In May 2001, severe stunting, lateral rot, and brown discoloration of taproots were observed in a field of direct-seed (no-till) pea cv. Columbia southeast of Lewiston, ID. The field had been previously cropped with direct-seeded spring barley. Roots were washed, plated on water agar containing benomyl at 1 μ/ml and chloramphenicol at 100 μg/ml, and incubated at 22°C. Fungal colonies were identified as Rhizoctonia oryzae (teleomorph Waitea circinata Warcup & Talbot) based on hyphal and colony morphology (3) and anastamosis reaction with known tester isolates. Two isolates were grown on autoclaved oat seeds for 3 weeks to produce inoculum for pathogenicity testing. One colonized oat seed was placed below a seed of Pisum sativum ‘Little Marvel’ planted in pasteurized sandy loam soil. There were five pea seeds per 10-cm-diameter pot and three replicate pots per isolate. Both isolates caused severe damping-off and stunting. Both isolates were also tested in nonpasteurized (natural) sandy loam in 4 cm × 20 cm plastic pine seedling tubes. Eight colonized oat seeds were placed in a band 1 cm below a single pea seed planted in each tube. Tubes were watered with metalaxyl (0.1g/liter, technical grade) to inhibit Pythium. Control treatments consisted of soil amended with either autoclaved oat seeds or nothing. Two isolates of R. oryzae were tested with two pea cultivars (B160 and Marjorette), with five replicates per treatment. R. oryzae did not significantly reduce emergence but did cause necrosis and browning of root tips and reduction in lateral root formation. R. oryzae was reisolated from infected roots. To our knowledge, this is the first report of R. oryzae causing disease on a dicot in North America. In Australia, a Waitea sp. was weakly virulent to subterranean clover producing constrictions of the taproot but did not affect plant survival and growth (4). W. circinata also caused damping-off of tobacco seedlings in India (2). In the Pacific Northwest, peas are often grown in rotation with wheat and barley, and R. oryzae can be virulent on these cereal crops (1). This finding may have important implications for disease management in wheat and legumes in crop rotation systems.
References: (1). M. Mazzola et al. Phytopathology 86:354, 1996. (2) C. A. Raju. Tob. Res. 19:92, 1993. (3) B. Sneh et al. Identification of Rhizoctonia Species. The American Phytopathological Society, St. Paul, MN, 1991. (4) D. H. Wong et al. Trans. Br. Mycol. Soc. 85:156, 1985.
