Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/20/2002
Publication Date: 8/20/2002
Citation: PAULITZ, T.C. FIRST REPORT OF RHIZOCTONIA ORYZAE ON PEA.. PLANT DISEASE. 86: 442. 2002. Interpretive Summary: Rhizoctonia oryzae has recently been shown to cause root rot on cereals in the Pacific Northwest. We also show that it can attack field peas. Diseased peas were observed in Idaho and we isolated R. oryzae. We did greenhouse inoculations to show that it was pathogenic to peas. This is the first report of this fungus causing a disease on dicots in N. America, and suggests that use of broadleaf crops such as peas in cereal rotations may not be useful for disease management.
Technical Abstract: 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 1 ug/ml benomyl and 100 ug/ml chloramphenicol, and incubated at 22 C. Fungal colonies were identified as Rhizoctonia oryzae (teleomorph = Waitea circinata Warcup and Talbot) based on hyphal and colony morphology (1) and anastamosis reactions with known tester isolates. Two isolates were grown on autoclaved oat seeds for three 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, three replicate pots per isolate. Both isolates caused severe damping-off and stunting. Both isolates were also tested in a non-pasteurized (natural) sandy loam in 4 cm X 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/L, technical grade) to inhibit Pythium. Control treatments consisted of either autoclaved oat seeds or nothing amended to the soil. 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 caused necrosis and browning of root tips and reduction in lateral root formation. R. oryzae was re-isolated from infected roots. 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 (2). W. circinata also caused damping-off of tobacco seedlings in India (3). In the Pacific Northwest, peas are often grown in rotation with wheat and barley, and R. oryzae can be virulent on these cereal crops (4). This finding may have important implications for disease management in wheat and legumes by crop rotation.