Resistance in South African and foreign wheat cultivars to pathotypes 6E16A- and 6E22A- of Puccinia striiformis f. sp. tritici

Abstract

Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici Eriks., has become an endemic disease of wheat (Triticum aestivum L.) in South Africa after being observed for the first time near Moorreesburg, Western Cape, during August 1996. The reaction of 55 South African and 18 foreign Triticum aestivum L. cultivars was determined to pathotypes 6E16A- and 6E22A- in the seedling and adult plant stage. The occurrence of stripe rust head infections was studied in 16 spring wheat cultivars and 17 supplemental lines. Six of the 55 local wheat cultivars expressed seedling resistance (infection type < 2+), 18 appeared heterogeneous and 31 were susceptible (infection type ≥ 2+). The mean area under the disease progress curve (AUDPC) determined in the field for 42 cultivars over a three year period showed that 11 cultivars expressed high levels of complete or adult plant resistance (AUDPC < 200). Twelve cultivars displayed intermediate levels of resistance (AUDPC 200 to 500) and 19 displayed AUDPC values of 500 to 1598. Terminal severity ratings were highly correlated with AUDPC for both winter (R2 = 0.91, P > 0.001) and spring (R2 = 0.82, P < 0.001) wheat cultivars. Stripe rust resistance expressed by local wheat cultivars appeared stable over five different environments. The percentage head infection was positively correlated (R2= 0.78, P < 0.001 during 1997 and R2= 0.84, P < 0.001 during 1999) to stripe rust severity on flag leaves. Cultivars and lines with seedling resistance showed no or a very low percentage (0 to 2%) head infection, whereas cultivars susceptible in both seedling and adult plant stages were severely infected in the heads. Cultivars and lines expressing adult plant resistance showed intermediate to low percentages of head infection. Of the 18 foreign cultivars evaluated 10 were resistant in both seedling and adult plant stages. The remaining eight cultivars were susceptible as seedlings but showed high levels of adult plant resistance in the field.