Soilborne Plant Diseases

Francis E. Clark, William J. Zaumeyer, and John T. Presley.

Many parasitic micro-organisms live below ground. They destroy about a billion dollars' worth of crops annually. The root rots and wilts of cotton alone cause yearly losses estimated at more than 100 million dollars. All major agricultural crops are at times seriously damaged by root diseases.

Some fungi that attack plant roots can grow only on the roots of their appropriate hosts. Others can grow in the soil even in the absence of a living root. Many of the root-inhabiting fungi can remain alive in the soil in a resting or dormant condition, sometimes for years, after the death of their host, even though they cannot multiply in the soil.

Fungi that can grow in the absence of roots are termed soil inhabiting. Usually they are not highly specialized parasites. Many of them grow readily on many kinds of organic debris. They are best known for their role in the damping-off of seedlings and as the agents responsible for plant wilts.

Identifying the causal organism or parasite is one step in dealing with a soilborne disease. You also must determine the relation of the parasite to its soil environment and the extent to which you can manage the soil to destroy the organisms that cause plant diseases.

Control measures for soilborne root rots are of two main types those that attempt to fortify the resistance of the plant to attack and those that attempt to inhibit or destroy the parasite.

The most economical and permanent method of controlling microorganisms that attack plants above or below ground is to develop plant varieties that resist them.

Plants resistant to many root diseases have not been discovered. Good farming practices then are valuable control measures. The judicious use of fertilizers has helped to cut down losses.

WELL-FED PLANTS usually are less susceptible to soilborne organisms than are poorly nourished plants. Good fertility may so enhance the resistance of the host plant that the parasite cannot successfully attack the roots.

Sometimes fertilizer reduces the damage by increasing the root-forming ability of the plant. In such instances the parasite readily enters and destroys individual roots, but meanwhile other roots are formed and they in turn support plant growth.

It is not always true that a fully nourished plant is less susceptible than a poorly nourished one. Crop damage by some diseases is increased by fertilizer treatment. The fertility practices required for root disease control vary so markedly for different parasites and for differing soil and farming conditions that they must be defined individually and regionally for each soil-borne disease.

Nitrogen fertilization reduces the severity of some root rots and increases the severity of others. The increased resistance of sugar beets to root rot after fertilization with nitrogen seems to be associated with an excess consumption of the element and not simply with the correction of any nitrogen deficiency. The addition of nitrogen is almost as effective in fields of high nitrogen fertility as in fields with less nitrogen. The Aphanomyces root rot of peas also is controlled by nitrogen fertilization.

The Pythium root rots of cereals and sugarcane, the Fusarium rot of gladiolus, and the Verticillium wilts of many plants are aggravated by applications of nitrogen. That may be due partly to the occurrence of more turgid and thinner walled plant cells after nitrogen application and their greater susceptibility to parasitic invasion. Also involved is a disturbance of the normal nitrogen and phosphorus ratio in the plant. The nitrogen fertility of the soil needs to be kept in balance with that of phosphorus, at least as far as the Pythium root rots are concerned.

Phosphorus fertilization usually helps control seedling and root diseases of cereals. The more vigorous development of new roots in phosphate-fertilized plants permits the plants to escape destruction. In many places where the take-all disease of wheat is severe, good response to phosphate fertilization is sometimes obtained. Phosphates also are beneficial in controlling both Fusarium yellows and black root in sugar beets.

A deficiency of potassium makes many diseases more severe, presumably because plant sugars accumulate in the cells in the absence of potassium instead of being built into new tissue.

Among the few diseases that are suppressed by potash deficiency are the crown and root galls, the clubroot of cabbage, and (in some soils) the Verticillium wilt of cotton. Potash fertilization helps control tomato wilt, cabbage yellows, the Fusarium wilt of cotton, the powdery mildew of cereals, and wildfire of tobacco.

The lack of trace elements, or micronutrients, in the soil, particularly boron, copper, iron, manganese, molybdenum, and zinc, often causes abnormalities, which are known as mineral-deficiency diseases. Sometimes the plant symptoms caused by shortage of a trace element can be mistaken for symptoms caused by a parasitic organism. Usually, however, the symptoms are sufficiently unique to be recognized. Nutrient deficiencies sometimes permit damage to roots by otherwise harmless soil organisms.

