Different severities of northern leaf blight were induced by
inoculating maize plants once at GS 4 and 5, three time at GS 4,5 and
6, allowing natural disease development, and by applying fungicides to
deter disease development on cultivars with different levels of
resistance to Exserohilum turcicum . Percentage leaf area affected by
northern leaf blight on the whole plant, ear, first, and second leaf
above the ear leaf were assessed weekly for a total of six times at GS
8,9.0,9.1,9.2,9.3, and 9.4. Data obtained from the ear leaf, first leaf
above the ear leaf, and second leaf above the ear leaf were used to
develop a one-three-two weighted scale to account for leaf position and
time of infection for the three leaves assessed and were related to
yield by regression analysis. The average disease severity estimated on
the ear leaf was significantly (P 64 0.001) correlated with
severities estimated using other disease assessment methods and loss in
grain yield. Overall, AUDPC yield-loss models using percentage leaf
area affected on the ear leaf gave the best relationship (Y= 5835-135
AUDPC, R2 = 0.42, P 64 0.001). Further analysis using leaves of
various positions on the plant did not improve the yield-loss models.
Critical point models, using percentage leaf area affected at GS 9.1 on
A619xA632 also gave good fit (R2 = 0.53)