Disease Severity and Yield of Sweet Corn Hybrids with Resistance to Northern Leaf Blight

Northern leaf blight (NLB), caused by Exserohilum turcicum (Pass.) K. J. Leonard & E. G. Suggs), is a prevalent disease of sweet corn (Zea mays L.) in the United States. Fungicides are used routinely to prevent severe epidemics of NLB on nearly 20,000 ha of supersweet (sh2) hybrids grown each spring in southern and central Florida. Epidemics of NLB on sweet corn grown in the midwestern United States are associated with wet weather. Northern leaf blight can be extremely severe in Illinois, Wisconsin, and Minnesota in August and September on susceptible, sugary (su1) sweet corn hybrids grown for processing. Northern leaf blight typically develops first on lower leaves and spreads to leaves in the middle and upper thirds of the canopy, as illustrated by a rating scale developed in the 1940s by Elliott and Jenkins (4). Reductions in the yield of field and sweet corn have been associated with severe necrosis or chlorosis of leaves in the upper two-thirds of the canopy (2,6,1316,22-26). Slope coefficients from linear regressions of percentage yield on NLB severity have ranged from –0.2 to –0.8, indicating a 2 to 8% reduction in yield for each 10% NLB severity (2,6,14-16). Hooker (9) proposed an index of photosynthesis based on the percentage of healthy leaf tissue at various reproductive growth stages, and assigned values of 10, 5, and 1 to the photosynthetic contributions to field corn grain yield derived from the upper, middle, and lower thirds of the leaf canopy. Hooker and Perkins (10) proposed that economic losses from NLB could be avoided in field corn if 75% or more of the upper leaf canopy remained uninfected for three-fourths of the grain-filling period. They suggested that blight on the lower leaves early in the season had a greater effect on hybrid appearance than on hybrid performance. In sweet corn, NLB had little or no effect on yield of partially resistant hybrids when NLB severity a week before harvest was less than 8% in the upper 75% of the leaf canopy (15), or less than 20% for the entire leaf area (16). Sweet corn hybrids differ in their reactions to NLB (18,21,22). Reactions of many sweet corn hybrids are classified as resistant or partially resistant based on significantly less symptomatic leaf area than those classified as susceptible (18,20,21,22), but few evaluations of resistance consider the distribution of NLB symptoms within the canopy. Severity of NLB usually is less than 20% when partially resistant hybrids are inoculated in disease nurseries in Illinois (21); however, NLB severity may be considerably higher on these hybrids under extremely conducive environments such as those typical of southern and central Florida. In the past decade, the levels of partial resistance to NLB in sweet corn hybrids and the number of resistant hybrids sold commercially have increased, particularly among supersweet (sh2) hybrids developed primarily for Florida markets (18; J. K. Pataky, personal observation). Symptom development is delayed until a week or two before harvest in a few sh2 hybrids which have resistance conveyed by the dominant gene HtN (8,19,20). Incubation period is normal but severity is relatively low for several sh2 hybrids with high levels of partial resistance (19,20). Symptoms are less severe on partially-resistant sweet corn hybrids as the result of smaller and/or fewer lesions. NLB lesions on several “2132-type” sh2 hybrids are smaller than those on susceptible hybrids. On certain sugary enhancer hybrids (e.g., Miracle, Merlin, Argent, Tuxedo), the number of lesions is less than on susceptible hybrids, but lesions are not as small as those on some of the 2132-type sh2 hybrids. With the introduction of new sh2 hybrids that carry HtN or have high levels of partial resistance to NLB, it would be useful to approximate the level of resistance needed to prevent yield reductions due to NLB under environments conducive to severe epidemics. The objectives of this study were to evaluate the reactions of sh2 sweet corn hybrids to NLB, and to associate yield with reactions of hybrids to NLB. Preliminary analyses of reactions of new hybrids have been reported (20).