Kurt J. Leonard
U.S. Department of Agriculture, Agricultural Research Service
Cereal Disease Laboratory, University of Minnesota, St. Paul, 55108
Differential varieties distinguish pathotypes (races) by their qualitative differences in reactions to different pathogen strains. In the simplest case showing unequivocal specificity of virulence, variety A is resistant to pathotype 1 but susceptible to pathotype 2, whereas variety B is susceptible to pathotype 1 but resistant to pathotype 2. In this example, A and B are differential varieties.
The first example of this type of differential reactions was reported by Barrus (1911), although he did not use the term ‘differential variety’ in developing his concept that varieties with different reaction types can be used to distinguish pathotypes. As a class exercise, graduate students at Cornell University inoculated a series of bean varieties with Collectotrichum lindemuthianum. To their surprise, several varieties that had been resistant in previous tests, were susceptible to the strain of C. lindemuthianum that they used. Furthermore, two varieties that previously had been susceptible, were resistant to their new strain of C. lindemuthianum. Barrus confirmed their results and speculated that “various strains of other fungi may differ in their power to infect the host plant and that certain plants now heralded as immune may be susceptible if attacked by a strain of the fungus from another source” (Barrus, 1911).
Levine and Stakman (1918) first used the term ‘differential host’ in identifying pathotypes of Puccinia graminis f. sp. tritici, cause of wheat stem rust. Their set of differentials included varieties of hexaploid bread wheat, Triticum aestivum; tetraploid durum, T. turgidum; and diploid einkorn, T. monococcum. Mains and Jackson (1926) used a set of 11 varieties of the single species T. aestivum to identify pathotypes of wheat leaf rust (P. triticina). Mains and Jackson used the term ‘differential variety’, but apparently they preferred the term ‘differential strain’ because many wheat varieties at that time were not genetically homogeneous. Over time with greater emphasis on varietal purity in crops, the term ‘differential variety’ became generally accepted and ‘differential strain’ disappeared from the literature.
From the beginning, it was clear that knowledge of the pathotypes in pathogen populations was essential for developing disease resistance effective over diverse locations and years. Uniform sets of differential varieties were established to compare pathotypes internationally both to predict effectiveness of new forms of resistance and to track long distance movement of pathogen populations. With increased knowledge of the genetics of resistance and virulence, differential varieties began to be used to monitor changes in virulence gene frequencies in pathogen populations. Flor’s (1955) discovery of the gene-for-gene relationship between host resistance and pathogen virulence showed that virulence genotypes of pathotypes can be inferred when resistance genotypes were known for each differential variety. Person (1959) expanded on Flor’s analysis by demonstrating that maximum numbers of pathotypes that can distinguished when each differential variety has a different single gene for resistance.
Roelfs and Martens (1988) established an international standard set of 12 differential varieties, each with a different single resistance gene, to replace Stakman’s wheat stem rust differentials, because Stakman’s set included several varieties with multiple resistance genes. Roelfs and Martens selected their differential varieties for clear distinctions between high and low infection types, and they changed Stakman’s pathotype nomenclature to a dichotomous system by reducing infection type choices to either high (susceptible) or low (resistant) instead of the three choices of high, low, or mesothetic reaction types in the Stakman system. The dichotomous system has been adopted generally in setting up differential varieties and designating pathotypes for other plant pathogens. Roelfs and Martens’ use of supplemental differential varieties in addition to the standard set also has become common practice with other pathogens when additional information is desired for specific situations.
Although any number of differentials may be used, standard sets generally include about 8 to 16 differential varieties. Larger sets become unwieldy for routine use. The best sets of differential varieties, such as for barley powdery mildew (Blumeria graminis f. sp. hordei), have differentials with different single resistance genes backcrossed into a common genetic background (Køster, et al., 1986). Near-isogenic differentials eliminate confounding effects of modifier genes acting on resistance in different host backgrounds.
The utility of differential varieties is most obvious in supporting disease resistance breeding in cultivated crops, but differential varieties also are used to study coevolution of natural host pathogen systems. For example, Harry and Clarke (1986) established a set of differential varieties of the wild species Senecio vulgaris to demonstrate the great diversity of powdery mildew (Erisyphe fischeri) pathotypes present in natural host-pathogen systems.
Barrus, M.F. 1911. Variation of varieties of beans in their susceptibility to anthracnose. Phytopathology 1:190-195.
Flor, H.H. 1955. Host-parasite interactions in flax rust - its genetics and other implications. Phytopathology 45:680-685.
Harry, J.B., and D.D. Clark. 1986. Race-specific resistance in groundsel (Senecio vulgaris) to the powdery mildew Erisyphe fischeri. New Phytologist 103:176-175.
Køster, P., L. Munk, O. Stølen, and J. Løhde. 1986. Near-isogenic barley lines with genes for resistance to powdery mildew. Crop Sci. 26:903-907.
Levine, M.N., and E.C. Stakman. 1918. A third biologic form of Puccinia graminis on wheat. J. Agric. Res. 13:651-654.
Mains, E.B., and Jackson, H.S. 1926. Physiologic specialization in the leaf rust of wheat, Puccinia triticina Erikss. Phytopathology 16:89-119.
Person, C. 1959. Gene-for-gene relationships in host:parasite systems. Can. J. Bot. 37:1101 1130.
Roelfs, A.P., and J.W. Martens. 1988. An international system of nomenclature for Puccinia graminis f. sp. tritici. Phytopathology 78:526-533.