The heart rot and dry rot of sugar beets, caused by too little boron in the soil, are common in parts of the United States. A breakdown of the fleshy root is the primary symptom. It is followed by the formation of a root canker, where secondary organisms may enter. A similar breakdown of the roots of table beets is called internal black spot. The disease can be controlled in most soils by disking in 12 to 20 pounds of borax an acre before the crop is planted. Soils that are neutral to slightly alkaline and high in organic matter require 40 to 60 pounds an acre. Other boron-deficiency diseases are crack stem of celery, internal brown spot of sweetpotato, internal cork of apples, top sickness of tobacco, and root necrosis of radish. Cabbage and cauliflower are also susceptible to boron deficiency.

Plant abnormalities can be listed for each of the other micronutrients. To mention a few: The typical chlorosis in many plants due to iron deficiency; chlorosis and necrosis, such as gray speck disease of oats and Pahala blight of sugarcane, due to manganese deficiency; whiptail of cauliflower, due to molybdenum shortage; and little-leaf, due to lack of zinc.

The amounts of fertilizer or spray material needed for correction in most instances are small. In some alkaline soils all one has to do is to create a neutral or slightly acid reaction by adding sulfur or an acid-forming fertilizer. Accurate diagnosis is important. An excess of any one of several nutrient elements causes toxicities, and You cannot always distinguish clearly between deficiency of one element and excess of another, particularly because the lack of one element may lead to excess accumulation in the plant of some other element.

GOOD MANAGEMENT PRACTICES are especially important in controlling a group of seedling diseases known as damping-off. The organisms that cause damping-off are widely distributed in the soil, but usually it is only in the presence of unfavorable environmental factors that their damage becomes significant.

Excessive soil moisture or an unfavorable growing condition, such as an unsuitable temperature, poor lighting, or excessive soil acidity, commonly increases the susceptibility of seedlings to the damping-off fungi. Under identical soil conditions, seedlings from seed of low vitality are more susceptible to damping-off than those that develop from good seed.

Influences of any one environmental factor may differ with the crop. Sugar beet seedlings are less susceptible to damping-off at soil temperatures below 60 F. than at 75 or 80 , but tomato seedlings are more susceptible. In common bunt of wheat, the greatest infection occurs at soil temperatures of 40 to 60 . If fall wheat is planted early in northern Idaho and eastern Washington while warm soil temperatures still prevail, the wheat seedlings largely escape infection. To the extent that marketing considerations and the length of the growing season permit, the date of seeding for each crop should be chosen to take advantage of a favorable soil temperature.

Drainage or bedding-up to eliminate excessive wetness reduces the damping-off losses of a number of field and vegetable crops. The use of soil that is well drained controls the Aphanomyces seedling rot of peas in the North.

Conditions of soil moisture that are ideal for plant growth also favor the fungus of the Rhizoctonia seedling disease of cotton. Allowing the soil to become dry at the surface simply causes the fungus attack to occur an inch or more below the surface. In extremely wet soil, the fungus causes only negligible damage. In irrigation farming, where the soil moisture can be regulated, the fungus can be controlled by planting the cotton in dry ground and then irrigating.

Water management also can be used to influence soil temperature. Verticillium wilt of cotton, which causes severe losses in the irrigated Southwest, is favored by low soil temperature. Incidence of the disease has been reduced by planting the crop on high ridges or cantaloup-type beds and irrigating every other middle.

Treatment of seeds with fungicides is a widely used and practical way to control damping-off. Because only the seed is treated, the cost is reasonable.

A number of seed protectants are on the market. Among them are the organic mercuric formulations, such as Ceresan and Panogen; the nonmercuric organic formulations, such as Arasan, Phygon, Captan 50 W, Spergon, Dow 9B, and Vancide, 51; and the metallic inorganic compounds, such as basic copper carbonate.

Because nearly all soils are infested with the damping-off fungi, the need for protectants probably will continue or increase. Attachments for the seeding shoe that permit spraying a fungicide on the temporarily open faces of the seeding furrow are a means of providing additional protection for the emerging seedling without the expense that would be involved in drenching the entire soil.

The vascular wilts are diseases caused by free-living members of the soil flora. Soil-inhabiting fungi are commonly involved. They enter the root system, grow into the water-conducting tissues, and choke off water movement or induce excessive loss of water.

Many wilt fungi can grow on organic residues in the soil. They usually are restricted in their parasitic growth to single or closely related species of higher plants. This specificity often extends to individual varieties of plants, and outstandingly successful control of many wilts has been obtained by selection of varieties and breeding wilt resistance into them. Plant wilts can be measurably reduced in many soils by fertilization and soil-management practices, but by themselves these practices do not seem adequate to control most wilts.