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Common Forage Pests and Problems

By Dr. Harold Willis


Believe it or not, a healthy, vigorous plant can ward off or resist the attack of infectious diseases and pests. This is a known fact that is being supported by more and more research, although the chemical companies may not want you to know about it.

Of course, it makes good sense to plant varieties of crops that have genetic resistance to certain pests and diseases, at least until you do get your soil in good condition.

A large book could be written about all of the various diseases and pests that afflict alfalfa and other forages. They range from verticillium wilt to Texas root rot to alfalfa weevils to spotted alfalfa aphids. The book Alfalfa Science and Technology (1972) lists 3 bacterial, 24 fungal, and 3 viral and mycoplasmic diseases; 66 kinds of insects and mites; and 9 nematodes that attack alfalfa.

Pity the poor plant pathologists and entomologists who spend all their time looking at sick plants, identifying the critter that’s causing the damage, and then using toxic rescue chemistry to try to save the crop.

The diseases and bugs are there for a reason—to clean up the unfit and unhealthy, and to tell us something’s wrong. The best approach to diseases and pests is to prevent them, to build up your soil fertility so that crops will be vigorous and healthy, able to resist attack. It’s just like human or animal health. Even though we are exposed to germs every day, we don’t get sick unless we let our natural defenses down and don’t get the right food or enough sleep or exercise. Also, stress can make you sick. The same is true of plants. They can be weakened by stresses—whether from unbalanced soil fertility, toxic substances, waterlogged soil, or adverse weather—and soon fall prey to diseases and/or pests.

Many studies prove this. For example, an excellent summary article by J. L. Dodd in the June 1980 issue of Plant Disease, is entitled “The Role of Plant Stresses in Development of Corn Stalk Rots.” He points out that stalk rot fungi do not attack corn plants until they are under some kind of environmental stress or are of a susceptible variety. Corn has been found to produce chemical substances that inhibit corn borer larvae, and an anti-fungus chemical has been found in wheat and corn. A study by S. D. Kindler and R. Staples found that two alfalfa varieties were more susceptible to the spotted alfalfa aphid when the soil fertility was out of balance, either too little calcium or potassium, or too much magnesium or nitrogen.

Non-infectious diseases

There are a number of problems which are not caused by disease germs or pests. They are sometimes called abiotic, nonparasitic, or physiogenic diseases. Some are beyond the farmer’s control, but others can easily be prevented by proper soil conditions and fertility. They include weather factors, nutrient deficiencies and toxici-ties, pollution, and anaerobic soil conditions.


Crop health and growth can also be damaged by air pollution (“smog,” sulfur dioxide, ozone), acid rain, and waterlogged or tight or crusted soil, which leads to anaerobic soil conditions (no oxygen). Anaerobic soil damages roots from lack of oxygen and also causes anaerobic bacteria to release toxins. Damaged roots often react by developing gummosis, a sort of “hardening of the arteries” in which vital water and food-conducting vessels become plugged by gums and other secretions

Crisis management

If your soil is not yet in good condition, or if the weather isn’t cooperating, and your plants are under stress and are attacked by a disease or pest, all the Utopian theories in the world will not help. You have to save that crop! So go ahead and follow the advice of your local extension agent or whoever, but if you can, don’t apply the recommended insecticide or fungicide at the recommended rate. Try reducing the rate by one-third or even one-half. Usually the recommended rates are plenty high. Also, since only a small fraction of sprays actually reach their intended target, the use of a surfactant or wetting agent will help the pesticide penetrate more readily and allow you to use less poison. If you don’t want to gamble on a whole field, at least try a test strip at reduced rates.

Remember that most pesticides upset something—either the plant’s functions or the soil’s balance of microorganisms, or something. There are usually kickbacks which you don’t need. They only make it harder to get your soil in healthy condition later. So use as little toxic chemicals as possible.

Source: How to Grow Great Alfalfa