Cytogenetic analysis of the susceptibility of the wheat line Hobbit sib (Dwarf A) to Septoria tritici blotch.

Septoria tritici blotch, caused by Mycosphaerella graminicola (anamorph Septoria tritici), is one of the most important foliar diseases of wheat in much of the world. Susceptibility of host plants to septoria was investigated by cytogenetic analysis. A line of Hobbit sib (Dwarf A) in which translocated chromosome 5BS-7BS was nominally substituted by chromosome arms 5BS and 7BS from Bezostaya 1 had a much lower mean level of septoria than Hobbit sib itself. By the use of microsatellite markers, it was shown that the 5BS arm of this line had in fact been substituted by the homologous arm of Chinese Spring. Further investigation of substitution and nullitetrasomic lines demonstrated that chromosome arm 5BS of Hobbit sib possesses genes, which either promote susceptibility to septoria or suppress resistance. This chromosome arm has previously been shown to carry genes for resistance to yellow (stripe) rust and powdery mildew, implying a trade-off between resistances to these two diseases and to septoria in wheat breeding. Bezostaya 1 was found to have specific resistance to M. graminicola isolate IPO323, probably controlled by the gene Stb6 on chromosome arm 3AS, present in numerous wheat cultivars. It also had partial resistance to septoria distributed over several chromosomes, which may explain the value of this cultivar as a source of septoria resistance.