|JAAFFAR, AHMAD KAMIL - Washington State University|
|SCHROEDER, KURTIS - Washington State University|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/25/2015
Publication Date: 5/20/2016
Citation: Jaaffar, A.M., Paulitz, T.C., Schroeder, K.L., Thomashow, L.S., Weller, D.M. 2016. Molecular characterization, morphological characteristics, virulence and geographic distribution of Rhizoctonia spp. in Washington State. Phytopathology. 106:459-473.
Interpretive Summary: Rhizoctonia root rot and bare patch, caused by R. solani AG-8 and R. oryzae, are chronic and important yield-limiting diseases of wheat and barley in the Inland Pacific Northwest (PNW) of the USA. The severity of Rhizoctonia is exacerbated when small grains are sown with reduced or no tillage (direct seeding) which is needed in order to control soil erosion. Unfortunately, no wheat or barley varieties have resistance to Rhizoctonia. Major gaps remain in our understanding of these diseases because at least seven other types of Rhizoctonia besides the known pathogens can be isolated from wheat and barley. Important questions remain about the distribution of the different Rhizoctonia, their virulence on small grains and other crops, and their interactions with the known pathogens. In addition, it can be difficult to distinguish among cultures of the various types of Rhizoctonia. Therefore, we assembled a large a collection of Rhizoctonia isolates from a wide range of cereal production fields throughout the Inland PNW. To identify the isolates, we used a combination of molecular biology techniques and classical microbiological and phytopathological approaches. In addition, the pathogenicity/virulence of each isolate on wheat and canola was determined. The results of this research described the distribution of Rhizoctonia isolates throughout the Inland PNW and conclusively demonstrated that the different types of Rhizoctonia and their pathogenicity/virulence can be easily predicted from these assays. This research provide new approaches to distinguish Rhizoctonia in PNW fields and a means to assess the potential threat of these fungi to crop production in cereal-based cropping systems.
Technical Abstract: Rhizoctonia root rot and bare patch, caused by R. solani AG-8 and R. oryzae, are chronic and important yield-limiting diseases of wheat and barley in the Inland Pacific Northwest (PNW) of the USA. Major gaps remain in our understanding of the epidemiology of these diseases, and because multiple Rhizoctonia anastomosis groups (AGs) and species can be isolated from the same cereal roots from the field, it is challenging to identify the causal agents. In this study, a collection totaling 498 isolates of Rhizoctonia was assembled from surveys conducted from 2000-2009, 2010 and 2011 over a wide range of cereal production fields throughout the Inland PNW. To determine the identity of the isolates, PCR with anastomosis group or species-specific primers and/or DNA sequence analysis of the internal transcribed spacers was performed. R. solani AG-2-1, AG-8, AG-10, AG-3, AG-4, and AG-11, comprised 157 (32%), 70 (14%), 21 (4%), 20 (4%), 1 (0.2%), and 1 (0.2%), respectively, of the total isolates. AG-I-like binucleate Rhizoctonia sp. comprised 44 (9%) of the total; and 53 (11%), 80 (16%) and 51 (10%) were identified as R. oryzae genotypes I, II and III, respectively. Isolates of AG-2-1, the dominant Rhizoctonia, occurred in all six agronomic zones defined by annual precipitation and temperature within the region sampled. Isolates of AG-8 also were cosmopolitan in their distribution but the frequency of isolation varied among years, and they were most abundant in zones of low and moderate precipitation. R. oryzae was cosmopolitan, and collectively the three genotypes comprised 37% of the isolates. Only isolates of R. solani AG-8 and R. oryzae genotypes II and III (but not genotype I) caused symptoms typically associated with Rhizoctonia root rot and bare patch of wheat. Isolates of AG-2-1 caused only mild root rot and AG-I-like binucleate isolates and members of anastomosis groups AG-3, AG-4, and AG-11 showed only slight or no discoloration of the roots. However, all isolates of AG-2-1 caused severe damping-off of canola, resulting in 100% mortality. Isolates of Rhizoctonia AG-8, AG-2-1, and AG-10, AG-I-like binucleate Rhizoctonia, and R. oryzae genotypes I, II, III could be distinguished by colony morphology on potato dextrose agar, by PCR reactions with specific primers, or by the type and severity of disease on wheat and canola seedlings, and results of these approaches correlated perfectly. Based on cultured isolates, we also identified the geographic distribution of all of these Rhizoctonia isolates in cereal-based production systems in the Inland Pacific Northwest.