Disease resistance and crop rotation

Decisions made this winter on crop rotation and variety selection are very important for 1996 disease management. Crop rotation is a wise practice for many reasons, disease avoidance being a major one. Most pathogens survive in crop residue, but only for a limited time, and most pathogens do not infect multiple crops. Therefore, their populations and the risk of disease can be decreased by crop rotation. Fields in which disease problems were noticed in 1995 should be planted to a different crop in 1996 if possible. Crop rotation is effective for most diseases, but a few diseases (rust, Stewarts wilt of corn, yellow dwarf of wheat and oats) do not survive this way, so crop rotation will not affect them.

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, you should recognize situations when specific disease resistance would be beneficial. Disease resistance 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 reoccur. 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 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. This type of resistance is well-defined and more easily measured than the second type. 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 is often called quantitative, because there are intermediate levels ranging from resistant to susceptible. It also is harder to measure than major-gene or single-gene resistance. 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 the common diseases. This resistance is usually adequate, although some yield loss may occur. When disease pressure is high, this resistance is not enough. Plant breeders specifically breed for high levels of resistance to certain diseases. Check with your seed company representative for specific information on the seed you buy.

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

Gray leaf spot of corn: Resistance is polygenic, and most hybrids do not have a high level of resistance. However, there is a range of partial resistance available, and most seed companies publish gray leaf spot ratings.

Northern corn leaf blight: Both monogenic 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.

Other corn leaf diseases: Most hybrids have good polygenic resistance to common rust, anthracnose, and northern leaf spot. Single-gene resistance to rust is available in a few hybrids. Most hybrids do not have high levels of resistance to eyespot and southern rust, but some do. Some seed companies publish ratings for these diseases.

Corn root and stalk rots: Most hybrids have some resistance, often reported as a stalk rating or lodging percent. This is not a direct measure of the level of infection, but a measure of the tendency of the hybrid to suffer lodging as a result of stalk rot. Major genes for anthracnose resistance have been reported, but are not yet available. Root rot susceptibility is closely related to stalk rot susceptibility, since most stalk rots originate as root infections.

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. Check with your seed company representative for less susceptible hybrids if you have had severe ear 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.

Sclerotinia stem rot: Varieties vary in their susceptibility, but little is known about the genetic basis. Varieties that do not lodge readily generally are less susceptible.

You should make disease resistance a priority if a potentially recurring disease was a problem in 1994 or 1995. Most diseases tend to recur due to pathogen buildup. However, some diseases do not overwinter here (corn rust, for example) 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 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.

A new publication, Disease Resistant Soybean Varieties for Iowa, supported by funds from the Iowa Soybean Promotion Board, is now available from ISU Extension Distribution. It also will be included in the January issue of the Iowa Soybean Review.

This article originally appeared on pages 2-4 of the IC-476 (1) -- January 26, 1996 issue.

Updated 01/25/1996 - 1:00pm