Isolation, identification, pathogenicity and sensitivity of Rhizoctonia spp. to phenazine-1-carboxylic acid (PCA)-producing Pseudomonas spp

Rhizoctonia root rot and bare patch is the most important disease of direct-seeded wheat and barley in the Inland Pacific Northwest. Major gaps remain in understanding the epidemiology of this disease and the biology and ecology of Rhizoctonia solani AG-8 and R. oryzae, the causal agents. In these studies, a collection of 498 isolates of R. solani, AG-I-like binucleate Rhizoctonia sp., and R. oryzae groups was assembled from fields throughout the Inland Pacific Northwest. Isolates were identified by PCR with primers specific to internal transcribed spacers of the nuclear ribosomal DNA and/or by DNA sequence analysis of the ITS regions. The results revealed the geographic distribution of Rhizoctonia isolates in cereal-based production systems. R. solani AG-8 and R. oryzae groups II and III (but not group I) caused severe root rot on wheat. R. solani AG-2-1 caused only mild root rot and the other groups showed trace discoloration of the roots. In contrast, R. solani AG-2-1 caused severe damping-off of canola and killed seedlings in greenhouse assays. Distinctive morphological characteristics were described for isolates of R. solani AG-8, AG-2-1, and AG-10, AG-I-like binucleate Rhizoctonia, and R. oryzae groups I, II, III. These results demonstrated for the first time that colony morphology and amplification by specific PCR primers are predictive of the identity of an isolate of Rhizoctonia on wheat and canola.;The distribution of fluorescent Pseudomonas spp. capable of producing the antibiotic phenazine-1-carboxylic acid (PCA) was shown to overlap closely with that of R. solani AG-8 but not that of R. oryzae AG-2-1 and to exhibit a highly significant inverse correlation with annual precipitation. Sensitivity of R. solani AG-8 and AG-2-1 to PCA was not correlated with exposure in nature or with virulence, indicating that tolerance of these pathogens to the antibiotic does not develop in nature.;Representative isolates from each of the four major phylogenetic groups of Phz+ pseudomonads were shown to control root rot of wheat caused by R. solani as well or better than did the model PCA producer P. fluorescens 2-79.