I wish I could draw a clear-cut, one-solution picture of plant diseases; if only organocultists were right and all soil-borne diseases of plants-caused by bacteria, viruses and fungi-could be eliminated simply by not using chemical (inorganic) fertilizers 1 Unfortunately, the answer is not that simple.
Feeding practices do have definite effects on many diseases- sometimes causing them and sometimes preventing them. In the U.S.D.A. Yearbook of Agriculture for 1953, Dr. George L. McNew cites a most interesting example of confused effects. He describes wheat grown on moderately fertile soil and fed with an extra supply of nitrogen. This special feeding helped the wheat escape seedling diseases but made it more susceptible to Pythium root rot; protected it against "take all" disease but made the plants more subject to leaf rust and mildew. If, instead of an extra supply of nitrogen, extra phosphorus and potash had been used, a different series of effects and diseases would have resulted.
An application of manure to a poor soil in Arkansas might protect cotton against wilt by supplying nitrogen and potash, yet this same application of manure made on nitrogen-rich soil in the Egyptian Delta of the Nile would encourage wilt in cotton.
A LOT OF DIFFERENCE
A similar case of "different places-different diseases" is that of the use of sewerage sludge in California to cure a certain turf disease. But in my own work in Illinois, the snow mold disease was far more prevalent in turf fed with this same organic fertilizer.
Potato scab, a soil-borne disease, is much more severe in alkaline
soils. It can be prevented by the use of fertilizers and soil treatments that bring pH so low that the scab organism cannot grow. The reverse is true of wilt and club root diseases of cabbage, which is made worse by acid soil; the use of alkaline materials helps control these diseases. (For a more complete discussion of diseases, see Plant Disease Handbook by Cynthia Westcott, published by D. Van Nostrand Co.)
At the U.S.D.A.'s research station in Beltsville, Maryland, Drs. McClelland and Stuart proved that gladiolus diseases were much more severe when organic fertilizers were used than when only chemical plant foods were used. To cite an opposite, the use of organic matter around pineapple plants in Hawaii saved them from injury by a soil fungus. The plowed-in organic matter (various waste products from sugar refining) stimulated organisms that used up excess nitrogen, thus preventing Pythium fungus from propagating rapidly.