Scab

Scab is an important disease in China with its own characteristics. It prevails in the middle and lower reaches of the Yangtze valley, coastal areas in southeastern China and eastern parts of Helongjiang province with a severe epidemic nearly once every four years. Since 1950's more than 100 thousand accession-times of wheat materials have been tested against scab isolates by Shanghai AAS and its partners and none is found immune or highly resistant to the disease. Only a very few landraces such as Liyang Wansuibai, Pinghu Jianzimai, Wengzhou redbeardless, etc. are found to be resistant to the extension of infection. But these landraces possess very poor agronomic traits and can hardly be used in breeding. Fortunately in 1970, a relatively resistant (to extension) variety known as Sumai 3 was developed by Suzhou Prefectural Agricultural Research Institute from a cross of two susceptible varieties (Funo/Taiwan). Since then numerous crosses were made between Sumai 3 and various improved varieties, and a series of new cultivars were developed and released for commercial production but rarely their resistance could surpass that of Sumai 3 (Liu et al 1992). Recently breeders from Fujian Agricultural College has produced some new improved wheat lines designated as Fan 60096 and the like from Jingzhou 1/Sumai 2 (a sister line of Sumai 3) whose resistance to scab is slightly better than that of Sumai 3 (Wang et al 1992). Although their difference in resistance to extension of infection is statistically significant yet such a small improvement is not enough to enhance yield stability.

There are arguments regarding the mode of inheritance in varietal resistance to scab. Most authors recognise it is polygenic in nature. Monosomic analyses of scab resistance in Sumai 3 showed that many chromosomes were involved with differential contributions (Li and Yu 1990, Yu 1990). A study of gene actions on scab resistance by generation means analysis from biparental crosses between the susceptible and a series of varieties with differenct degrees of resistance indicated that although additive effect was of primary importance, dominance should not be neglected and epistasis was unimportant (Wang et al 1992). But some authors provided evidence that scab resistance was controlled by a few major genes with a certain number of modifiers, since it usually showed in F₂ a segregation pattern of two peaks with one large and one small (Zhou et al 1987, Bai et al 1990). The discrepency may be attributed to the ways of expressing the infection indices and related also to the fact whether infection data are taken at various stages of disease development or not. If observations are taken in a dynamic pattern then the conclusion would be polygenic; but as far as the sympton of rachis infection is concerned it is of major genes inheritance (Li and Yu 1990).

A study on the components of varietal resistance to scab extention of infection through path analysis showed that yield loss depended on the number of kernels infected by the fungus and the rate of reduction in kernel weight from disease-free kernels on the infected head, the first component playing a more important role. Both of them then were affected by the number of spikelets infected which, in turn, was influenced by the number of rachis internode infected and the time of infection on the diseased head. Their relation is shown in Fig.2.

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