Effects of Three Parastagonospora nodorum Necrotrophic Effectors on Spring Wheat under Norwegian Field Conditions

The wheat (Triticum aestivum L.) disease Septoria nodorum blotch (SNB) is caused by the necrotrophic fungus Parastagonospora nodorum (Berk.) Quaedvlieg, Verkley & Crous and causes significant yield and quality losses in several wheat growing regions. The resistance mechanisms are quantitative and progress in resistance breeding has been slow. However, gene-for-gene interactions involving necrotrophic effectors (NEs) and sensitivity genes (Snn) are involved, providing hope for more effective breeding. Although the interactions are significant determinants of seedling SNB susceptibility, their role in adult plant leaf blotch resistance in the field is less understood. In this study, the frequency of SnTox genes was investigated in 62 P. nodorum isolates collected in Norway. A panel of Norwegian and international spring wheat lines and cultivars was screened under natural SNB infection in a mist-irrigated field nursery across 7 yr. The lines were infiltrated in the greenhouse with the purified NEs SnToxA, SnTox1, and SnTox3, and the prevalence of corresponding sensitivity was investigated, as well as correlation between NE sensitivity and resistance level in the field. The frequencies of SnToxA, SnTox1, and SnTox3 in the isolates were 0.69, 0.53 and 0.76, respectively. Sensitivity to SnToxA, SnTox1, and SnTox3 was present in 45, 12, and 55% of the plant material. Sensitivity to SnToxA was associated with significantly higher disease severity in the field than insensitivity. This indicates that elimination of SnToxA sensitivity in the breeding material by effector infiltrations or marker-assisted selection can be an effective way to increase field resistance to SNB.