Variations in field populations, disease development, and yield losses associated with Exserohilum turcicum on maize in Uganda

Virulence patterns, mating types, disease development, and yield losses associated with Exserohilum turcicum from Uganda were investigated. Maize differentials containing Ht1, Ht2, Ht3 or HtN genes were resistant to 215 Ugandan isolates tested, whereas those without Ht genes showed necrotic susceptible reactions, indicating all isolates were race 0. The mating type of the Ugandan isolates was determined by pairing with tester isolates of known `mating types, A, or a. Forty-four of the 189 isolates studied formed fertile Pseudothecia; 36 were MATa and 8 were MATA. Several assessment methods were used to characterize the type and level of disease reactions of Ugandan maize genotypes to E. turcicum in seedling and adult plants. On adult plants, rates of disease progress were similar for moderately resistant and susceptible genotypes, Mo17, B73Ht1, and B73 but were significantly P≤0.001 lower for Babungo 3, KWCA, and population 42. Significantly lower apparent infection rates indicated that rate-reducing resistance characterized resistant genotypes. In seedling studies, the genotype Babungo 3 was most resistant. Babungo 3 delayed formation of necrotic lesions, had fewer and smaller lesions, supported low sporulation, and delayed sporulation by at least 3 days compared to susceptible genotypes. Dispersal gradients of E. turcicumfrom inoculated foci were studied on three maize genotypes. The main effect of genotype was significant for the slope (b) and intercept (a) of the linearized disease gradient (P≤0.007). However, spread was limited to <3 m from inoculated foci on B73xMo17 and KWCA-SR, while disease spread up to 7.2 m on EV8428-SR. Relationships between yield and severity of northern leaf blight were investigated. Average disease severity estimated on the ear leaf highly (P≤0.001) correlated with other disease assessment methods and grain yield (Y). AUDPC disease-yield models using ear leaf data produced the best linear relationship between disease and yield (Y=5835-135 AUDPC, R2=0.42, P≤0.001 ). Critical point models also gave acceptable fits.