Disease Management

In the chapter “Plant Diseases,” we discussed the concept of a disease pyramid. The concept of the disease pyramid is an important one because it helps us make decisions regarding disease management. If we expect any one or more of the points on the pyramid to be lacking (e.g., a field is planted to a non-host crop), then we could assume that a given disease will not occur and act accordingly. If, on the other hand, we expect disease to occur, all of our available management options are directed at one or more of the three items at the base of the pyramid; that is, we can control the pathogen directly (e.g., with a fungicide), reduce host susceptibility (e.g., plant a resistant variety), or manipulate the environment (e.g., maintain a low soil pH to prevent potato scab).

As we discuss the need for and methods of disease control in this appendix, keep the concept of the disease pyramid in mind.

Predict the Need for Disease Control

Predicting whether or not a disease will occur, and to what extent, can be as difficult as it is important. However, because most control measures rely on stopping a pathogen before or very soon after disease develops, you will benefit greatly by being able to determine when disease is likely to occur. To do so, you will need to consider each point on the disease pyramid.

The Pathogen “Pyramid” Point

Your first clue as to whether a disease might occur is whether or not it has occurred in the same field before. This is especially true for soilborne pathogens, which usually remain in a field once they infest it. Pathogens that overwinter in crop debris may also be a problem unless the debris decays fully before you plant a susceptible crop.

You can check for the pathogen itself by submitting soil or plant samples for analysis. Scout fields frequently so as to detect any disease symptoms as soon as possible. Quite often, a disease may be present but not at a high enough level to warrant control.

The Host “Pyramid” Point

Although planting a resistant variety is in itself a control measure, it can also help reduce the need for other control measures. However, this is not always the case; soybean varieties that are tolerant or have some resistance to Phytophthora root rot actually benefit more from an application of a seed treatment than do fully susceptible varieties.

If you did not rotate to a nonhost crop for at least the previous growing season, you can expect more problems with pathogens that are soilborne or that survive in crop debris. Likewise, if you were not effective in controlling weeds that serve as alternative hosts of a pathogen, disease is more likely when you rotate back to a susceptible crop.

The growth stage of the crop is also an important consideration in predicting both disease occurrence and the need for control. For example, a number of diseases, such as corn rust, can occur so late in the growing season that they might not cause sufficient loss to yield or quality to warrant control. The fungus that causes white mold in green beans primarily infects plants during the bloom period. Also, seedlings tend to be more susceptible than older plants to a number of diseases.

The Environment “Pyramid” Point

Understand which conditions favor the development of a particular disease.

Site Characteristics
Corn leaf diseases are commonly seen in fields located in valleys between ridges and in lowland areas along streams and rivers. Such fields can have prolonged periods of leaf wetness (from dew and fog) and of high relative humidity and low or moderate temperatures that favor most leaf diseases.

Your crop management plan can also influence disease. For example, reduced tillage in continuous corn and overhead irrigation both favor corn leaf diseases by providing debris on which to overwinter and the moisture needed for infection. It follows, then, that you can manipulate field conditions to help reduce the likelihood of disease.

Weather
Weather during intercrop months may be a good predictor of whether certain diseases will be a problem. For example, a very cold winter may reduce the survival of overwintering insect vectors (e.g., corn flea beetles that transmit Stewart’s wilt). Heavy snow insulates potato tubers left in a field after harvest and allows them to survive the winter; volunteer plants that sprout from these tubers in the spring provide a source of inoculum of the late blight organism.

Weather during the growing season is critical to disease development and can help you predict if control will be needed. For example, soybean diseases that affect soybean grown for seed can be important when warm, wet weather prevails during the pod-fill stage. Note that severe weather can also damage a crop and thus increase its susceptibility to infection, as occurs when hail injury to corn ears increases the risk of ear rot.

The Time “Pyramid” Point

The conditions that favor disease development must exist long enough for a problem to actually occur. The duration of leaf wetness (or high relative humidity) and adequate temperature are often among the best predictors of disease. For example, infection by the fungus that causes white mold of green beans can require 48 hours of continuous moisture at 60°F to 70°F but only 9 hours of wetness at 77°F.

