Disease resistance is the most effective, efficient way to reduce disease losses in corn and soybeans. It is an important factor in the performance of all cornhybrids 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, learn to recognize situations when specific disease resistance would beo beneficial. For example, if you know that a certain disease has been a problem and has a potential to reoccur, specific disease resistance should be a priorityu in hybrid or variety selection.

Disease resistance can be good insurance against diseases that are known to be present, even if losses have not previously been excessive. There are several types of disease resistance dealing with the effects of genes on the plant and pathogen. However, in terms of the number of genes involved, there are just 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 conferi 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, thed plant needs different major genes for resistance to each race. This type of resistance is well-defined and more easily measured. It is sometimes called qualitative resistance because plants are either resistant or susceptible, without intermediate levels.

The second type, in which several or many genes are involved, is called polygenic resistance. 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 is often called quantitative, because there are intermediate levels ranging from resistant to susceptible. It is also harder to measure. Often, polygenic resistance does not give a plant as high a level of resistance as major gene resistance.

Another word that describes the reaction of a plant to a disease is tolerance. This word has two meanings. Sometimes, it refers to a moderate level of polygenic resistance. Other times, it refers to the ability of a plant to maintain yield when the plant is diseased. This is a separate characteristic 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 common diseases. This resistance is usually adequate, although a small amount of yield loss may occur.

When disease pressure is very high, more resistance may be needed. Plant breeders breed for high levels of specific resistance to certain diseases. Check with your seed company representative for information on the best seed for your situation.

A summary of the types of resistance you can expect to find in common hybrids and varieties follows.

Corn foliar diseases

Most hybrids grown in Iowa have some polygenic resistance to common foliar diseases including rust, northern leaf blight, anthracnose, gray leaf spot, and northern leaf spot. Some hybrids are more resistant than others. Stay-green is related to this characteristic. Single gene resistance may also be available, but is not as common.

Corn stalk rots

Most hybrids have some tolerance, often reported as a stalk rating or lodging percent. This measures the ability of the plant to remain standing even when infected. Resistance is available for some stalk rots, including anthracnose and4 Diplodia. This is usually polygenic resistance, so it will vary among hybrids. Major genes for anthracnose resistance have been reported, but are not yet 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.

Corn root rots

Little is known about resistance to root rots in corn, but hybrids with better root strength ratings are probably more resistant to root rots.

Corn seedling blights

Little is known about resistance to seedling blights, but hybrids with more vigorous early growth may escape damping off to some extent.

Brown stem rot of soybeans

Both polygenic and major gene resistance are available.

Phytophthora root rot of soybeans

Both polygenic and major gene resistance are available. Tolerance to Phytophthora root rot refers to polygenic resistance. Major gene resistance is available for several races of the pathogen; a different gene may be required for each race. Races 1, 3, and 4 are the major races in Iowa.

Soybean cyst nematode

There are two sources of major gene resistance. There are several races of SCN, but both sources of resistance are effective against the Iowa populations.

Soybean foliar diseases

Most varieties have some polygenic resistance to brown spot and bacterial blight, but this varies among varieties. Relative susceptibility may not be known because these diseases do not consistently cause losses. Major gene resistance exists for soybean mosaic virus.

Sclerotinia stem rot

Varieties differ in their susceptibility, but little is known about the genetic basis. Earlier maturity groups and varieties that do not lodge readily generallyU are less susceptible.

Sudden death syndrome

Little is known about resistance to this disease.

Make disease resistance a priority if a potentially reoccurring disease was a problem in 1992 or 1993. Most diseases tend to reoccur due to pathogen buildup. However, some diseases do not overwinter here (corn rust, for example), so the risk of reoccurrence 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 quite as well as susceptible varieties, but the difference is usually small, especially when compared to the substantial yield benefit of resistant varieties in the presence of disease.