The Roles of Rhizoctonia fragariae and Terbacil in the Development of Black Root Rot in Strawberry, Poster Board #282

Black Root Rot (BRR) is a disease complex that causes tissue decay in strawberry roots leading to loss of function and death over time.  As a result of root death, above ground tissue cannot be supported. Organisms most commonly associated with BRR development are lesion nematode and soil-borne fungal pathogens such as Rhizoctonia fragariae, Pythium spp., and Cylindrocarpon spp.  However, other factors may contribute to disease development by disrupting normal plant function and fine root development. These factors include dry or waterlogged soils, winter injury, nutrient deficiencies, and some chemical applications. The objectives of this research are to discover which strains of R. fragariae are present in Pennsylvania soils and to identify plant stresses that influence BRR development in strawberry plantings. Three commonly reported strains of R. fragariae (AG-A, AG-G, and AG-I) were isolated from symptomatic strawberry plants at 7 out of 8 farms sampled in Pennsylvania.  An unreported strain was isolated from symptomatic plants at 1 farm location. Also, the application of a commonly used herbicide, terbacil (Sinbar WDG, Tessenderlo Kerley, Inc.), at various field rates (roughly equivalent to 0, 2, 4, 6, and 8 oz/acre in 25 gallons of water), significantly decreased photosynthetic rates of ‘Jewel’ leaves causing disruption in normal physiological function and fine root development.  Photosynthesis in leaves less than 10 days old was fully inhibited; however, photosynthesis in leaves older than 10 days was not affected by terbacil application. Photosynthetic levels of leaves returned to normal with 10 to 14 days after application. The inhibition of photosynthesis may stress the plant leading to increased susceptibility to R. fragariae infection and BRR development. These results suggest that strawberry growers should use a low concentration of terbacil or the equivalent to 1oz./acre in 25 gallons of water.  Growers should apply terbacil only when plants have produced multiple older leaves to limit the impact on photosynthesis.  Also, growers should implement management strategies, such as crop rotation, to decrease populations of R. fragariae in strawberry plantings.