Growing healthier plants

The Nature of Resistance to Disease

S. A. Wingard.

We can get a double economic gain if we can introduce into our agriculture crops that are naturally immune to diseases and thereby avoid both the loss from disease and the cost of sprays and other ways of combating disease.

To do that as W. A. Orton pointed out in 1908 one has to know the problems of heredity, the nature of a disease, its governing factors, and the type of resistance involved in order to adopt the most promising lines of approach in breeding.

Disease resistance in plants is not an easy subject to understand. It involves the intricate relations between the plant the host that is being attacked and the fungus or bacterial organism the parasite; that is doing the attacking.

The terms "disease resistance" and "immunity" can be used to denote different degrees of the same thing. Various degrees of disease resistance are possible. Immunity means complete resistance to disease; immune means not subject to attack by a pathogenic organism or virus.

We might consider other definitions here at the start: A pathogen is a parasitic organism or virus whose activity causes disease in the host. The host is a living organism that harbors another organism or virus that depends on it for existence. Pathogenic, the adjective, means having the ability to induce disease. A parasite is an organism or virus that lives on the tissues of another living organism. To inoculate is to introduce a micro-organism or virus or a material containing either into an organism, a culture medium, soil, or something like them. To infect is to invade an organism and to bring about infection. A suscept is an organism that is affected or can be affected by a given disease. Resistance is the ability of a plant to withstand, oppose, lessen, or overcome the attack of a pathogen. Susceptibility is the inability of a plant to defend itself against an organism or to overcome the effects of invasion by a pathogenic organism or virus.

Immunity is absolute. Resistance and susceptibility are relative: A plant is either immune or not immune to a pathogen, but it may be more or less susceptible or resistant. A plant may be "slightly susceptible," "moderately resistant," or "extremely susceptible" but not "moderately immune" or "highly immune."

The ability of a susceptible plant to avoid infection because it possesses some quality (such as earliness of maturity) that prevents successful inoculation is called escape, or klendusity. Escape must be clearly distinguished from resistance. Tolerance is the ability of a plant to endure the invasion of a pathogen without showing many symptoms or much damage. A degree of resistance great enough so that no serious economic loss results (although there might be considerable invasion by the pathogen) is termed "practical resistance." Certain varieties of wheat, for example, for practical purposes are resistant to leaf rust the rust causes little loss although the plants may become heavily rusted as they approach maturity.

We use the term hypersensitiveness to denote such a violent reaction of a plant to an attack by an obligate parasite (a pathogen that depends on living tissue for its nutrition) that the invaded tissues of the host are quickly killed, so that there is no further spread of infection. In essence, hypersensitiveness is extreme susceptibility, but its practical effect, as far as crop loss is concerned, amounts to extreme resistance. This type of reaction is common in cases of infection of many plants by rust fungi and some of the viruses.

The definitions help us avoid some wrong ideas about the nature of resistance and help us understand what can and cannot be done in plant breeding. For instance, a variety that escapes a particular disease is not necessarily a resistant variety.

William A. Orton long ago pointed out the differences between what he termed disease-escaping, disease-enduring, disease-resisting, and immune varieties.

He said: "Disease endurance sometimes results from the ability of the plant to grow in spite of an attack, either through exceptional vigor or through a hardier structure, as in the case of certain melons which better survive the attacks of leaf-blight because the leaves do not dry out as quickly as do those of the ordinary melons. Drought-resistant plants are often disease-enduring. Watermelons from semiarid Russia were for this reason the last to succumb to the wilt disease when planted in our Southern States.

"Finally, we have disease-escaping varieties. Such, for example, are the extra early cowpeas which mature before the season for wilt and root-knot to develop. These varieties which escape disease through earliness or lateness are often really very susceptible. The Early Ohio and other early potatoes, which commonly mature before the appearance of the late blight disease, are among the first to succumb to this disease if planted so late as to be still immature when the moist weather of the late summer or early fall enables late blight to spread."

E. M. Freeman, a professor in the University of Minnesota and one of the pioneers in this subject, pointed out that little difference exists in the real resistance powers of oat varieties to the common oat rusts, and when a grower is told that an oat variety is resistant because it usually escapes the rust through earliness of ripening, he Is led into a serious mistake. The essential character of true resistance lies in a protoplasmic activity and is independent of inoculation accidents.

A variety may escape a disease through certain peculiarities of the host. To prove that, Dr. Freeman grew a variety of barley in different soils, which varied from the normal garden soil to those containing about 2 percent of alkaline salts. Plants in the different soils, inoculated by sprays in the greenhouse, showed different amounts of rust. Those in the stronger alkaline soils generally showed less rust. The latter, however, when attacked outside the greenhouse, exhibited large and vigorous growths of the rust. That was undoubtedly due to the greater development of "bloom" on the barley foliage when it was grown in the strong alkali; the bloom caused the drops of water to roll off and the inoculating material to be lost. The bluish color of the plants in the alkaline soils and the greater tendency of the water drops to run off were quite pronounced. There seems to be no reason for assuming that there was any difference in real resistance to rust.

The economic importance of disease-enduring varieties ought not to be overlooked by pathologists and plant breeders. Effective work in selection and breeding may be accomplished in the production of disease-enduring plants. Some of the best varieties of the hard spring wheats commonly used in the North Central States, for example, come under this class in regard to stem rust, since they do not possess any appreciable amount of resistance. They can endure moderate attacks of rust, but all go down in a heavy epidemic of rust. No sharp distinction may exist between disease-enduring and disease-resistant varieties, but for practical purposes well-marked resistance can be detected readily under proper experimental conditions.

Immune varieties have perfect resistance. Their production will probably always remain rare in comparison with those possessing only a partial resistance or high powers of endurance.