Bacterial Leaf Blight (bacterium – Erwinia Stewartii): This disease is also called Stewart’s bacterial wilt, Stewart’s leaf blight and maize bacteriosis, and is rarely found in Texas. Leaves show linear, pale green-yellow streaks with irregular margins that parallel veins. These streaks become dry and brown. Plants produce premature, bleached, dead tassels. Severely infected plants form cavities in the pith of the lower stalk and the bacteria spread through the vascular system into the kernels. Chewing insects, such as flea beetles, wireworms, root worms and May beetles can spread the bacterium. Field corn is more resistant than sweet corn. Control by using resistant hybrids and control chewing insects.
Brown Spot (fungus – Physoderma maydis): Brown spot is generally not a serious problem. Symptoms are small, round, yellow spots on leaves and sheathes that appear in bands across the leaf (See Photo). Lesions turn brown and blend together to form larger blotches . Infected leaf tissue decomposes, releasing brown spores. Stalks often break and lodge.
Common Rust (fungus – Puccinia sorghi): The fungus erupts from the leaf tissue in round to oval areas called pustules, with cinnamon brown spores (See Photo). These spores eventually turn black. Rust is favored by cool temperatures and high humidity. Older tissue is generally resistant. There are numerous physiologic races of this fungus. Control disease with slow rusting resistant hybrids (See Photo). Fungicides are usually not economically practical. High summer temperatures usually reduce the disease.
Common Smut (fungus – Ustilago maydis): All parts of the plant are susceptible. Galls are covered with a white membrane. The inside of the gall turns into a mass of powdery black spores (See Photo). Early infection may kill small plants but this is rare. A gall on the lower part of the stalk can make the ears small or non-existent. Dry weather with 78oF – 95oF is favorable. High nitrogen or heavy manure is also favorable. Injuries from blowing sand, hail damage, cultivation, or buggy-whipping will increase smut. Avoid mechanical injury, avoid highly susceptible varieties, and maintain balanced fertility.
Crazy Top (fungus – Sclerophthora macrospora): Symptoms vary with time of infection and degree of colonization. First symptoms are excessive tillering, rolling, and twisting of upper leaves. The tassel proliferates until it is just a mass of leaves, hence the name crazy top . Leaves may also be stunted with yellow stripes. Flowering before plants reach the four- to five-leaf stage is usually a factor. Losses are usually confined to small areas. Avoid wet fields with poor drainage.
Ear and Kernel Rots (fungi – several species): Corn ear and kernel rots are a greater problem in areas with high rain fall from silking to harvest. Symptoms range from pink mold on ears with Fusarium to green mold with Tricoderma (See Photo 1 and Photo 2). Moldy corn may contain toxic materials (see section on mycotoxins for detailed information). Bird and insect damage to ear and stalks along with lodging will increase stalk rots. Rots are reduced in ears that mature hanging downward. Yield, quality, and feed value can be reduced. Controls include resistant hybrids, early harvest, and proper storage below 15% moisture.
Head Smut (fungus – Sporisorium reilianum): This soil-borne fungus attacks both corn and sorghum. It appears on the ear and tassels. Tassel infection may be from individual spihelets or a large mass of black spores (See Photo). Smutted ears are small and tear-drop shaped, with no evidence of a cob or kernels inside (See Photo). Usually if the tassels are smutted, all the ears will be smutted. Controls include resistant varieties or hybrids and rotation.
Nematodes: In general, nematodes decrease the efficiency of the root system, resulting in reduced growth, yellowing and reduced yields. More than 40 species have been reported on corn. The main symptoms on corn are lack of vigor and dark, discolored areas on the roots. Rotate with non-host plants and use chemical infurrow treatments where practical (See Table 1 Below).
Northern Corn Leaf Blight (fungus – Exserolilium turcicum): Long, elliptical, tan lesions develop first on older lower leaves (See Photo). It progresses up the plant and resembles frost or drought injury (See Photo). Lesions form on husks but ears are not infected. Lesions on resistant hybrids have a yellow margin (See Photo). It overwinters on leaves, husks, and other plant parts in debris. In wet weather, black spores are produced on the lesions that are wind-blown long distances. Secondary spread is common within and between fields. Moderate temperatures, heavy dews, and frequent showers are favorable. High losses will occur with infection before silking. Infection six weeks after silking will cause minimal loss. Control with resistant hybrids. Check for approved fungicides. Section 18 emergency exemptions for Tilt have been available in the past.
