UC Riverside

Macrophomina phaseolina is an economically important pathogen that causes charcoal rot of strawberries (Fragaria x ananassa). The disease cycle on strawberries is not well understood and there are currently very few ways to manage charcoal rot. We performed a field study to investigate the role of soil moisture in charcoal rot development and M. phaseolina colonization on strawberries. Bare-root transplants were inoculated or uninoculated and maintained at either a high, optimal, or low soil moisture level based on tensiometers placed in each treatment. Randomly selected plants from each treatment were sampled for pathogen colonization every 2 to 4 weeks and all plants were rated for disease severity every 2 weeks starting at symptom onset. In two seasons, low soil moisture significantly increased charcoal rot mortality among inoculated ‘Monterey’ and ‘Fronteras’ plants, but high soil moisture only significantly affected mortality among inoculated Fronteras plants in one year of the study. Among inoculated Monterey plants, high soil moisture reduced colonization of crowns in two years of the study. Soil moisture did not influence root colonization in any year of the study. These results indicate that the role of soil moisture in colonization of M. phaseolina on strawberries may be cultivar dependent and the extent of pathogen colonization may not directly relate to plant mortality.To investigate whether M. phaseolina directly colonizes the roots or crown of the strawberry plant, we performed a greenhouse study with Monterey strawberry plants and 3 inoculation treatments (control, root-, and crown-inoculated) sampled at 5 timepoints (3, 7, 14, 21, and 28 days post-inoculation). To quantify the number of M. phaseolina colonies/g root or crown tissue, tissue was dried, ground, bleached, rinsed, and poured into Petri dishes in a mixture with molten media. All plants were asymptomatic throughout the experiment, however M. phaseolina was detected in plant roots and crowns within 28 days after inoculation. This study begins to elucidate early events in the disease cycle of Macrophomina charcoal rot on strawberries.