A Test of Taxonomic Predictivity: Resistance to White Mold in Wild Relatives of Cultivated Potato

Amajor justification for taxonomic research is its assumed ability to predict the presence of traits in a group for which the trait has been observed in a representative subset of the group. Taxonomy is regularly used by breeders interested in choosing potential sources of diseaseresistant germplasm for cultivar improvement. We designed this study as an empirical test of prediction by associating resistance to white mold [caused by the fungal pathogen Sclerotinia sclerotiorum (Lib.) de Bary] to diverse potato (Solanum spp.) taxonomies and biogeography, using 144 accessions of 34 wild relatives of potato in Solanum sections Petota and Etuberosum. Tremendous variation for resistance to white mold occurs both within and among species. No consistent association was observed between white mold resistance and taxonomic series (based on a phenetic concept), clades (based on a cladistic concept), ploidy, breeding system, geographic distance, or climate parameters. Species and individual accessions with high proportions of whitemold-resistant plants have been identified in this study, but both often exhibit extensive variation and designation of either as resistant or susceptible must take this variation into account. Therefore, taxonomic relationships and ecogeographic data cannot be reliably used to predict where additional sources of white mold resistance genes will be found. Aprimary functionof classification is to construct classes about which we can make inductive generalizations (Gilmour, 1951). One of the greatest assets of a sound classification is its predictive value (Mayr, 1969). TAXONOMY HAS LONG STRIVED to construct predictive classifications (Vavilov, 1922; Gilmour, 1951; Michener, 1963; Rollins, 1965; Warburton, 1967; Mayr, 1969; Sokal, 1985; Stuessy, 1990; Daly et al., 2001). Warburton (1967) stated this goal clearly as, “[Prediction] means that one can describe a trait as characteristic of all members of a taxon before it has been verified for all. It also means that if organisms have been classified together as a taxon because they have all been found to share certain traits, they will later be found to share other traits as well.” For example, plant breeders use taxonomy to make their initial choice (or avoidance) of related (or unrelated) germplasm based on such statements as Species X is resistant to a particular disease, by choosing other accessions of this or related species. Clearly, not all accessions of a species share traits, but lacking prior evaluation data, taxonomy provides a useful guide to make inferences on unevaluated accessions, based on knowledge of a limited subset of the group. The idea that traits should be associated with related organisms is perhaps universally accepted. As stated by Warburton (1967), “[This idea] is so deeply ingrained in the common sense of biologists that it sounds strange when formally stated.” This very utilitarian predictive component of taxonomy has long been used to justify taxonomy grant proposals, and cladograms serve as useful hypotheses to associate to a variety of traits as diverse as morphology, anatomy, ecology, development, disease epidemiology, and behavior (Harvey et al., 1966; Baum et al., 2005). While the association of traits to taxonomy is only as reliable as the taxonomy, revised molecularbased phylogenetic results are uncovering previously unexpected associations and have shown tremendous economically important discoveries of use to society. For example, Daly et al. (2001) summarize the enhanced predictive component of new phylogenies to synthesis of a nonprotein amino acid, glucosinolate production, N fixation, and taxol biosynthesis. Adams et al. (2001) show associations to new phylogenies and the loss of a telomere repeat in the plant order Asparagales. Crandall (1999) shows the use of an AIDS phylogeny to detect viral recombination and to identify modes of transmission. The present study is the first test of the association of potato taxonomy todisease resistance (here, to the fungal disease white mold) and, to our knowledge, the first experiment designed to test predictionwithout prior knowledge of traits. For example, while the studies mentioned above clearly demonstrate the enhanced value of molecular phylogenies and prediction, they do not mention the many instances surely overlooked where prediction does not occur. All of the above studies (except the AIDS example) show associations at the ordinal level, and the present study investigates associations at a much lower taxonomic level (sections Petota and Etuberosum of Solanum). Perhaps the predictive power of taxonomy varies with traits or taxonomic levels. While white mold is not a widespread problem in potato, it can be a serious disease in certain environments. It is becoming more important in production systems with high fertility regimes and sprinkler irrigation, where dense foliage can remain wet long enough for the fungus to become established (Powelson, 2001). White mold is a serious and widespread disease in soybean [Glycine max (L.) merr.] (Kim et al., 2000), so it is especially prevalent in potato in regions where soybean is grown as a rotation crop. While white mold is controlled to some extent with frequent fungicide applicaS.H. Jansky and D.M. Spooner, USDA, Agricultural Research Service, Dep. of Horticulture, Univ. of Wisconsin, 1575 Linden Drive, Madison, WI 53706-1590 USA; R. Simon, International Potato Center, P.O. Box 1558, La Molina, Lima 12, Peru. Received 8 Dec. 2005. *Corresponding author ([email protected]). Published in Crop Sci. 46:2561–2570 (2006). Plant Genetic Resources doi:10.2135/cropsci2005.12.0461 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: PDA, potato dextrose agar; PET, potential evapotranspiration; QTL, quantitative trait loci. R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 2561 Published online November 21, 2006