Leaf Spots (Helminthosporium, Cochliobolus, Cercospora)
Symptoms and the appearance of lesions can vary widely by pathogen and corn cultivar. Lesions will appear on leaves, leaf sheaths, and husks. Plant resistant varieties. Improve air circulation by increasing plant spacing and choosing sites with good airflow. Plow under crop debris promptly after harvest to speed decomposition.
FRAC Group 3:
- propiconazole (Tilt): 2 to 4 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 24h.
- pyraclostrobin + metconazole (Headline AMP, aka BAS 556 SC): 8 to 10 fl oz/A; PHI 7 to 20d, REI 12h, Groups 11 & 3.
- trifloxystrobin + propiconazole (Stratego): 10 to 12 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 & 3.
FRAC Group 7:
- fluxapyroxad + pyraclostrobin (Priaxor Xemium): 4 to 8 fl oz/A; PHI 7d sweet corn, 21d popcorn, REI 12h, Groups 7 & 11.
FRAC Group 11:
- azoxystrobin (Quadris F, aka Abound F): 6 to 15.5 fl oz/A; PHI 7d, REI 4h.
- azoxystrobin + propiconazole (Quilt Xcel): 10.5 to 14 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 & 3.
- fluxapyroxad + pyraclostrobin (Priaxor Xemium): 4 to 8 fl oz/A; PHI 7d sweet corn, 21d popcorn, REI 12h, Groups 7 & 11.
- pyraclostrobin (Headline): 6 to 12 fl oz/A; PHI 7d, REI 12h. Apply in sufficient water to ensure thorough coverage.
- pyraclostrobin + metconazole (Headline AMP, aka BAS 556 SC): 8 to 10 fl oz/A; PHI 7 to 20d, REI 12h, Groups 11 & 3.
- trifloxystrobin + propiconazole (Stratego): 10 to 12 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 & 3.
FRAC Group M03:
- mancozeb (Dithane F45): 1.2 qt/A; PHI 7d, REI 24h.
FRAC Group M05:
- chlorothalonil (Bravo Weather Stik, aka Bravo 720): 0.75 to 2 pt/A; PHI 14d, REI 12h. Fresh-market sweet corn only. See label for livestock restrictions.
Northern Corn Leaf Blight (Exserohilum [Helminthosporium] turcicum)
Northern corn leaf blight is an emerging problem on farms up and down the Connecticut River Valley. Symptoms begin as long, narrow, tan lesions that run parallel to leaf veins. Lesions coalesce and affect larger areas, and may cause a complete blighting of leaves. E. turcicum overwinters in and on leaf debris and conidia (spores) can be carried long distances by wind. Under the right conditions (prolonged leaf wetness, extended rainy and humid weather, moderate temperatures), conidia are produced abundantly and disease spread is rapid. Reduce primary inoculum by crop rotation and tilling under of crop residue. Evidence suggests more severe disease in reduced-till systems; a 1-year rotation is suggested for conventional practices and a 2-year rotation for reduced-till. Resistant hybrids of all maturity groups are available. Lesions may vary in morphology when certain genes for resistance to this disease are present. The presence of these genes results in long, yellow, linear streaks that resemble the symptoms of Stewart's wilt. Growing susceptible varieties will cause the resistance of nearby resistant varieties to break down.
FRAC Group 3:
- azoxystrobin + propiconazole (Quilt): 10.5 to 14 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 & 3.
- propiconazole (Tilt): 2 to 4 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 24h, Group 3.
- trifloxystrobin + propiconazole (Stratego): 10 to 12 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 &3.
- pyraclostrobin + metconazole (Headline AMP): 10 to 14.4 fl oz/A; PHI 7 to 20d, REI 12h, Groups 11 & 3.
FRAC Group 7:
- fluxapyroxad + pyraclostrobin (Priaxor Xemium): 4 to 8 fl oz/A; PHI 7d sweet corn, 21d popcorn, REI 12h, Groups 7 & 11.
FRAC Group 11:
- azoxystrobin (Quadris F, aka Abound F): 6 to 15.5 fl oz/A; PHI 7d, REI 4h.
- azoxystrobin + propiconazole (Quilt): 10.5 to 14 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 & 3.
- pyraclostrobin (Headline): 6 to 12 fl oz/A; PHI 7d, REI 12h.
- trifloxystrobin + propiconazole (Stratego): 10 to 12 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 &3.
- pyraclostrobin + metconazole (Headline AMP): 10 to 14.4 fl oz/A; PHI 7 to 20d, REI 12h, Groups 11 & 3.
FRAC Group M03):
- mancozeb (Dithane F45): 1.2 qt/A; PHI 7d, REI 24h.
FRAC Group M05:
- chlorothalonil (Bravo Weather Stik aka Bravo 720): 0.75 to 2 pt/A; PHI 14d, REI 12h. Fresh-market sweet corn only. See label for livestock restrictions.
Rust (Puccinia spp.)
The common rust fungus that occurs on sweet corn does not overwinter in the northeastern United States; each year it travels here on southerly winds. Rust appears as blister-like pustules called uredinia on the leaf epidermis. Uredinia produce abundant reddish-brown spores, hence the name “rust.” The disease can cause significant reduction in ear weight and yield on susceptible varieties. Corn is more susceptible to infection prior to tasseling so later maturing plantings may have higher losses. Fungicides will control this disease but should be applied before tasseling. Apply fungicides when 80% of the plants show 1 or more pustules per leaf. Resistant varieties are available.
