One more aspect: The utilization of disease resistance is not merely the stopping of leaks here and there in an otherwise profitable agriculture. The disease situation with our crops very frequently is serious and, almost without exception, control measures add to farming costs. The farmer, as he pays for fungicides and their application and as he employs other disease-control measures such as sterilization of seed and soil pays a heavy impost to the plant pathogens. Control of disease by use of a disease-resistant variety has been described as the "painless method" that does not levy on the farmer's pocketbook except as he has to pay for the care and harvest of a larger crop. As increasing crop production costs tend to make more and more crops marginal, the lessened expense for disease control may mean the difference between profit and loss from the farming business.
With some crops, notably the sugar plants, cereals, and potatoes, disease-resistant varieties may spell the difference between success and failure. Without the kinds resistant to curly top, western United States would have abandoned the culture of sugar beets, and this keystone crop would have been lost to irrigation agriculture. Without resistance to mosaic and root rot, sugarcane culture would have disappeared in the Sugar Belt of the South. The citrus industry must depend on trees reworked on resistant stocks if tristeza should develop in the United States at all comparably to its advance in Argentina, Brazil, and other countries. Wheat production has been threatened by a new race of the black stem rust, 15B, and new resistant varieties are imperatively needed.
The contribution of a disease-resistant variety includes other things. .Improvement in yield and quality usually has accompanied the resistant variety as a result of better and more nearly normal growth or as intrinsic improvement, irrespective of the factors of resistance. The farmer gives better culture to a crop that shows promise. Certainly the varieties that have gained ready acceptance and have moved into position of standards in our agriculture such as Washington asparagus; Wisconsin yellows-resistant cabbage; the numerous wheat varieties; Bond, Victoria, and other disease-resistant oats: Michigan golden celery; the Robust bean; mosaic and root rot resistant C. P. sugarcane; U. S. 22/3 sugar beet, which is resistant to curly top; U. S. 215 x 216/3 and U. S. 216 x 226, the varieties of sugar beet that are resistant to leaf spot; Katandin and Chippewa potatoes; and mildew-resistant cantaloups 40 (to cite only a few) all have had improved commercial quality and high capacities for yield, along with disease resistance. With the resistant varieties, even if the crop is somewhat reduced under epidemic conditions, there is certainly something to harvest which offsets labor costs of production a striking contrast to crops of some of the old varieties that were not worth harvesting at all. Those intangible contributions stability in rotation systems, permanence in the agricultural program of an area, and the increased security that comes from lessening of hazards in crop production; permit the farmer to plan with greater confidence.
ALMOST ALL of our cultivated plants trace back to primitive man. During the ages, as people wrested the plants from nature and conserved them, they must have improved them whenever disease outbreak retarded or eliminated the less resistant ones.
The early experiences in which resistant host plants were found in regions where a given disease or insect pest is endemic have shaped our thinking and forced recognition of the first and fundamental principle in breeding for disease resistance, namely, that where host and parasite are long associated, then in the evolutionary process resistant forms are developed by natural selection.
Conversely, when an introduced parasite enters a new environment and finds new host plants, conditions may be conducive to its growth and spread; above all, the pathogen may find a host plant in which no resistance has ever been developed. Such plants are attacked with great virulence. Many of our serious outbreaks of plant disease trace to the introduction of parasites to which our nonresistant crop plants immediately succumb. Faced with such emergencies, it is almost axiomatic to apply this basic concept about host and parasite relations that came to light 75 years ago when grape culture in Europe was threatened with extinction.
WILLIAM A. ORTON inaugurated in the Department of Agriculture breeding for disease resistance as an effective and practical means of disease control. His investigations met the specific disease-control problems and gave the guiding principles to this branch of plant pathology. When Erwin F. Smith, famed plant pathologist and bacteriologist of the Department, had completed his studies on the pathology of the fusarium wilts of cotton, watermelon, and cowpea, he assigned to young Orton fresh from the University of Vermont the job of developing control measures. Each disease problem was solved by the application of disease resistance, but each crop required a different approach. To combat the wilt of cowpea, Orton utilized the natural resistance of an existing variety after his comparative tests on infested soil had shown the "Iron" cowpea not only resistant to wilt but nearly immune to the root knot nematode.
