Repeatability and Diallel Analysis of Resistance to Septoria Tritici Blotch in Winter Wheat

Septoria tritici blotch, caused by Septoria tritici a fungal foliar disease in wheat (Triticum aestivum L.), has been a major constraint to wheat production worldwide. In the Great Plains, reduced-tillage (or no-tillage), and lack of resistance among available wheat cultivars are largely responsible for the recent increase of disease incidence in winter wheat. The development and use of resistant cultivars is the most economical approach to reduce losses caused by this disease. A well-established disease resistance screening technique, and understanding of the inheritance of resistance, would potentially result in more efficient selection for resistance in a breeding project. The objectives of this research were (1) to estimate the repeatability of current greenhouse seedling screening procedures, as a part of the effort to establish a standard protocol of screening wheat for septoria tritici blotch resistance; and (2) to investigate combining ability and mode of inheritance of septoria tritici blotch resistance in winter wheat genotypes of diverse origin by a full diallel scheme. Thirty-four winter wheat genotypes were used in the repeatability study, and eight genotypes and their F1 and reciprocal F1 crosses derived from a full diallel scheme were used in the diallel study. The repeatability study consisted of five planting dates (sets) conducted at three-day intervals. In each planting date, five seeds of each experimental unit were planted in the greenhouse; materials were arranged in a randomized complete block design (RCBD) with two replicates. Materials for the diallel analysis were seeded at three different dates. Within each planting date, three to five seeds of each experimental unit were planted in the greenhouse; materials were arranged in a RCBD design with three replicates. Plants at the second-leaf stage were inoculated with a bulk of six S. tritici isolates. In the repeatability study, analysis of variance revealed that differences among genotypes were highly significant (P\u3c0.001). There were no significant differences between replicates within each set or among sets. The repeatability of septoria tritici blotch score was high (94.0 ± 2.0% ). The results of this study suggest that our greenhouse seedling screening procedure provides consistent information on host response and repeated evaluations over time may not be necessary to obtain reliable information. In the diallel analysis of resistance to septoria tritici blotch, significant general combining ability (GCA), specific combining ability (SCA), and reciprocal effects were observed in the analysis of variance. The ratio of GCA sum of squares relative to SCA sum of squares suggested that GCA was more important than SCA. Additive gene effects played the major role in host response to septoria tritici blotch, while non-additive effects were also detected. General combining ability effects of individual genotypes were in close agreement with parental performance. KS94U338, a genotype with resistance derived from T. tauschii, had the lowest septoria tritici blotch score and the highest general combining ability. This result indicates that this genotype, possessing resistance distinct from other known sources, should prove useful in breeding efforts to improve septoria tritici blotch resistance in wheat