Damage from damping-off and root rot is avoided by promoting disease-escaping growth habits, such as rapid development of roots. The critically important nutrients for such growth are nitrogen and phosphorus; consequently properly balanced nitrate and phosphate fertilizers are beneficial.

The wilt diseases are most critically affected by the ratio of nitrogen and potassium. The gall and overgrowth diseases are appreciably affected by the calcium-potassium ratios, partially modified by hydrogen-ion effects. Potassium and nitrogen are decisive factors in influencing infection of leaves by obligate parasites.

None of these treatments completely alters the inherent reaction of a plant to pathogens. Immune plants are not attacked regardless of the soil they are grown in. Extremely susceptible varieties cannot be immunized by fertilizing the soil. The greatest benefits from soil fertilizers have been observed on moderately susceptible to partially resistant varieties. Most horticultural varieties are in this latter class since they often escape infection or recover from disease. Proper fertilizers improve their opportunities to escape destruction and produce a profitable crop.

The concentration of properly balanced nutrients in the soil may influence the severity of disease. As growth is promoted by more liberal fertilizer applications, the damping-off and root rot diseases become less serious, but the galls, clubroot, and scab diseases become more conspicuous and leaf rust and powdery mildew caused by obligate parasites flourish. If sufficient balanced fertilizers are applied to injure the plant and retard growth, damping-off and seed decay are more destructive and the obligate parasites may be less active.

The balance of nutrients may be more important than concentration of total fertilizer when plants are exposed to attack by parasites. A deficiency or surplus of any one element often promotes disease.

All classes of diseases from those caused by the facultative saprophytes that destroy stored fruits and vegetables to those caused by the obligate parasites are most severe when nitrogen is overly abundant. Although it encourages infection of wheat roots by the fungi that cause the browning disease and take-all, its over-all effects may be beneficial if it promotes growth of new roots. If phosphorus, however, is not available to facilitate root development, the excess nitrogen is fatal to the root system of the plant.

An excess of nitrogen promotes wilt diseases by providing better nourishment for the vascular parasites. Ammonium salts are usually more readily used by the parasite so they are more damaging than nitrates unless the latter are reduced to nitrites, which are poisonous to plants. A deficiency of potassium, which automatically creates an excess of nitrogen and carbohydrates in the plant, also increases wilting by most vascular parasites.

A deficiency of potassium increases the severity of many diseases. Potash fertilizers have alleviated damage from more diseases than any other nutritional treatment. The importance of potassium has not been explained, but it is probably due to its ability to regulate chemical reactions in the cells of the plant. A deficiency of potassium under most circumstances implies an excess of nitrates and phosphorus; thinner cell walls in epidermal tissues; reduced production of amino acids because nitrate reduction is suppressed; accumulation of carbohydrates which cannot be synthesized into proteins; failure to produce new cells for want of essential amino acids for the protoplasm; and slower growth of meristematic tissues that would permit replacement of diseased tissues.

These changes may facilitate penetration of the epidermis by plant parasites, increase their metabolism and growth in plant tissues, or promote destruction of the entire plant because it cannot develop new tissue to replace those lost by ravages of the pathogens.

The only diseases that are consistently suppressed by potassium deficiency are the galls and overgrowths that depend upon multiplication of cells. Most of the overgrowths increase conspicuously as the potassium supply is increased while wilts and leaf diseases diminish.

Soil reaction and organic matter content of soils must be considered as regulants of soil fertility because they affect the availability of the nutrient elements. Such materials as the phosphates, calcium, iron, manganese, and boron are not available to plants in alkaline soils since they may be precipitated as insoluble salts. Applications of organic matter contribute nutrients, mostly nitrogen and potassium, but the immediate effect is to remove all surplus available nitrogen by combining with it or by promoting growth of soil micro-organisms that assimilate nitrogen to the point of temporarily depleting the soil supply. The effect on diseases may be good or bad, depending upon the type of disease and the stage of its development.

Soil reaction and organic matter also influence the prevalence of plant pathogens. Some pathogens prefer alkaline soils and others thrive in acid soils so no general rule can be laid down. If soils are extremely acid or alkaline soil micro-organisms may be eliminated. Organic matter is essential for growth of most soil inhabitants irrespective of whether they are beneficial or destructive.

Many plant parasites increase and are disseminated on organic matter, such as plant refuse, manures, and compost. The addition of organic matter to soil, however, often suppresses pathogens if they are poor soil invaders. The organic matter stimulates growth of soil saprophytes that deprive the less aggressive pathogens of mineral nutrients or else they secrete toxic antibiotics. Only a few plant disease agents are affected appreciably, since most of them are effective soil inhabitants and compete successfully with the saprophytic micro-organisms.

Probably the greatest benefits derived from organic matter are in soil stabilization. The humus from lignified tissues improves the physical structure of soil so it does not erode and will hold more moisture. Application of barnyard manure, straw, crop residues, or even carbohydrates such as starch and sugar stimulates the soil microorganisms to growth so they often synthesize all available nitrates into their bodies. This removes surplus nitrates from the soil; later they are released gradually so that the plants have a more continuous supply of nitrogen; excessive concentrations, which predispose them to so many diseases, are avoided.

This ability of organic matter to stabilize the soil solution and make the soil into a desirable medium for root growth fully justifies its use. The material may not have any unusual properties that would make plants resistant to all diseases. It can and does increase the severity of some diseases, particularly when applied to soils at the wrong time, but it can also be used to advantage for other diseases. There is no evidence that it encourages soil bacteria and fungi to produce antibiotics that are taken into the plant and help immunize it. As a matter of fact, there is evidence that some antibiotics are inactivated when added to soil.

No general rules can be laid down about fertilizing soils so as to avoid disease. Each disease must be considered by itself.

Any sound recommendation must be based on the type of soil, the availability of essential nutrients in the soil, and the character of the disease agents that are most likely to strike plants in the area. Conspicuous soil deficiencies, particularly in potash, should be corrected.

Every effort should be made to avoid surpluses of nitrogen that are not needed for steady and strong growth of the plant. Fertilizers may often be used to advantage in controlling acidity so the soil reaction will be favorable to the crop and unsuited to maximum development of the plant pathogens.

GEORGE L. McNew is managing director of the Boyce Thompson Institute for Plant Research at Yonkers, N. Y. Before taking that position in 1949, he was professor and head of the botany department at Iowa State College and manager of agricultural chemical research and development of the United States Rubber Company at Bethany, Conn. Earlier he conducted research on vegetable crop diseases at the New York Agricultural Experiment Station, Geneva, N. Y., and on the fundamental nature of parasitism in bacterial plant pathogens at the Rockefeller Institute for Medical Research at Princeton, N. J.

Diseased root of cabbage plant.