Population Diversity and Fungicide Sensitivity of Gaeumannomyces graminis var. graminis from St. Augustinegrass in Texas

Abstract

Gaeumannomyces graminis var. graminis (Ggg) is an ectotrophic root infecting fungus that causes root-decline of warm-season turfgrasses and black sheath rot of rice (Oryza sativa). A total of 76 Ggg isolates were collected from St. Augustinegrass (Stenotaphrum secundatum) throughout Texas. Variations of colony morphology, hyphopodia production, teleomorphic and anamorphic structures, and growth rate of isolates were examined on potato dextrose agar (PDA), and rice seedling pathogenicity assays were performed. A multilocus phylogeny reconstruction was developed to support phenotypic data and to resolve the phylogeny of Ggg. Fungicide sensitivities were also evaluated. Azoxystrobin 50% effective concentration (EC50) values of nine isolates were determined in the presence and absence of salicylhydroxamic acid (SHAM) for determination of alternative respiration. Tebuconazole and azoxystrobin EC50 values from 15 and 20 isolates, respectively, were used to establish single discriminatory concentrations for resistance screening and EC50 determination of remaining isolates. Isolates were also screened for thiophanate-methyl resistance.

Three phenotypic groups were found on PDA: (i) highly melanized with round colony formation (termed M group), (ii) non to slightly melanized with round colony formation (termed L group), and (iii) highly melanized with irregular colony formation (termed H group). All isolates produced lobed hyphopodia and a Phialophora-like anamorph, where phialospore lengths were significantly longer among M and H groups compared to the L group. M group isolates were adapted to warmer temperatures compared to L and H group isolates, and groups were differentiated based on growth rates at 35°C. The M group was most aggressive. A multilocus phylogeny reconstruction supported the differentiation between phenotypic groups and distinguished Ggg and G. graminis varieties avenae and tritici.

EC50 values from azoxystrobin-amended PDA were significantly lower in the presence of SHAM, indicating utilization of alternative respiration. Azoxystrobin and tebuconazole discriminatory concentrations were 0.1 μg a.i. ml-1 and were effective for rapid EC50 determination. Azoxystrobin EC50 values for 75 isolates ranged from <0.014 to 0.399. Tebuconazole EC50 values for 76 isolates ranged from 0.021 to 0.296 with a mean of 0.049. All isolates were completely inhibited by thiophanate-methyl at 500 μg a.i. ml^-1.