Breeding for Resistance to Disease

George H. Coons.

In the botanical garden of the National School of Agriculture at Montpellier, France, stands a statue dedicated to Professor G. Foex, commemorating his success in saving the grape industry of France by resistant varieties from America. It symbolizes the spirit and aim behind man's effort to breed healthier plants.

The figure of the weak, old man in the statue represents the grape industry of southern France, about to die. Supporting and sustaining the man is a sturdy, young woman, America.

The statue commemorates the modern application of disease resistance as a control measure that began about 1870. The story of it has great meaning for us today.

Attempts to control powdery mildew of grape in Europe by introducing rooted American vines for breeding purposes had backfired because the American grapes had carried with them the grape root aphis, or phylloxera. The insect, native of United States, did no great damage to our wild and cultivated grapes because it and its host had reached an equilibrium over the centuries. Introduced into Europe some time before 186o, the phylloxera found the vinifera types of grape highly susceptible. Soon the insect spread throughout the entire grape-growing area.

When the vines were first found dying under attacks of the root aphis, the American varieties were remarkable in escaping injury. Attempts were then made to substitute the best American varieties to replace the dead vines. That was not satisfactory. Next, the old French vinifera varieties were grafted to the American varieties that had maintained vigorous growth despite the phylloxera. That procedure was more successful. It saved the grape industry until hybrids between the resistant or nearly immune American sorts and the French varieties could be bred to furnish better adapted, resistant stocks on which to graft the vinifera scions.

Powdery mildew, phylloxera, and downy mildew, introductions from the United States, revolutionized methods of grape culture in Europe. Some of the resistant stocks that were developed to meet the crisis still are being used wherever vinifera types are grown.

All that the statue at Montpellier tells. And more: It underscores the international aspects of the research program that seeks to control plant diseases by developing disease-resistant varieties.

That program has many facets.

Men the world over have searched within all species of crop plants subject to serious diseases and their cousins for breeding materials that carry the hereditary factors for resistance.

They have made worldwide collections of germ plasm and comprehensive assemblages of varieties and strains of crop plants and their related species.

The crop specialists have sought the wild forebears of the cultivated sorts at the places where the species presumably originated. They have combed the countries where the particular diseases are endemic in the search for strains and individuals which, through operation of natural selection, might have come to bear factors for disease resistance.

Upper: Mineral deficiency disease caused the discoloration of the old corn leaves. A, a normal leaf. B, C, D, and E lack nitrogen, phosphorus, potassium, and magnesium, respectively. Lower: Leaves from a cotton plant showing symptoms of magnesium hunger. The leaves A to D represent progressively the reactions on old to new growth.

For experiments and tests of resistance, they have chosen sites where incidence of disease may be expected to be extremely high, the exposure severe and uniform, and other conditions conducive to efficient research.

All in all, then, breeding healthier plants is a long-time program whose many phases require diversified yet balanced, strong, consistent, and thoroughly coordinated efforts.

The research has to be international and national. It must also be regional in scope: Seasons have to be telescoped; tests of breeders' strains and selections under a range of disease exposures have to be conducted in many areas at the same time. Delays in producing seed and multiplying clones of desirable stocks must be avoided. Increases of seed for commercial purposes have to go forward promptly and efficiently.

Breeding for disease resistance has constituted for many years a major part of the Department of Agriculture's plant pathological program, in which the Bureau of Plant Industry, Soils, and Agricultural Engineering works in cooperation with State agricultural experiment stations, the seed growers, and farmers.

Outstanding results have come from the research. Among them are fundamental biological discoveries that apply not alone to plants and their reactions but shed light and afford new techniques for solution of problems of human and animal disease. They have added to our knowledge, especially to the concepts concerning man and his relations to his environment. They have had immediate practical significance. The disease-resistant varieties that have been developed and introduced have added greatly to our own wealth and to the wealth of nations.

An attempt was made in 1936 to appraise the contribution of disease-resistant varieties to American agriculture. The crop statistics and the dollar values of the 8-year average, 1928-1935, that were compiled then of Course do not apply to today's farm production and values. But at that time the contribution to farm value for 17 leading farm and vegetable crops was placed at 10 percent.

In the years since 1935, the degree Of disease resistance in the new varieties has been greatly increased, and many other disease problems have been put in line for solution by the introduction of superior types. The acreage of many crops has increased, and on the expanded acreage the disease-resistant varieties have gained wider and wider acceptance among growers. Almost revolutionary changes have occurred as old varieties have been replaced by new productions whose chief superiority derives from disease resistance. The acreage occupied by varieties for which it is proper to apply the term "disease-resistant" has moved from approximately 25 percent in 1935 to more than 50 percent. No small part of the increase derives from the almost total swing in many States to varieties of hybrid corn and to the popularity of newer kinds of wheat, oats, and potatoes. Very generally in the areas of greatest production the higher yielding, better adapted, and more disease-resistant sorts have replaced the old varieties.

If we want to put the benefit from disease resistance in terms of dollars, we may use the early, conservative estimate of 10 percent, which allows for seasons with lessened disease outbreak. But we must take into account the greater utilization of the superior sorts. We assume that about one-half of the acreage is concerned. Our total farm value of crops ranges from about 12 to 15 billion dollars annually. So we get a figure of 600 to 750 million dollars as the annual benefit that comes from the use of resistant varieties.