Expression of adult resistance to stripe rust at different growth stages of wheat

Stripe, or yellow, rust (caused by Puccinia striiformis) is a common disease of wheat in cool, humid climates. Genes that confer adult resistance to stripe rust are known to be present in high-yielding spring wheats (Triticum aestivum). However, it is not known when in the growth of the plant adult resistance is first expressed. The objectives of the study were to evaluate the expression of adult resistances at various growth stages in the greenhouse, and to assess the effect of such resistances on stripe rust development in the field. Eight wheat cultivars known to carry different levels of adult resistance and three susceptible cultivars were evaluated for resistance in the greenhouse at six growth stages (from seedling to anthesis), using a Mexican race of I;I striiformis. Resistance was measured by infection type (IT) and latent period (LP). The cultivars were also evaluated in the field using the same race to determine disease progress. In all tests, Morocco was the most susceptible cultivar (IT 7 to 9, LP 11 to 13 days), followed by Avocet S (IT 6 to 9, LP 12 to 15 days) and Jupateco 73S (IT 6 to 9, LP 12 to 17 days). Seedling IT and LP for cultivars with adult resistance were similar to those of the susceptible cultivars (IT 6 to 8, LP 12 to 15 days). However, as plants grew, resistance increased and was expressed as lower IT and longer LP. The changes in IT displayed by HD2258 and PBW65 were the greatest; intermediate to low IT (2 to 3) were seen as early as the completion of tillering and changed to immunity (IT 0) at anthesis. Other cultivars with adult resistance, viz., Mexico 82, Pavon 76, Jupateco 73R, Apache 81, Anahuac 75 and Ciano 79, generally showed intermediate to moderately susceptible responses (IT 4 to 6) at growth stages later than tillering. The IT and LP displayed at anthesis by resistant cultivars were lowest and highest, respectively, indicating that adult resistance was best expressed at this growth stage. A strong negative rank correlation (r = -0.939, P < 0.01) between IT and LP at anthesis suggested that lower ITs of the cultivars were generally associated with longer LP. In the field study, HD2258 and PBW65 were highly resistant (relative AUDPC less than or equal to 5% of Morocco). Pavon 76 and Mexico 82 had acceptable resistance levels (relative AUDPC less than or equal to 20%), whereas other cultivars displayed moderate resistance levels (relative AUDPC 30 to 52%).