Complexity of <i>Gaeumannomyces</i> species causing take-all root rot of St. Augustinegrass in Texas

<i>Gaeumannomyces graminis</i> var. <i>graminis</i> (<i>Ggg</i>) has been the etiological agent of take-all root rot (TARR) in St. Augustinegrass (<i>Stenotaphrum secundatum</i>) and root decline of the other warm-season turfgrasses. Seventy-five <i>Ggg</i> isolates were obtained from St. Augustinegrass in central and east Texas. Evaluation of colony morphologies on potato dextrose agar (PDA) within 2 wk and follow-up multilocus phylogenic analyses revealed three phenotypic groups associated with different <i>Gaeumannomyces</i> species: (i) <i>G. floridanus</i>, highly melanized with round colony formation; (ii) <i>G. arxii</i>, none to slightly melanized with round colony formation; and (iii) <i>G. graminicola</i>, highly melanized with irregular colony formation. Further examination with representative isolates from each group revealed that their phenotypic characterizations supported the distinctive genetic groups within <i>Ggg</i> associated with St. Augustinegrass TARR. <i>Gaeumannomyces floridanus</i> isolates grew faster at warmer temperature (30 C) than <i>G. arxii</i> or <i>G. graminicola</i>. Pathogenicity assays using rice seedlings indicated that <i>G. floridanus</i> was more aggressive in disease symptom development than <i>G. arxii</i> or <i>G. graminicola</i>. A multilocus phylogeny reconstruction supported that most of <i>Gaeumannomyces</i> isolates tested in this study were separated into three phylogenetically distinct groups: <i>G. floridanus, G. arxii</i>, and <i>G</i>. graminicola. The resolution of intravarietal complexities of causal fungi of TARR is important for proper diagnostics and management strategies for TARR in St. Augustinegrass and other root-decline diseases in warm-season turfgrasses.