Transgressive segregation for resistance to yellow rust in wheat

Summary Crosses were made between wheat varieties Joss Cambier, Nord Desprez and Maris Bilbo, all classified as susceptible to yellow rust in field tests, and between Cappelle Desprez and Maris Huntsman, both classified as moderately and durably resistant. Selection for resistance to yellow rust among the progeny was carried out using races of Puccinia striiformis able to overcome all the known race-specific components of resistance in both parents of each cross. Lines with greater resistance than in both parents were obtained from each cross, those with greatest resistance being obtained from the cross between the moderately resistant parents. Three lines selected for resistance from the cross of Joss Cambier with Nord Desprez and one from the cross of Cappelle Desprez with Maris Huntsman, together with the parents, were tested in the field with 12 races of P. striiformis. Nord Desprez possessed a previously undetected race-specific component. The selected lines also displayed race-specific resistance, some of which was clearly related to race-specificity of the parents, and a component of resistance, greater than in both parents, that was effective against all 12 races. The possible origin and potential durability of this transgressive level of resistance is discussed. It is suggested that such transgressive resistance is more likely to be durable if it is derived from parents that have shown durable resistance.