FRAC Group 3:
- azoxystrobin + propiconazole (Quilt): 10.5 to 14 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 & 3.
- propiconazole (Tilt): 4 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 24h.
- pyraclostrobin + metconazole (Headline AMP): 10 to 14.4 fl oz/A; PHI 7 to 20d, REI 12h, Groups 11 & 3.
- trifloxystrobin + propiconazole (Stratego): 10 to 12 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 & 3.
FRAC Group 7:
- benzovindiflupyr (Aprovia): 4 to 10.5 fl oz/A; PHI 7d, REI 12h. Do not exceed 2 applications per year.
fluxapyroxad + pyraclostrobin (Priaxor Xemium): 4 to 8 fl oz/A; PHI 7d sweet corn, 21d popcorn, REI 12h, Groups 7 & 11.
FRAC Group 11:
- azoxystrobin + propiconazole (Quilt): 10.5 to 14 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 & 3.
fluxapyroxad + pyraclostrobin (Priaxor Xemium): 4 to 8 fl oz/A; PHI 7d sweet corn, 21d popcorn, REI 12h, Groups 7 & 11.
- pyraclostrobin (Headline): 6 to 12 fl oz/A; PHI 7d, REI 12h.
- pyraclostrobin + metconazole (Headline AMP): 10 to 14.4 fl oz/A; PHI 7 to 20d, REI 12h, Groups 11 & 3.
- trifloxystrobin + propiconazole (Stratego): 10 to 12 fl oz/A; PHI 14d sweet corn, 30d popcorn, REI 12h, Groups 11 & 3.
FRAC Group M03:
- mancozeb (Dithane F45): 1.2 qt/A; PHI 7d, REI 24h.
FRAC Group M05:
chlorothalonil (Bravo Weather Stik, aka Bravo 720): 0.75 to 2 pt/A; PHI 14d, REI 12h. Fresh-market sweet corn only.
Seed Decay
Buy treated seed. Do not use treated seed for food, feed, or oil purposes.
Common Smut (Ustilago maydis)
Smut occurs sporadically, and typically less than 2% of the plants are affected. The disease is favored by dry conditions when temperatures are 80°F–90°F and nitrogen fertility is high. Injury due to cultivation, hail, or wind-blown sand can increase the incidence of disease when environmental conditions are favorable. Symptoms will appear as silver-gray galls on the ears or stalks. When mature, the galls produce abundant black spores. Fungicides will not provide effective control of common smut. Remove and dispose of infected stalks. Plant resistant varieties. Avoid injuring plants during cultivation. Rotate away from corn for at least three years
Stalk Rots (Gibberella, Stenocarpella, Macrophomina, Colletotrichum, and Fusarium spp.)
Fungal stalk rots are serious diseases of corn, though their occurrence and severity vary from year to year. Symptoms often begin with wilting, followed by leaf discoloration, browning of the lower stalk, and development of a hollow pith. Crop losses due to difficulties with mechanical harvesting, ear rot caused by contact of ears with the soil, and premature plant death can occur. Factors that influence stalk rot incidence and severity include varietal susceptibility, weather conditions, the presence of foliar diseases, plant density, insect damage, moisture availability, soil fertility, and tillage practices. The disease is more severe and more common where nitrogen levels are high: a balanced and continuous supply of N reduces the occurrence of stalk rot. Adequate K reduces disease severity by strengthening cell walls. Practice proper spacing of plants and rows. Grow resistant hybrids. Manage insect pests and weeds.
Stewart's Wilt (Pantoea [Erwinia] stewartii)
This bacterial disease is spread by the corn flea beetle and is particularly damaging when susceptible varieties are planted following mild winters. Symptoms include narrow lesions, 5 cm or greater, with wavy margins. P. stewartii overwinters in the alimentary tract of adult corn flea beetles, not in the soil or in plant debris. Severity of Stewart's wilt depends on three factors: the winter temperatures prior to planting, the amount of disease the previous season, and cultivar susceptibility. If frigid winter temperatures occur, fewer flea beetles survive to transmit the disease. The amount of disease in the previous season determines the percentage of emerging beetles carrying the bacterium. Hybrids with greater levels of resistance can tolerate more severe infections with less yield loss. Resistance restricts the movement of the bacteria in the plant. Use insecticides to control flea beetles, particularly on susceptible varieties in the seedling stage. This is not as effective as resistant varieties but reduces losses where susceptible hybrids must be planted. Scout early and frequently for flea beetles as rapid growth of leaf tissue makes thorough insecticide coverage difficult.
Maize Dwarf Mosaic Virus (MDMV)
There are several strains of MDMV which are spread by more than 20 species of aphids. The virus can be seedborne in corn and possibly other annual grasses. The virus is nonpersistent but is retained by the aphid for up to 18 hours. This allows time for the vector to be transported from southern states where MDMV is more prevalent and where many weed hosts occur. Early symptoms are chlorotic spots and streaks; leaves will later develop a mosaic pattern. Plant resistant or tolerant corn varieties; see the Sweet Corn section for resistant varieties. MDMV is more likely to be a problem for later plantings. Plant late plantings away from peach trees, as peach is an overwintering host of the green peach aphid, which can vector the virus. Johnson grass is an important perennial weed host for MDMV in southern New England and it should be eradicated.