Comparative analysis of powdery mildew development on leaves, seedlings and flower panicles of different genotypes of cashew

Resistance of cashew ( Anacardium occidentale ) genotypes to powdery mildew ( Oidium anacardii ) was assessed on detached leaves, seedlings and flower panicles. Leaves were more susceptible before the development of a waxy cuticle, which confers a dull appearance. Differences in susceptibility/resistance were observed on detached leaves inoculated in the laboratory. Some genotypes supported dense growth of mildew, whereas on others only slight infection was observed under comparable conditions. Mature leaves were immune to colonization. In the field the rate of mildew infection, as estimated from disease progress curves on both seedlings and flower panicles, varied significantly between genotypes allowing their categorization into susceptible and partially resistant clones. A similar rank order for degree of susceptibility/resistance was determined from studies on detached leaves, seedlings and flower panicles. Brazilian introductions of A. occidentale and the AZA2 and AC6 genotypes proved to be partially resistant, while AM6, ATA19 and AIN62 were highly susceptible. The relationship between leaf and flower mildew infection showed positive and highly significant correlations. It is concluded that each of the three approaches to the assessment of disease susceptibility can be used to screen genotypes against mildew attack. Screening in the laboratory provides a rapid indication of potential resistance providing that leaves are compared at the same stages of maturity.