Mechanism of durable resistance: a new approach.

Wheat genotypes, including backcross derivatives of 'Thatcher' carrying Lr10 and Lr23, substitution lines for Lr10 and Lr23 in Chinese Spring background and Chinese Spring and Thatcher were analysed against 21 pathotypes of leaf rust in seedling tests. Adult plant responses in all these stocks were observed in the field nurseries under exposure to the inoculum of the Indian virulent races of leaf rust. The seedling data demonstrated that both the substitution lines and the backcross derivatives for each gene carry identical pattern of infection for resistance. The high level of adult plant resistance in the substitution lines, in contrast to the backcross derivatives in Thatcher, has been postulated to be due to the combination of resistance contributed by Lr10 and adult plant Chinese Spring resistance or to Lr23 and Chinese Spring adult plant resistance. It has been suggested that genes Lr10 and Lr23 added to the Chinese Spring background provide sources for durable resistance, since Chinese Spring has continued to provide a moderate level of adult plant resistance to leaf rust for a very long time.