Assessment of damage and grain yield loss in maize caused by northern leaf blight in western Ethiopia

The effect of northern leaf blight (NLB) caused by Setosphaeria turcica (anamorph Exserohilum turcicum) on three open-pollinated maize cultivars ('MDRST', resistant; 'Pool32 C19', susceptible and 'Belerech', tolerant) was assessed in field experiments on inoculated and non-inoculated plants for 2 years at Bake and for 1 year without inoculation at Kelala Bero. NLB incidence and severity, lesion number and area, yield components and grain yield were recorded. Equivalent yield, yield loss, area under the disease progress curve (AUDPC) and apparent infection rare were calculated to investigate the pattern of disease development and its effect on yield. The highest mean NLB severity (57.0 %) was recorded on 'Pool32 C19' in 1993 with artificial infection and the lowest (1.0 %) on 'MDRST' in same year with natural infection. Disease incidence and lesion number were both significantly correlated with early morning humidity and low temperature, particularly in 'MDRST', indicating that in Ethiopia cool nights with long dew periods facilitate infection of partially resistant cultivars. 'Pool32 C19' had the highest mean grain yield loss (43.1 %) and kernel weight loss (16.4 %) whereas 'MDRST' had the lowest losses (10.4 % grain yield and 9.4 % kernel weight). Simple regression analyses indicated that yield losses can be best predicted (R(2) = 0.47) by disease severity ratings taken at 5 weeks after inoculation, i.e., about 3 weeks after mid-silk. AUDPC was significantly correlated with grain yield loss for 'Pool32 C19' and 'Beletech' with coefficients of 0.69 and 0.58, respectively. 'Beletech' was more tolerant of NLB than 'Pool32 C19', as its coefficient of regression of yield loss on AUDPC was smaller (0.023 vs. 0.033). The yield components ear number and kernel size were involved in the tolerance of cv. 'Beletech' suggesting that in maize, tolerance to foliar diseases may rely on similar mechanisms as tolerance to drought stress (maintenance of ear formation under stress and rapid dry matter accumulation).