The disease triangle is a visual diagram used to explain the development of plant disease. In order for disease to occur a pathogen must interact with a host plant. If these two components do not come into contact, disease cannot develop. However, the development of plant disease is further regulated by environmental conditions, with ideal conditions favoring the development of disease and unfavorable conditions deterring disease. These three components make up the plant disease triangle.

Corn
Although more than 30 different diseases affect corn in Ohio, only about half cause economic losses. Approximately 5 percent to 15 percent of the state’s corn crop is lost to disease each year, which amounts to nearly $100 million in lost farm income. Many factors determine the severity of disease in a particular year or a specific field. Weather affects dissemination of disease agents and rate of disease spread. Crop rotation and tillage practices are important in determining pathogen survival. Plant nutrition strongly influences hybrid susceptibility to certain diseases. Soil type and soil moisture influence many diseases, and hybrid resistance is the single most important factor in the preservation of plant health.

The major diseases affecting corn in Ohio include leaf blights, stalk rots, ear and kernel rots, seedling diseases and viral diseases. Some miscellaneous diseases occur, such as common smut and crazy top, but they usually cause little economic loss. Disease can occur on the corn plant during any stage of growth, from germination to maturity.

Soybeans
Diseases of soybeans in Ohio have increased in incidence and severity over the past few years as a result of intensified soybean production. Soybeans are now continuously cropped in many areas. In addition, soybean diseases have also been affected by the increased use of reduced tillage. Reduced tillage systems decrease the amount of soil loss from erosion but also allow for the buildup of pathogens that survive on crop residues.

Wheat
Wheat has been an excellent rotation crop for Ohio, and it also provides a source of cash income mid-season. Wheat provides a break in the crop rotation to permit fungi and nematodes that affect corn and soybeans to be managed. Many growers question the economics of growing wheat because of the low prices of grain in the past. Farmers need to focus on the long-term profitability of their operation and the important role wheat plays as a component for sustained yields in corn and soybeans. There is at least one field on every farm that could use the beneficial properties of a wheat crop.

Foliar fungicide use
The best chance that a foliar fungicide will result in an economic gain for corn or soybeans occurs when there’s enough disease to cause significant yield reductions. Fungicides vary in their activity on diseases. They can be excellent on one while having no impact on another. Reserve fungicide treatment for fields that are at risk for significant disease development. The following should be taken into consideration when making decisions regarding fungicide application:

•Susceptibility and yield potential of the hybrid/cultivar;
•The amount of disease in the field;
•The growth stage of the crop;
•Weather conditions (recent and near term);
•Fungicide and application cost;
•Grain price;
•Directions and restrictions on product label.

Learn more about Ohio corn, soybean and wheat diseases by clicking on to www.oardc.ohi-state.edu/ohiofieldcropdisease

Readers with questions or comments for Roger Bender may write to him in care of this publication.