The time to take care of many of your crop disease problems is when you buy your seed. Disease resistance is the most effective and efficient way to reduce disease losses in corn and soybeans. It is an important factor in the performance of all corn hybrids and soybean varieties. Even when you choose a variety that is not specifically resistant to a particular disease, you are buying a product that has some built-in disease resistance. In many cases, a high level of resistance to a specific disease is not needed. However, one should recognize situations when specific disease resistance would be beneficial. It should be a priority in hybrid or variety selection if you know that a certain disease has been a problem and has a potential to recur. Disease resistance can be good insurance against diseases that are known to be present, even if losses previously have not been excessive.

There are several types of disease resistance in terms of the effects of genes on the plant and pathogen. However, in terms of the number of genes involved, there are two general types of resistance. The first is called "major-gene" or "single-gene" resistance. Plants with this type of resistance usually have one or a few specific, well-defined genes that confer a high level of resistance to a specific pathogen. Often, the gene gives the plant resistance to only one race of a pathogen. If other races are present, the plant needs different "major genes" for resistance to each race. It is sometimes called qualitative resistance because plants are either resistant or susceptible, without intermediate levels.

The second type, called "polygenic" resistance, involves several or many genes. This type of resistance is harder to define; exactly which genes are involved may be unknown. It usually is effective against all races of a pathogen. This type of resistance is often called quantitative because there are intermediate levels ranging from resistant to susceptible. Often, polygenic resistance does not give a plant as high a level of resistance as major-gene resistance.

Tolerance also describes the reaction of a plant to a disease. Tolerance has two meanings. It may refer to a moderate level of polygenic resistance or to the ability of a plant to maintain yield when the plant is diseased. This characteristic is a separate from resistance to infection.

The type of resistance available depends on the specific disease. Most hybrids and varieties have some level of polygenic resistance to the common diseases. This resistance is usually adequate, although some yield loss may occur. When disease pressure is very high, this resistance may not be enough. Plant breeders specifically breed for resistance to certain diseases, and high levels of resistance are available for these diseases. Check with your seed company representative for specific information on the seed you buy. Following is a brief summary of the types of resistance you can expect to find in common hybrids and varieties.

Gray leaf spot of corn. Resistance is polygenic. Most hybrids do not have a high level of resistance, but in recent years, some moderately resistant hybrids have become available. Most seed companies publish gray leaf spot ratings for their hybrids, and Iowa State University publishes gray leaf spot ratings for hybrids entered in the Crop Performance Test (Pm-660-GLS).

Northern corn leaf blight. Both single-gene and polygenic resistance are available, but the polygenic type is more common. There are many hybrids available with good resistance to northern leaf blight, and most seed companies publish ratings.

Eyespot of corn. Resistance is polygenic. Most hybrids do not have a high level of resistance, but in recent years, some moderately resistant hybrids have become available. Some seed companies publish eyespot ratings for their hybrids.

Other corn leaf diseases. Most hybrids grown in Iowa have some polygenic resistance to common rust, anthracnose, and northern leaf spot. Some hybrids will be more resistant than others. Single-gene resistance to rust and anthracnose may be available in a few hybrids. Most hybrids do not have much resistance to southern rust. Some companies publish ratings for some of these diseases.

Corn stalk rots. Resistance is usually polygenic. Hybrids differ but all have some resistance to stalk rots. The stalk rating or lodging score is related to general stalk rot resistance or tolerance, but it is not a direct measure of resistance to infection. Many factors affect the stalk rating of a hybrid, including its partial resistance to the major stalk rot pathogens. Stress tolerance also affects stalk rot occurrence in hybrids. Major genes for anthracnose resistance have been reported but are not widely available.

Corn ear rots. Some polygenic resistance exists in hybrids, but high levels of resistance are not common in most hybrids. Single-gene resistance to Gibberella has been reported but is not yet available. Hybrids vary in their susceptibility to Fusarium, Gibberella, and Diplodia. Check with your seed company representative for less susceptible hybrids if you have had severe ear rots.

Seedling blights. Little is known about resistance to seedling blights in corn, but hybrids with more vigorous early growth may escape damping-off to some extent. In soybeans, resistance to Phytophthora can be an important step toward controlling damping-off.

Brown stem rot of soybeans. Both polygenic and major-gene resistances are available. Most seed companies offer varieties with good levels of resistance.

Phytophthora root rot of soybeans. Both polygenic and major-gene resistances are available. "Tolerance" to Phytophthora root rot refers to polygenic resistance. Major-gene resistance is available for several races of the pathogen; different genes are required for different races. Races 1, 3, and 4 are the major races in Iowa. Most seed companies offer Phytophthora-resistant varieties.

Soybean cyst nematode. There are three or more sources of major-gene resistance, but one (PI88788) is the most common. There are several races of soybean cyst nematode (SCN), but any of the three sources of resistance is effective against the Iowa populations. Iowa State University publishes results of SCN variety trials each year (IPM-52).

Sclerotinia stem rot (white mold). Resistance is polygenic and there are some varieties with good white mold resistance. Growth characteristics of different varieties also may influence their tendency to be infected. Iowa State University publishes results of white mold variety trials each year.

Sudden death syndrome. Resistance is available in late-maturing varieties, but in maturity groups 0 to III, resistant varieties are not yet available.

You should make disease resistance a priority if a potentially recurring disease was a problem in 1997 or 1998. Most diseases do tend to recur due to pathogen buildup. However, some diseases do not overwinter here (e.g., corn rust) so the risk of recurrence is not high. Resistance is most effective if used in combination with crop rotation and other management practices. Continuous planting of one resistant variety can lead to a breakdown in resistance to some diseases, so there is some risk to overuse of resistance. In the absence of disease, some resistant varieties may not yield as well as susceptible varieties, but the difference is usually small, especially compared with the substantial yield benefit of resistant varieties in the presence of disease.

For more information on disease resistance, see ISU Extension publication PM-1649, Soybean: Disease-resistant Varieties for Iowa, or visit the ISU Plant Pathology Website.