Management of Corn Diseases in New York
Diseases of corn in New York are seldom dramatically obvious, yet they constitute an important production constraint because they reduce yield and quality of grain and silage. Fungal leaf blights, stalk rots, and ear rots are the major diseases affecting New York corn. Check with your local Cornell Cooperative Extension office for recent fact sheets and other information available to aid in identification of corn diseases.
Integrated corn disease management involves the selection of hybrids with genetic resistance to diseases, application of seed fungicides, the adoption of sound crop management practices, and the occasional application of foliar fungicides when warranted by disease risk.
Seed Treatment Fungicides
During the developmental stage between seed germination and seedling establishment, the corn plant often requires chemical protection from seed decay, seedling blight, and damping-off caused by fungi on seed or in the soil. Treatment of seed with fungicides is always good insurance against these problems, particularly if the soils are cold and wet or excessively dry. Under these conditions the rate of germination and seedling emergence is retarded, and the seed and seedlings are more susceptible to attack. Except for organic seed, all commercial hybrid seed comes pretreated with fungicide. Planter box products are also available for supplemental disease and insect control.
NOTE: Some seed corn is treated with an insecticide before storage to guard against stored grain insects. This chemical treatment does not protect against soilborne insects such as seed corn maggot or wireworms, nor does it protect against seed rots or seedling blights. Also, it is important to pay attention to the percentage of various component chemicals in insecticide-fungicide combination products. Where the percentage of fungicide in the mixture is low, the product may be labeled only for supplementary disease control on seed previously treated with a fungicide.
Selection of Disease-Resistant Varieties
Resistance to several diseases common to New York State is often incorporated in modern hybrids. Because no hybrid is resistant to all diseases and the importance and prevalence of diseases vary over time and location, it is important to keep up to date on what diseases are currently causing problems in your area. Even a moderate level of resistance is enough to prevent losses to certain diseases; other diseases warrant the highest level of resistance available. Hybrids can also be selected for tolerance - the ability to produce acceptable yields even though symptoms develop. Your seed dealer can help you select hybrids that have appropriate levels of resistance or tolerance to specific diseases.
All New York hybrids should possess good standability. Strong stalk rind characteristics may be as important as, or more important than, resistance to internal stalk-rotting organisms, although hybrids do vary in stalk-rot resistance. Gibberella stalk rot is endemic in New York and harvest losses occur annually, especially in fields that are otherwise stressed. Anthracnose stalk rot can be severe in certain seasons, especially in fields heavily infested with European corn borer.
Although heavy losses are rare, leaf blight epidemics are potentially devastating if they develop during or before the first four weeks after pollination. In addition to directly reducing grain yields, leaf blights can predispose corn plants to stalk rots. Resistance to debris-associated diseases, such as anthracnose leaf blight, eyespot, and gray leaf spot, is most important for hybrids being sown into or near infested corn debris, as in no-till corn production.
Two leaf diseases (northern leaf blight and northern leaf spot) can threaten corn yields even at a considerable distance from infested corn debris. This is because the spores of the causal fungi can be dispersed long distances by high air currents.
Traditionally, hybrid resistance to foliar fungal diseases has been based on single genes that condition strong resistance to the predominant fungal races. The hybrids are susceptible, however, to new races of these highly variable fungi, which occasionally appear. Corn hybrids are now available with partial resistance effective against all races of a pathogen. These hybrids avoid serious losses by slowing down the rate of epidemic development within the crop.
Diseases such as common smut and common rust are relatively minor problems today because most modern field corn hybrids have been selected for resistance to them. They could become severe again, however, if we don’t continue to grow resistant hybrids.
Sound Crop Management
Management decisions throughout the season can affect the prevalence of corn diseases. Attention to the following is important:
Cropping sequence. Because many diseases of corn are perpetuated by contaminated crop debris, the practice of continuously planting corn on the same land is discouraged. This applies particularly to reduced-tillage culture. Debris-borne diseases that can build up in systems of corn monoculture include northern leaf blight, northern leaf spot, eyespot, gray leaf spot, and anthracnose.
Selection of planting site. Avoiding poorly drained soils can help reduce stand losses caused by seed rots and seedling blights. Likewise, if a farmer has a choice, it is good policy not to plant corn in low-lying, shaded fields with poor air drainage and constantly high relative humidity. These conditions allow serious development of leaf diseases.
Seedbed preparation. Important considerations include thorough incorporation of previous crop debris, especially if corn follows corn and diseases have been serious. Correct soil pH (6.0 or above) and fertilizing according to soil test results will help plants withstand several diseases such as stalk rot, common smut, and leaf blights. Balanced fertility is the key; in particular, avoid too much nitrogen and too little potassium.
Date of planting. From the standpoint of disease control, early planting is recommended unless soils are excessively cold and wet. Grain that is thoroughly mature at harvest is least likely to become moldy.
Plant population. Dense stands can increase stalk rot, particularly if nitrogen fertilization is excessive. Where fields have a history of stalk rot, it may be advisable to reduce plant populations from those of previous years.
Varietal maturity. The tendency in New York is to select early-maturing hybrids. Provided they are harvested promptly after maturity and are not allowed to stand until late fall, early hybrids alleviate ear rots associated with immature grain and early frost. But remember that stalk rot will be more severe if mature corn is allowed to stand for too long in the field.
Foliar Fungicide Application
A rather new practice in the Northeast is the application of foliar fungicides at late vegetative stages through kernel blister stage (R2) to control foliar fungal diseases, in particular gray leaf spot and northern leaf blight, on the upper leaves during grain filling. The primary fungicides being applied in New York are products that combine a strobilurin and a triazole fungicide. The cost of product plus application is currently in the range of $35 to $45 per acre. There are no firm disease thresholds to guide fungicide application at this time, though foliar fungicide application is generally indicated in situations where fungal diseases have developed in a majority of plants at or just below the ear leaf by the time of tassel emergence (VT). The risk of foliar diseases (and therefore the potential benefit of fungicide) is increased in susceptible hybrids, in continuous corn under reduced tillage, where there is a previous history of disease, and in humid environments with persistent morning dew. Since economic return on foliar fungicide investment is still poorly understood, growers who apply fungicide are urged to leave non-sprayed strips in their fields for comparison of yield and disease severity. The below table lists labeled fungicides based on their relative efficacy in controlling foliar diseases that occur in New York. Follow label directions carefully for all fungicide applications. Indiscriminate application of foliar fungicides to all corn acres would likely hasten the development of pathogen strains that are resistant to these fungicides and is therefore discouraged.