An In Planta Method for Assessing the Role of Basidiospores in Rhizoctonia Foliar Disease of Tomato

A tomato (Solanum lycopersicum) foliar blight disease of unknown etiology was observed in North Carolina (NC) during 2005 to 2006. Symptoms included necrotic lesions and blighted leaves, with signs of white mycelial growth on abaxial leaf surfaces. The morphology of isolates from symptomatic leaves was consistent with that of Rhizoctonia solani. Because the pattern of symptom expression suggested that basidiospores were the primary inoculum source, Koch's postulates were fulfilled using a method to generate basidiospores in planta. Isolates were characterized by morphology, DNA sequence analysis, hyphal anastomosis, and somatic hyphal interactions. Phylogenetic analyses and hyphal anastomosis criteria support placement of the isolates in R. solani anastomosis group 3 (AG-3). Tomato foliar blight isolates from NC form a single phylogenetic group with tomato isolates of R. solani AG-3 from Japan and are more closely related to R. solani AG-3 isolates from potato than tobacco. Isolates exhibited both compatible and incompatible hyphal interactions when paired in vitro. To our knowledge, this is the first detailed report of tomato foliar blight caused by R. solani AG-3 in North America. A comprehensive description of the technique employed for producing basidiospores is presented with potential utility for understanding foliar disease etiology in other Rhizoctonia pathosystems.