Abstract
Selected elite cowpea lines with different levels of resistance to brown rust ( Uromyces vignae) and false rust ( Synchytrium dolichi) were used to study the relationship between false and brown rusts and grain yield. Varying intensities of brown and false rust were obtained with different fungicide treatments in field experiments. The correlation between brown rust severity and yield was significant ( r = − 0.57; p = 0.001). There was no significant relationship between false rust severity and yield. Thus, brown rust probably accounted for the yield reductions observed. Although critical point models of the form L = a + bX ∗ , where L = loss, a = intercept, b = slope and X ∗ = percentage leaf area diseased at a given time, resulted in significant relationships between percent brown rust severity and yield, the area under disease progress curve (AUDPC) multiple-point model, was appropriate for explaining the relationship between brown rust severity and yield in each of three seasons. The regression equations for yield and AUDPC for percentage leaf area affected by brown rust on TVU-465 were y = 4347 − 48.1 X ( p = 0.01, R 2 = 0.57), y = 1197 − 59.3 X ( p = 0.02, R 2 = 0.33) and y = 2943 −51.8 X ( p = 0.03, R 2 = 0.49) for the first and second rains of 1992, and first rains of 1993, respectively.
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