Wheat powdery mildew resistance genes

The wheat powdery mildew disease arises as the result of interaction betwee n wheat (the host plant) and Erysiphe graminis DC. f.sp. tritici Marchal (a biotrophic parasitic fungus). Powdery mildew can be found to a greater or lesser extent wherever wheat is grown in the world. In some cases (in epide mic years) the damage may be substantial, so it is necessary to control the pathogen. If plant protection is to be both effective and acceptable as re gards the environment and human health, a knowledge of resistance genes, va rieties bearing these genes, the efficiency of the genes and their territor ial distribution is of great assistance. Major and minor resistance genes which could be effective in reducing epide mics or keeping them at an acceptable level are described on the basis of w orld literature. Some varieties bear one, two or more resistance genes. Gen e combinations (gene pyramids) are thought to be effective in overcoming th e pathogen. To date 24 major resistance genes have been identified, all of which are li sted in detail by the author. Many resistance genes have only been identifi ed in a small number of genotypes. Of these 24 resistance genes, 9 have been identified in cultivated varietie s, alone or in combination. These genes are much the same in most countries , so the generic diversity of powdery mildew resistance genes is small. The se genes no longer provide satisfactory protection, as is proved by the fre quency with which the pathogen exhibits virulence. As the result of their i neffectiveness, epidemics have arisen on large areas. As the result of research carried out in recent years, new resistance genes have been identified. Further experiments will be required to prove how ef fective they are. If breeding is to be successful, it is essential to disco ver new, efficient resistance genes and to apply them in breeding programme s. Breeders can handle major resistance genes with relative ease, but they have the disadvantage that they are easy prey for new virulence genes in th e pathogen.