The Mode of Inheritance of Factors for Resistance

It is difficult to generalize about the mode of inheritance of factors for resistance except to say that determinations must be made in each specific case. In his pioneer studies, Biffen (1907) demonstrated that resistance to yellow rust of wheat was inherited as a simple recessive.

It was at first thought that this mode of inheritance might be universal, but it is not. The immunity of Kanred wheat from a considerable number of races of wheat stem rust is due to a single dominant gene. In other cases of resistance to stem rust two or more factor pairs are involved;

in still others there are multiple factors, some major, some minor. The mode of inheritance, then, within one kind of crop plant to one kind of pathogen may differ greatly in different crosses.

Almost every type of genie relationship has been reported in connec-tion with the genetics of bunt resistance in wheat (Fischer and Holton, 1957). There are many physiologic races of the pathogen and many wheat varieties have been crossed in attempts to checkmate them. In the many crosses between resistant and susceptible varieties, many factors apparently are involved. In some cases a single factor was involved, with or without modifiers; in others there are two, three, or more factors for resistance. Multiple factors have been recorded in many cases. Factors may be strong or weak, linked or independent, effective against a single race or against several. There may be inhibitors of factors for resistance to certain smut races but not to others. In resistant X resistant crosses, there may, of course be no segregation; in some, there is transgressive segregation.

Generalization regarding mode of inheritance cannot be applied to specific cases. Each must be considered separately. It can only be said, therefore, that the number of genes for resistance varies greatly with the crop plant and with the pathogen, and the way in which they are inherited varies greatly also.

Naturally there are many factors for virulence in species of pathogens that comprise many physiologic races. The final outcome of breeding for disease resistance will be determined by the ability of breeders to find and combine genes for resistance that counteract the genes for virulence in pathogens. As different genes for resistance are often dis-persed among many varieties of crop plants and those for virulence often are widely dispersed among many physiologic races of pathogens, it is

not an easy task to prognosticate the outcome of many breeding pro­

grams. Information is continually being accumulated about the genes for resistance, and some has been accumulated about the genes for virulence, but much more is needed.

Much more needs to be known about the genetics of pathogens. The writers prepared a brief summary in 1953 (Stakman and Christensen, 1953). Flor (1954, 1956) has made an especially comprehensive study of virulence in flax rust, Melampsora lini, and of resistance in flax. Flor suggested the presence in flax of a gene for susceptibility or resistance to match each gene for virulence or avirulence in the pathogen. He obtained 30 lines of flax, each apparently homozygous for an individual gene that governs reaction to rust. By using 18 of the lines as differential varieties, it was possible to distinguish 239 races of rust. In crosses between races of rust it appeared that avirulence was dominant over virulence and that from one to three factor pairs were involved. In crosses between resistant and susceptible varieties of flax, resistance was dominant and one to three factor pairs were involved. According to Flor, varieties of flax that carry " L L " genes are resistant to all North American races of rust. More work of this kind is needed.

V. F U T U R E PROSPECTS

Progress in the future should be more rapid and more nearly permanent than in the past. Many breeding programs must be inter­

national in scope to be successful, simply because different genes for resistance and for virulence may exist in different parts of the world.

Varieties with different genes for resistance must be assembled and then tested extensively in different geographical areas and intensively under optimum conditions for disease development. Breadth of experimentation must be combined with intensity of research. Facilities for both types of effort are being improved, but they must be expanded. There must be facilities for more travel in order that all possible information may be obtained from the international disease nurseries that are now grow­

ing in number. Large phytotrons are needed at strategic locations so that the most promising plant materials may be tested under necessary combinations of environmental conditions. Appropriate elimination plots are needed where plant materials can be assembled and exposed to infection by a world-wide collection of pathogens. There must be much more intensive research on the genetics, ecology, and physiology of hosts and pathogens and on the nature and variability of disease resistance.

These things are prerequisite to most rapid and assured progress. In the meantime progress is being made.

It is possible that attitudes must be changed. It is entirely possible

that a succession of varieties may have to be produced to control some diseases, as there can be no assurance that new pathogens or new races of old ones may not appear periodically to disrupt the status quo. The important thing is to anticipate the changes and prepare to meet them.

Even if permanent immunity is not attainable, practical field resistance or tolerance may reduce the destructiveness of many diseases. Quality may sometimes have to be sacrificed in order to attain greater resistance and productivity, especially when the standard of quality is not based on real values.

If the efforts to produce resistant varieties are really intelligent, efficiently systematic, and world-wide in scope, there is great promise for the future. Above all, the lessons of the past must not be forgotten and many new ones must be learned in the future. Progress in scientific attitudes and attainments can guarantee progress in the production of resistant varieties.

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In document The Problem of Breeding Resistant Varieties (Pldal 53-58)