Biology and control of bean anthracnose in Ethiopia

Common bean (Phaseolus vulgaris L.) is an important food legume for millions of Ethiopian people. However, diseases are major constraints in bean production with anthracnose, caused by Colletotrichum lindemuthianum (Sacc. & Magn.) Briosi and Cav. (teleomorph = Glomerella cingulata (Stonem.), being of particular concern. Anthracnose is the most important disease of bean in Ethiopia and can cause complete yield losses on susceptible cultivars when contaminated seed are planted. The impact of epidemics is further enhanced when favourable environmental conditions prevail. The main objective of this study was to provide information that can be used as part of an integrated control program for anthracnose in Ethiopia. Field surveys were conducted in the foremost bean growing areas of Ethiopia to acquire information on the geographical distribution of anthracnose. Similar to bean rust, anthracnose is favoured by high moisture and relative humidity. In surveys of the different agro-ecological zones (AEZ) anthracnose was found severe at Ambo (08°58'N, 03r26'E), Meki (08°05'N, 038°51 'E), Arsi-Negele (OrOO'N, 038°35'E), Areka 0 and Bako (09°06'N, 37°09'E), intermediate at Awassa (06°52'N, 038°27'E), Jimma (07°28'N, 035°37'E), and Metu (07°02'N, 35°63'E), and slight to absent at Melkassa (08°23'N', 039°19'E). Other diseases observed included bacterial blight, angular and floury leaf spot and rust. The success rate in breeding for resistance depends to a large extent on the degree of variability in a pathogen. Using the standard differential cultivar set and binomial system proposed by CIAT (Centro Internacional de Agricultutra Tropical), eight races of C. Iindemuthianum, viz. 65, 73 128, 296, 511, 589, 961, and 1027, were identified from Ethiopia and compared to races 3, 6, 81,323,390 and 593 from Southern Africa. Races 128 and 511 constituted more than 50% of all isolates characterised. Considering the differential cultivars, G 2333 was resistant to all races in Ethiopia, and AB 136, G 2333, Kaboon, Cornell 49-242 to all races in Southern Africa. Field studies were conducted at Ambo and Bako (Ethiopia) to evaluate resistant and susceptible cultivar mixtures as a means of anthracnose control. Results showed that disease incidence and severity were consistently lower in mixtures containing 50%, 67% and 80% of the resistant component at both locations. Disease progress curves and disease gradients indicated that disease development and spread were slower or reduced in all treatments that contained an increased proportion of the resistant cultivar. It was thus found that anthracnose could be controlled by growing cultivar mixtures containing at least 60% of a resistant cultivar. In an assessment of the efficacy of chemical control of anthracnose, a benomyl seed dressing followed by a foliar difenoconazole spray, or difenoconazole application alone, showed the most promising results. Finally, greenhouse and multi location field experiments were carried out to identify bean genotypes as possible sources of resistance to Ethiopian isolates of C. lindemuthianum. The study established that cultivars Widusa, GLP X 1132, A 482, A 193, G-7, HAL 5 and G 2333 were valuable sources of anthracnose resistance.