. In the first season, mortality was significantly lower in the high compared to the optimal soil moisture treatment. Colonization of crowns was increased by low soil moisture among inoculated plants in the first season, but soil moisture did not influence root colonization in either year of the study. In the greenhouse, charcoal rot severity was highest in the low soil moisture treatment. These results indicate that soil moisture has a limited influence on colonization of strawberries by M. phaseolina and that maintaining optimal soil moisture can help prevent excess charcoal rot mortality.