Principles of Disease Management

There are very few plant diseases that can be totally controlled, much less eradicated. The goal of disease management, then, is to prevent disease or reduce its incidence or severity. While efforts are directed at protecting the crop as a whole, you must keep in mind that a single plant may serve as a source of infection for an entire field. Plant disease management involves the application of one or more of the following principles:

• Resistance involves planting varieties that are able to resist or withstand infection.
• Eradication technically means the complete removal of a pathogen from an area. In practice, reducing the size of a pathogen population at its source (e.g., field, debris) is a more realistic goal.
• Protection involves establishing a chemical barrier between a host and a pathogen to prevent infection.
• Exclusion keeps a pest out of an uninfested area using such methods as planting disease-free seed and cleaning equipment before entering a field.
• Avoidance is accomplished by planting at times when, or in areas where, a pathogen is rare or ineffective.
• Therapy involves reducing disease severity in an already-infected plant.

Each control method we discuss will fit into one of these management strategies.

Methods of Disease Control

A single procedure may control a plant disease. Most management strategies, however, use several methods and usually involve an integrated program of genetic, cultural, mechanical, and chemical control measures. The following discussion will summarize general approaches to the control of plant diseases.

Resistant Varieties

If available, planting disease-resistant varieties is usually the most effective and economical way to control plant diseases. Again, there are variable levels of resistance and no one variety is resistant to all pathogens.
New races of pathogens can result in disease on what had been a resistant variety. For this reason, you should consult annual updates for varietal reactions to specific diseases.

Disease reactions of a given crop can differ with each locality because of different weather conditions, changes in cultural practices, tillage operations, soil type and fertility. Use varieties that are well adapted to your
growing conditions. In corn, full-season hybrids tend to have fewer problems with stalk rot and ear rot than early-maturing hybrids.

Cultural Control and Crop Management

As stated earlier, some fields are more favorable for disease development: lowland fields; fields with waterlogged soils, and fields known to be infested with a pest, such as soybean cyst nematode or the Verticillium wilt fungus. By planting resistant or nonhost crops in fields that favor a certain disease, you can avoid the disease this year and perhaps reduce its likelihood in subsequent years.

Crop Rotation
Disease severity generally will increase for crops that you grow continuously. In planning a crop rotation, you must consider a pathogen’s host range; ideally, only one crop in a cropping sequence will be susceptible to the pathogen. For example, few fungi that attack soybean infect other crops, so rotating to corn, small grains, or forages will deprive the soybean pathogens of food. In years when nonhosts are planted, debris on which the pathogen survives will eventually decompose.

Crop rotation is most effective for controlling pathogens that survive in the soil or on crop debris; it is less effective for those that are carried by wind each year from other areas.

Planting Time and Methods
Cool, wet soils favor seed rot and seedling diseases; delay planting until the soil moisture and temperature are favorable for seedling development. Adjusting planting time can also help you avoid diseases that are transmitted
by insect vectors. Planting spring wheat early and winter wheat late (e.g., after mid-September) can help avoid high populations of the aphids that transmit barley yellow dwarf virus.

Improper planting depth can lead to poor stands because of seedling diseases. Likewise, excessively high plant populations can increase the chance of stalk rot in corn and white mold in green beans. The higher plant populations increase the relative humidity in the field and can stress the plants, both of which favor disease development.

Choice of Seed or Propagation Material
Seed choice is also critical in preventing disease. Bacteria that cause blights in snap beans overwinter in infected seed, so the use of disease-free seed is critical in avoiding disease. When such seed is available, plant only certified disease-free seed or propagation material.

Proper Plant Nutrition
Many plant diseases are less severe when there is a proper balance of soil nutrients. An excess of nitrogen can increase the risk of stalk rot in corn and smut in sweet corn; be sure to account for nitrogen credits when using manure or planting corn after a legume crop.

