Prevalence and Importance of the Necrotrophic Effector Gene <i>ToxA</i> in <i>Bipolaris sorokiniana</i> Populations Collected from Spring Wheat and Barley

<i>Bipolaris sorokiniana</i> is a necrotrophic fungal pathogen that causes foliar and root diseases on wheat and barley. These diseases are common in all wheat- and barley-growing regions, with more severe outbreaks occurring under warm and humid conditions. <i>B. sorokiniana</i> can also infect a wide range of grass species in the family Poaceae and secrete <i>ToxA</i>, an important necrotrophic effector also identified other wheat leaf spotting pathogens. In this study, the prevalence and virulence role of <i>ToxA</i> were investigated in a collection of 278 <i>B. sorokiniana</i> isolates collected from spring wheat and barley in the Upper Midwest of the United States or other places, including 169 from wheat leaves, 75 from wheat roots, 30 from barley leaves, and 4 from wild quack grass leaves. <i>ToxA</i> was present in the isolates from wheat leaves, wheat roots, and wild grass leaves but was absent from isolates collected from barley leaves. Prevalence of <i>ToxA</i> in wheat leaf isolates (34.3%) was much higher than that in wheat root isolates (16%). Sequencing analysis revealed the presence of two haplotypes, with the majority being BsH2. All <i>ToxA+</i> isolates produced the functional effector in liquid cultures. Pathogenicity assays revealed that <i>ToxA+</i> isolates caused significantly more disease on spring wheat lines harboring <i>Tsn1</i> than their <i>tsn1</i> mutants, suggesting that the <i>ToxA-Tsn1</i> interaction plays an important role in spot blotch development. This work confirms the importance of <i>ToxA</i> in <i>B</i>. <i>sorokiniana</i> populations infecting wheat and, thus, the need to eliminate <i>Tsn1</i> from spring wheat cultivars to reduce susceptibility to spot blotch.