Seed Rots and Seedling Blight (fungi – several species): Soil and seed-borne fungi attack corn seed causing seed rots and seedling blight. These fungi are favored by cold (50oC – 55oC) wet, poorly drained soils. Factors that affect disease severity include genetic resistance, seed quality, planting depth, and soil type. Symptoms include yellowing and wilt of leaves, seed rot, damping-off of seedlings, seedling wilt, and root rot. Controls include planting high quality damage-free seed, planting in warm, moist soil, correct placement of infurrow pesticides and fertilizer, and seed treatment fungicides (See Table 1 Below).
Sorghum Downy Mildew (fungus – Peronoscherospora sorghi): Symptoms are yellow, stunted plants with occasional white striped leaves. Infection is systemic. Infected plants have narrow, more erect leaves. Plants will have bushy tassels and downy white growth will appear on both leaf surfaces. Tolerant plants show some symptoms but still produce seed. Some varieties show long, narrow, yellow lesions. Spores can blow from plant to plant or field to field. Other spores survive for years in the soil. Use resistant hybrids and systemic fungicide seed treatment.
Southern Corn Leaf Blight (fungus – Bipolaris maydis): This was a minor disease of corn for many years, with no economic effect on yield. In 1970, a highly virulent strain called Race T appeared on corn hybrids with Texas male sterile cytoplasm (See Photo). Losses were severe. Symptoms of Race lesions are spindle-shaped with yellow-green halos. They will later have dark, reddish-brown borders and occur on leaves, stalks, leaf sheaths, husks, and shanks. Cob Rot ear occurs as well as ear drop. The fungus overwinters in corn debris and on seed. Wind and splashing water spread the spores rapidly in the field under ideal conditions, cycling in about 72 hours. Control with resistant hybrids.
Southern Corn Rust (fungus – Puccinia polysora): Southern rust develops more rapidly and is more destructive to leaf tissue than common rust, but it occurs primarily along the Coastal Bend as far north as Hempstead. Southern rust pustules are smaller, more round and orange in color than common rust. The many pustules that form cause the leaf to turn yellow and die. Yields are reduced by heavy infection at ear filling. Unlike common rust, southern rust is favored by high temperature. Corn hybrids vary in susceptibility to this disease. Resistant hybrids should be planted. Chemical control is available to growers through mancozeb fungicides.
Stalk Rots (fungi – several species): Stalk rots are the world’s most destructive corn diseases. They are caused by a complex of fungi and bacteria that attack the stalks near maturity. Stalk rots are favored by conditions that encourage heavy kernel set followed by late season stress, such as leaf blights, extended cloudiness, heavy plant stands, drought, hail, and low K with high N. Most stalk rots are favored by dry conditions early and warm, wet weather after silking. Symptoms include destruction of pith in the stalk and lodging (See Photo). Interior discoloration ranges from pink with Gibberella to gray with charcoal rot (See Photo). Yield reductions of 10 – 20% are common with susceptible hybrids. Losses occur from both poorly filled ears and harvest losses due to lodging. Controls include using disease-free seed of resistant hybrids, crop rotation, balanced N and K fertility, sanitation, stress reduction such as pest control, and reduced plant populations.
Maize Dwarf Mosaic Virus (virus – maize dwarf mosaic virus (MDMV)): This is the most common and most damaging of corn viruses in Texas. Youngest leaves have a light to dark green mottle or mosaic in narrow streaks along veins (See Photo). Plants are stunted (See Photo), excessively tillered, with multiple ear shoots and poor seed set. Early infection can lead to root and stalk rots and death. The virus is transmitted mechanically by at least 12 different aphids. Symptoms appear 30 days after emergence. Many wild and cultivated grasses serve as hosts for MDMV. Johnsongrass is considered the main overwintering reservoir. Use resistant hybrids and control Johnsongrass.
Table 1: Chemical Infurrow Treatments
|Corn Seed Treatments||Corn Nematicides|
|Captan, Metalaxyl, Metalaxyl + PCNB, Carboxin, PCNB, Cargoxin + Maneb + Lindane||Carbofuran, Terbufos, Ethoprop|