The interaction of yellow rust ( Puccinia striiformis) with winter wheat cultivars showing adult plant resistance: macroscopic and microscopic events associated with the resistant reaction

Abstract

The development of race 104 E 137 of Puccinia striiformis was observed in leaves of four winter wheat cultivars which were susceptible at the seedling stage but showed a graded series of reactions at later growth stages. The degree of resistance, measured by the reduction in area of sporulating tissue, increased gradually with successively appearing leaves on the main axis of Cappelle Desprez, Holdfast and Maris Widgeon. Resistant reactions were associated with reduced fungal colony growth rates, the appearance of scattered cells showing hypersensitive necrosis and with more general areas of chlorosis and browning of host tissue. The differences in resistance between cultivars were correlated with different frequencies of hypersensitively necrotic cells but the gradual increase of resistance in successive leaves of Maris Widgeon did not correspond with the occurrence of hypersensitive necrotic cells which appeared suddenly in the second leaf and remained similar in successive leaves. By contrast the rate of fungal colony growth during the latter half of the infection process and the development of tissue chlorosis and browning were well correlated with increasing resistance of successive leaves, final reaction type and relative differences in resistance between cultivars. The significance of these observations in terms of resistance mechanisms is discussed.