Abstract
Field experiments were conducted in the Agricultural Research Institute, Uyole, using five susceptible maize varieties during 2013 and 2014 growing seasons. The varieties were used to determine yield losses due to northern leaf blight disease in Mbeya Region of Tanzania. The trials were laid out in two blocks of E. turcicum inoculated and Mancozeb treatment arranged in randomized complete block design in three replicates. Five fungicide sprays were done at weekly interval, starting from 35 days after planting (DAP) while disease inoculation was done twice at 35 and 45 DAP, using whorl placement technique. Data on disease severity index were collected using visual scale of 0-5, and grain yield of each treatment was recorded after harvest and drying. Such data were subjected to analysis of variance, correlation coefficient and coefficient of determination (R²). Means were separated using Turkey’s-Kramer simultaneous test at P≤0.05. Results indicated that Mancozeb sprays enhanced maize grain yield by 30 to 46.6% and 1000-grain weight by 19 to 24%. Disease severity index range of 78.7 to 95.7% indicated that Bora, Kilima, Situka-1, Staha and TMV-1 varieties were more susceptible to Northern leaf blight (NLB) disease. Yield losses ranged from 46 to 62.8% in grain yield (tons/ha) and 31.9 to 38.9% (g/plot) in 1000-grain weight. Disease severity index assessed at silking dry stage had a strong relationship to yield in all the varieties, but varied from Kilima (r = - 0.7617, R² = 0.580, P ≤ 0.078) in 2014 to Staha (r = - 0.9901, R² = 0.9803, P ≤ 0.001) in 2013 in grain yield. The minimum relationship between1000-grain weight and severity index of NLB was recorded in Staha (r = - 0.9300, R² = 0.8649, P ≤ 0.007) in 2013. The grain yield was enhanced and crop loss models indicated good fitness with strong and reliable validity in all the varieties, and as such can be used to estimate potential losses of maize caused by NLB disease in Mbeya, Tanzania. Key words: Zea mays, northern leaf blight, yield loss, Tanzania.  
Highlights
Maize is an important food crop extensively grown in both developed and developing countries of the world (International Maize and Wheat Improvement Centre (CIMMYT) and IITA, 2011)
Severity index of Northern leaf blight (NLB) ranged from 78.7% in Situka-1to 95.7% (Bora) in the E. turcicum inoculated plots, with relatively low disease severity index in Mancozeb treated plots (23%).The study revealed high E. turcicum pressureattributed tofavourable climatic factors in Mbeya
Northern leaf blight (NLB) disease progressed faster and was very high (88.9%) in the maize inoculated with E. turcicum, than in Mancozeb treated plots
Summary
Maize is an important food crop extensively grown in both developed and developing countries of the world (International Maize and Wheat Improvement Centre (CIMMYT) and IITA, 2011). About 100 million hectares are under cultivation in 125 developing countries (FAOSTAT, 2010), with an annual worldwide production of 822 and 817 million tons in 2008 and 2009, respectively (Food and Agriculture Organization (FAO), J. Northern leaf blight of maize (Zea mays L.) caused by Exserohilum turcicum (Pass.) Leonard and Suggs Helminthosporium turcicum (Pass.) is almost ubiquitous in all the countries where maize is grown and is a threat to maize production in many areas of the world (Pandurangegowda et al, 1993; Carlos, 1997; Harlapur et al, 2000; Muiru et al 2010)
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