Proper Irrigation
As we’ve mentioned, wet conditions tend to favor disease. Thus, you should manage your irrigation carefully. Overly wet soil will favor root rot and overhead irrigation increases the risk of leaf diseases in corn. If you do need to irrigate, timing can be critical. For example, when white mold of green beans may be a problem, time irrigation so that plants dry by evening; this prevents the long periods of continuous humidity and leaf wetness that favor disease development.

Harvesting Time and Methods
Improper harvesting methods can result in cracked seed and a high amount of foreign matter with the harvested product. These factors may lead to problems in storage or may adversely affect seed quality. You can reduce the severity of disease by adjusting your harvest schedule. For example, if stalk rot is a problem, harvest early to prevent further yield loss caused by lodging. Likewise, an early harvest is recommended when corn ear rot is prevalent because mycotoxin production is less, early in the harvest season. Harvesting alfalfa at the bud stage or no later than first flower can help minimize adverse effects on nutritional value caused by stem and leaf diseases, which often get worse after first flower.

Sometimes, you may want to change your harvest method. Another option for responding to an ear rot problem is to harvest the crop early and dry the grain quickly. The fungi associated with ear rots will cease activity in corn having lower than 15% moisture content. Problems would continue, however, if you stored the commodity at higher moisture levels.

Proper Storage of Harvested Plant Parts
Microorganisms cause decay in seed and other harvested plant parts that are improperly stored. They lower the nutritional value and also may produce toxins that harm animals and/or humans. Chemical control of such organisms is usually not an option for most crops already in storage, so proper storage conditions are critical.

Mechanical Control

Many pathogens that infect above-ground plant parts can survive only in plant debris. You can achieve at least partial control by removing or deep plowing crop residues; this hastens decomposition and leads to the death of many associated pathogens. Even chopping corn stalks in the fall can help, as many of the pathogens that overwinter in debris do not survive as well close to the soil surface.

If you use conservation tillage, you will need to counter the increased risk of disease with other management practices. For example, corn leaf diseases increase in
reduced-till fields, so you should rotate crops so that corn debris decomposes before you plant corn again or, if you plant continuous corn, select disease-resistant
hybrids.

The pathogen that causes potato late blight can overwinter on culled potatoes or those that were small enough to be missed at harvest. It is important to check each spring that all such potatoes have been destroyed or disposed of.

Another aspect to mechanical control involves the cleaning of farm equipment. Plant pathogens associated with soil and plant debris can be moved from field to field on tillage equipment, harvest equipment, and on tractor tires. Clean equipment before you bring it into a field, especially if you know that a particular problem (e.g., soybean cyst nematode) exists in the last field the equipment had been in.

Keep equipment and workers out of fields when the foliage is wet so that they do not inadvertently drag pathogens from diseased to healthy plants.

Control of Other Pests

Disease management is part of your overall pest management program. Other plant pests can increase the incidence and severity of plant diseases in the following
ways:

• Insects can transmit plant pathogens from infected plants to healthy plants or create wounds that facilitate infection by pathogens,
• Weeds can serve as alternative hosts for plant pathogens, and
• Stresses caused by plant pests reduce the plant’s natural abilities to fight off disease.

Controlling insect vectors can be the key to preventing the spread of plant viruses. After all, we have yet to develop a chemical that can kill a virus and yet leave
the host plant unharmed; that is, there are currently no “viricides.”

Weed control is essential to the success of a crop rotation. Many pathogens can infect or survive on weed hosts; if you do not control these weeds during years that you plant to a nonhost crop, the pathogen population will be maintained and possibly even build up on the weeds. For example, take-all of wheat infects quackgrass and lesion nematodes can feed on the roots of many weed species in addition to some of our common crops. Likewise, the bacteria that cause brown spot in snap beans can overwinter on the foliage of
hairy vetch.

Weed control is also important with respect to fungicides. A dense canopy of weeds will intercept fungicide sprays intended for the crop foliage and thus control will be reduced. High crop plant densities and narrow rows may be an effective way to control weeds by increasing crop competitiveness; however, the denser canopy will produce a higher relative humidity and reduce the drying effect of wind, thus possibly increasing the likelihood of plant disease.