The Effect of Terbacil, Root Injury, and Rhizoctonia fragariae on Strawberry and the Development of Black Root Rot

Black Root Rot (BRR) is a disease complex of strawberry plants in the mid-Atlantic region.  Multiple factors have been implicated in disease development such as Rhizoctonia fragariae, Pythium spp., Pratylenchus penetrans, and environmental and cultural stresses. There is anecdotal evidence that strawberry plants treated with the herbicide terbacil may be more susceptible to BRR. The effect of terbacil at 0 and 160 mg·L^-1, a.i., and R. fragariae anastomosis groups A, G, I, and an uncharacterized strain, from Pennsylvania, were observed on ‘Jewel’ strawberry plants and development of BRR symptoms.  Plants inoculated with AG-A had more leaves with phytotoxicity symptoms when terbacil was applied than other inoculation treatments. Terbacil and inoculation did not reduce leaf (including petiole), crown, root, and total dry mass, and final crown number.  In another experiment, the effect of root injury and inoculation with R. fragariae strains were observed on ‘Jewel’ strawberry plants. Roots were injured by scraping or crushing then before inoculation with isolates of R. fragariae strains, AG-A and AG-G, from Pennsylvania and Connecticut. Plants inoculated with CT-A, CT-G, and PA-A had higher plant survival than non-inoculated or PA-G inoculated plants, and uninjured plants had higher plant survival than injured plants.  Plants inoculated with PA-A and crushed roots had more primary roots than uninjured plants, and inoculated plants had a higher percentage of primary roots with lesions except for plants inoculated with PA-A. Plants inoculated with CT-G and PA-A had higher leaf, crown, root, and total dry mass than non-inoculated plants, and dry mass of plants inoculated with CT-A and PA-G was intermediate.