Against cotton wilt, Orton employed methods that are operative today in all attempts to improve cotton varieties the selection within desirable strains of individuals that survive under conditions of drastic exposure and the proving of the selections by subsequent tests of the progeny. Cooperating closely with growers and a practical cotton breeder E. L. Rivers, who had started some selections in 1895 at Centerville, S. C. Orton centered his attention on fields of highest infestation with the fusarium wilt fungus. He subjected to further tests the progenies from the individual plants that were selected, because he soon learned that mass selection alone was not effective. In less than to years he produced many highly resistant varieties Rivers, Centerville, Dillon, and Dixie varieties, each, in its day, a successful introduction that grew well where ordinary types failed and each a contributor of germ plasm for the use of cotton breeders to improve their varieties further.
In developing wilt-resistant watermelons, Orton had to go beyond just selection and progeny testing. Failing to find resistance in edible varieties of watermelon, he turned to the highly resistant citron melon, used only for feeding livestock, and incorporated genes for resistance from it into the "Eden" watermelon. Thus he synthesized a disease-resistant variety by hybridization. Those investigations antedated the rediscovery of Mendel's law, whose disciplines would have been extremely useful to the young scientist as he selected for desirable characteristics from the segregates in the F, generation. By further selection, Orton obtained the wilt-resistant watermelon, Conqueror, capable of giving good crops despite disease. In repeated tests in later years, Conqueror retained its qualities, and the problem apparently was solved, except, as Orton quaintly said, "styles in watermelons changed." Market demand for long melons of the Tom Watson type made the round type of melon unwanted. But the scientific contribution was there, and the genes from Conqueror still are used in research.
H. L. Bolley, another pioneer in plant pathological research, shares with Orton the distinction of bringing to the fore the possibility of meeting serious disease problems by resistance breeding. Bolley discovered in 1900 that flax wilt, most serious of all flax diseases, was caused by a soil-infesting fungus, Fusarium lini. Bolley added a new concept to plant pathology, namely, that of flax-sick soil that is, soil infested with Fusarium lini. Bolley extended this concept of soil infestation to apply to other crops. He pointed out that greatly reduced yields below those of virgin soils and the so-called "running out of soil" might have a biological explanation. He used experimental plots, notably famed Plot 30, that were highly infested with Fusarium lini to develop resistant flax varieties: North Dakota Resistant 52, North Dakota Resistant 114, and Buda, and later, with O. A. Heggeness, produced the variety Bison, which is still in wide commercial use.
The investigations of Lewis R. Jones and his students, J. C. Gilman, J. C. Walker, and W. B. Tisdale, at the Wisconsin Agricultural Experiment Station, in breeding yellows-resistant cabbage firmly established disease-resistance breeding as a control measure. This wilt disease, caused by Fusarium conglutinans, had practically ruined cabbage production in the rich bottom lands near Racine, Wis.
Building on the work of Orton and Bolley, the Wisconsin investigators selected in 1910 some individual plants from fields sustaining an almost complete loss. Only a few of the remaining plants produced heads. The plants to serve as seed bearers were critically selected from them and brought to seed. Then the individual progenies were tested on highly infested soil. By this technique, yellows-resistant strains of Hollander type were obtained. Later, Walker and his associates produced yellows-resistant strains of other standard cabbage varieties. The results with the resistant cabbages were dramatic. The new strains gave almost a full crop where the commercial type failed. Before the work was concluded, resistant types, equivalent in quality and productiveness to any nonresistant types previously grown, were made available to growers. This important vegetable crop was saved not alone for Wisconsin but for many other States where the disease had been introduced.
The work in Wisconsin stressed the influence of environment upon the parasitism of Fusarium. Soil temperature particularly was found to be highly significant in determining the ability of the fungus to attack and of the host to withstand the parasite. Here we have the beginning of the concept that disease resistance needs to be defined not alone in terms of the organism-host relation, but as well in terms that include the environmental conditions as they influence both host and parasite.
The work of W. H. Tisdale at the Wisconsin Agricultural Experiment Station with flax wilt developed that idea further. He showed why some varieties of flax resisted Fusarium ling only under certain conditions. Temperature relations determined the infecting powers of the fungus. Wilt-susceptible flax grown with soil temperatures below 60 F. escaped disease; grown at 68 F., it was completely susceptible.
By his experiments, conducted on a field that had grown flax continuously for a decade, H. D. Barker at the Minnesota Agricultural Experiment Station clarified a confused situation with respect to disease resistance by showing that resistance depends on the genotype and is not something acquired by mere association of host and parasite. The high incidence of infection in an experimental field at St. Paul, Minn., augmented by inoculation of the soil with pure cultures of the flax wilt Fusarium, allowed him to obtain clear-cut reactions with the resistant varieties then available. He showed that lines may be selected that breed true for resistance and that some lines are heterozygous. In his work we find the first research that indicated that the flax fungus itself breaks up into strains.