|JAAFFAR, KAMIL AHMAD - Washington State University
|PAREJKO, JAMES - Washington State University
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2017
Publication Date: 6/12/2017
Citation: Jaaffar, K.M., Parejko, J.A., Paulitz, T.C., Weller, D.M., Thomashow, L.S. 2017. Sensitivity of Rhizoctonia isolates from the Inland Pacific Northwest of the United States to phenazine-1-carboxylic acid and biological control by phenazine-producing Pseudomonas spp. Phytopathology. 107(6):692-703.
Interpretive Summary: Rhizoctonia root rot and bare patch, caused by R. solani AG-8, R. solani AG-2-1 and R. oryzae, are chronic and important yield-limiting diseases of wheat and barley throughout the Columbia Plateau of the Inland Pacific Northwest (PNW) of the USA. The severity of disease caused by these pathogens varies throughout the region and is exacerbated when small grains are sown with reduced or no tillage (direct seeding), which is needed in order to control soil erosion. In addition, there are no wheat or barley varieties with resistance to Rhizoctonia. R. solani AG-8 is most commonly found in the low precipitation zone where root rot and bare patch are most severe, whereas R. solani AG-2-1 is distributed throughout the low, intermediate and high precipitation zones. Bacteria of the genus Pseudomonas that produce the antifungal antibiotic phenazine-1-carboxylic acid (PCA) also are particularly abundant in the low precipitation zone, but it is unclear whether the differential severity of disease caused by R. solani is influenced by the presence of these indigenous PCA-producing bacteria. Therefore, we surveyed the distribution of Rhizoctonia AG-8, AG-2-1, R. oryzae, and PCA producers (Phz+) in 59 commercial fields in cereal-based cropping systems throughout the Columbia Plateau. We found that the geographic distribution of Phz+ pseudomonads overlaps closely with the distribution of R. solani AG-8 but not with that of R. oryzae or R. solani AG-2-1. Moreover, populations Phz+ bacteria declined markedly in locations with greater amounts of annual precipitation. In greenhouse studies, representative isolates of Phz+ pseudomonads from the Columbia Plateau effectively suppressed Rhizoctonia root rot of wheat. The effective dose of PCA for isolates of R. solani AG-8 and AG-2-1 from fields with high and low frequencies of Phz+ bacteria did not differ significantly, nor did we find a correlation between the virulence of an isolate and its sensitivity to PCA. These results demonstrate that Rhizoctonia isolates exposed to Phz+ pseudomonads in nature do not become tolerant to PCA and support the idea that the bacteria play a role in the natural suppression Rhizoctonia root rot and the sustainability of wheat production in the Columbia plateau.
Technical Abstract: Rhizoctonia solani AG-8, AG-2-1, and R. oryzae, causal agents of Rhizoctonia root rot and bare patch, are ubiquitous in cereal-based cropping systems of the Columbia Plateau of the Inland Pacific Northwest, yet the severity of this disease differs throughout the region. R. solani AG-8 is most commonly found in the low precipitation zone, and root rot and bare patch is most severe there, whereas R. solani AG-2-1 is more uniformly distributed throughout the low, intermediate and high precipitation zones. Fluorescent Pseudomonas spp. that produce the antibiotic phenazine-1-carboxylic acid (PCA) also are most abundant in the rhizosphere of crops grown throughout the low precipitation zone of the Columbia Plateau. We surveyed the distribution of PCA producers (Phz+) in 59 commercial fields in cereal-based cropping systems throughout the Columbia Plateau. Phz+ Pseudomonas spp. were detected in 37 of 59 samples and comprised from 0 to 12.5% of the total culturable heterotrophic aerobic rhizosphere bacteria. The frequency with which individual plants were colonized by Phz+ pseudomonads ranged from 0 to 100%. High and moderate colonization frequencies of Phz+ pseudomonads were associated with roots from fields located in the driest areas whereas only moderate and low colonization frequencies were associated with crops where higher annual precipitation occurs. Thus, the geographic distribution of Phz+ pseudomonads overlaps closely with the distribution of R. solani AG-8 but not with that of R. oryzae or R. solani AG-2-1. Moreover, linear regression analysis demonstrated a highly significant inverse relationship between annual precipitation and the frequency of rhizospheres colonized by Phz+ pseudomonads. Representative isolates from each of the four major phylogenetic groups of Phz+ pseudomonads from the Columbia Plateau suppressed Rhizoctonia root rot of wheat as well or better than did the model Phz+ strain P. synxantha (formerly P. fluorescens) 2-79. The 50% effective dose (ED50 ) of PCA for isolates of AG-8 from fields with high and low frequencies of Phz+ pseudomonads ranged from 5 to 13 µg/ml and 6 to 12 µg/ml. For AG-2-1 isolates from high and low frequency Phz+ fields, ED50 values ranged from 8 to 16 µg/ml and 5 to 12 µg/ml, respectively. Mean ED50 values for all isolates of AG-8 from high and low Phz+ frequency fields were 10.6 and 9.2 µg/ml, and for AG-2-1 the values were 10.8 and 9.1 µg/ml, respectively. Mean ED50 values for isolates of AG-8 and AG-2-1 from fields with high and low frequencies of phenazine producers did not differ significantly, nor was there a correlation between virulence of an isolate and sensitivity to PCA, resulting in rejection of the hypothesis that tolerance in Rhizoctonia to PCA develops in nature upon exposure to Phz+ pseudomonads.