Resistance of farmer preferred varieties and use of varietal mixtures to control angular leaf spot disease in common beans.

Abstract

In east Africa, angular leaf spot (ALS) disease of beans is one of the most important biotic constraint to common beans production. The disease is widely distributed across varying altitudes causing up to 70% yield losses on susceptible varieties. Although use of resistant varieties is the most realistic approach to an economically effective and environmentally friendly way to manage bean diseases, farmers still plant susceptible varieties because of certain good qualities they usually posses. This study was aimed at evaluating farmer preferred bean varieties for their reaction to angular leaf-spot disease of common beans and investigating the effect of different spatial arrangements of resistant/susceptible varietal mixtures on the development of angular leaf-spot disease in common beans. Screening of bean varieties was done both in the field and in the screenhouse. Forty farmer preferred local bean varieties were planted in both cases and the experiments repeated once. In the field disease development relied on the natural field inoculum whereas in the screenhouse plants were artificially inoculated with a mixture of Phaeoisariopsis griseola isolates. Disease assessment was carried out on the first trifoliate leaves and second trifoliate leaves for screenhouse and field experiments respectively. Results indicated that 2 varieties NABE 10C and Shemererwa were resistant to the disease. One resistant variety Shemererwa and a susceptible one Sugar 31 were selected from the screening experiments basing on their popularity. They were deployed in the field in five mixture combinations with two controls arranged in a Randomised Complete Block Design with three replicates. The mixture arrangements used were; (1) alternating rows of resistant and susceptible varieties, (2) two rows of resistant followed by one row of susceptible varieties, (3) one row of resistant variety followed by two rows of susceptible variety, (4) two rows of resistant variety followed by two rows of susceptible variety, (5) even mixture of resistant and susceptible variety, pure resistant variety and pure susceptible variety. Angular leaf spot severity data collected at the critical growth stages of flowering, podding, pod filling was used to compute area under disease progress curve (AUDPC) and the mixture efficiency of each mixture combination. The AUDPCs were then subjected to Analysis of Variance (ANOVA) using the SAS software. Results obtained showed significant differences (P<0.0001) in angular leaf spot severities and mixture efficiencies. However the even mixture arrangement produced the most reliable overall control of the disease. This study established that there are different levels of resistance to angular leaf spot among common beans in Uganda which can be exploited in varietal mixture experiments to manage the disease. The study also showed that different spatial arrangements of varietal mixtures reduce ALS disease to varying levels, although the even mixture arrangement produced the most reliable overall control of the disease. It also showed that varietal mixtures can be effective even when the resistant component is not completely resistant. The following recommendations arose from the study; that the varieties Shemererwa and NABE 10C be taken up for further evaluation of their reaction to angular leaf spot, that farmers choose their preferred spatial arrangement putting in to consideration market factors, and that future mixture studies be done in larger fields to avoid inter plot and environmental influences.