Testing Taxonomic Predictivity of Foliar and Tuber Resistance to Phytophthora infestans in Wild Relatives of Potato.

Potato late blight, caused by the oomycete phytopathogen Phytophthora infestans, is a devastating disease found in potato-growing regions worldwide. Long-term management strategies to control late blight include the incorporation of host resistance to predominant strains. However, due to rapid genetic changes within pathogen populations, rapid and recurring identification and integration of novel host resistance traits is necessary. Wild relatives of potato offer a rich source of desirable traits, including late blight resistance, but screening methods can be time intensive. We tested the ability of taxonomy, ploidy, crossing group, breeding system, and geography to predict the presence of foliar and tuber late blight resistance in wild Solanum spp. Significant variation for resistance to both tuber and foliar late blight was found within and among species but there was no discernable predictive power based on taxonomic series, clade, ploidy, breeding system, elevation, or geographic location. We observed a moderate but significant correlation between tuber and foliar resistance within species. Although previously uncharacterized sources of both foliar and tuber resistance were identified, our study does not support an assumption that taxonomic or geographic data can be used to predict sources of late blight resistance in wild Solanum spp.

Resistance to Alternaria solani in hybrids between a Solanum tuberosum haploid and S. raphanifolium.

Early blight of potato (Solanum tuberosum), caused by the foliar fungal pathogen Alternaria solani, is a major cause of economic loss in many potato-growing regions. Genetic resistance offers an opportunity to decrease fungicide usage while maintaining yield and quality. In this study, an early blight resistant clone of the diploid wild species S. raphanifolium was crossed as a male to a haploid (2n=2x) of cultivated potato. Hybrids were backcrossed to both parents. Eight families were created and evaluated for early blight resistance in the field. Families created by backcrossing to the wild species parent exhibited significantly lower relative area under the disease progress curve means than those from backcrossing to the cultivated parent, leading to the conclusion that S. raphanifolium contributes genes for early blight resistance. The mechanism of resistance in S. raphanifolium is unique because A. solani could not be recovered from lesions. Clones were identified with high levels of resistance and adaptation to the photoperiod of a temperate production region.

A test of taxonomic and biogeographic predictivity: resistance to Potato virus Y in wild relatives of the cultivated potato.

A major 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. Similarly, populations in similar environments are expected to be more alike than populations in divergent environments. Consequently, it is logical to assume that taxonomic relationships and biogeographical data have the power to predict the distribution of disease resistance phenotypes among plant species. The objective of this study was to test predictivity in a group of widely distributed wild potato species, based on hypotheses that closely related organisms (taxonomy) or organisms from similar environments (biogeography) share resistance to a simply inherited trait (Potato virus Y [PVY]). We found that wild potato species with an endosperm balance number (EBN) of 1 (a measure of cross compatibility) shared resistances to PVY more than species with different EBN values. However, a large amount of variation was found for resistance to PVY among and within species. We also found that populations from low elevations were more resistant than those from high elevations. Because PVY is vectored by aphids, we speculate that the distribution of aphids may determine the level of selection pressure for PVY resistance.

A test of taxonomic predictivity: resistance to the Colorado potato beetle in wild relatives of cultivated potato.

Wild relatives of potato offer a tremendous germplasm resource for breeders. Because the germplasm base of potato is so broad and diverse, we have undertaken a series of studies to determine whether we can predict the distribution of valuable genes in wild Solanum species based on taxonomic or biogeographic data. This is the third study in the series. Resistance to defoliation by Colorado potato beetle, Leptinotarsa decemlineata Say, larvae was evaluated in 156 accessions of 41 wild Solanum species. The highest frequencies of resistant accessions were found in diploid species with an endosperm balance number of 1. In contrast to previous studies on resistance to foliar fungal pathogens, there was little variability in defoliation scores among plants within an accession and among accessions within a species, at least for the most resistant species. There was no strong association of Colorado potato beetle resistance in wild potato species to biogeographic data. Resistance was confirmed in species previously characterized by high levels of glycoalkaloids or dense glandular trichomes. However, we have identified additional species with resistance to the Colorado potato beetle. Mechanisms of resistance are being studied in these species and attempts will be made to introgress them into the cultivated potato.

A test of taxonomic predictivity: resistance to early blight in wild relatives of cultivated potato.

Host plant resistance offers an attractive method of control for early blight (caused by the foliar fungus Alternaria solani), a widespread disease that appears annually in potato crops worldwide. We tested the assumed ability of taxonomy to predict the presence of early blight resistance genes in wild Solanum species for which resistance was observed in related species. We also tested associations to ploidy, crossing group, breeding system, and geography. As in a prior study of Sclerotinia sclerotiorum (white mold) resistance, tremendous variation for resistance to early blight was found to occur within and among species. There was no discernable relationship between the distribution of resistant phenotypes and taxonomic series (based on an intuitive interpretation of morphological data), clade (based on a cladistic analysis of plastid DNA data), ploidy, breeding system, geographic distance, or climate parameters. Species and individual accessions with high proportions of early blight resistant plants were identified, but high levels of inter- and intra-accession variability were observed. Consequently, the designation of species or accessions as resistant or susceptible must take this variation into account. This study calls into question the assumption that taxonomic or geographic data can be used to predict sources of early blight resistance in wild Solanum species.

Multiplex Real-Time Quantitative PCR to Detect and Quantify Verticillium dahliae Colonization in Potato Lines that Differ in Response to Verticillium Wilt.

ABSTRACT Potato early dying (PED), also known as Verticillium wilt, caused by Verticillium dahliae, is a seasonal yield-limiting disease of potato worldwide, and PED-resistant cultivars currently represent only a small percentage of potato production. In this study, we developed a real-time quantitative polymerase chain reaction (Q-PCR) approach to detect and quantify V. dahliae. The efficiency of the designed primer pair VertBt-F/VertBt-R, derived from the sequence of the beta-tubulin gene, was greater than 95% in monoplex Q-PCR and duplex (using Plexor technology) procedures with primers PotAct-F/PotAct-R, obtained from the sequence of the actin gene, designed for potato. As few as 148 fg of V. dahliae DNA were detected and quantified, which is equivalent to five nuclei. Q-PCR detected V. dahliae in naturally infected air-dried potato stems and fresh stems of inoculated plants. Spearman correlations indicated a high correlation (upward of 80%) between V. dahliae quantifications using Q-PCR and the currently used plating assays. Moreover, Q-PCR substantially reduced the variability compared with that observed in the plating assay, and allowed for the detection of V. dahliae in 10% of stem samples found to be pathogen free on the culture medium. The described Q-PCR approach should provide breeders with a more sensitive and less variable alternative to time-consuming plating assays to distinguish response of breeding lines to colonization by V. dahliae.

Considerations for Verticillium Wilt Resistance Evaluation in Potato.

Verticillium wilt (Vw), caused by the soilborne fungi Verticillium dahliae and V. albo-atrum, is an important disease of potato (Solanum tuberosum). Host plant resistance is a promising method of Vw control. Culture-based methods that quantify the pathogen in host tissue often are used for Vw resistance screening. To evaluate the processing time, accuracy, and precision of these methods, 46 clones were planted in a field naturally infested with V. dahliae to collect data on visual disease symptoms, pathogen colonization, and yield. In 2002, disease severity explained 4.34% of the variability of yield loss, but the linear relationship between stem colonization and yield loss was not significant. In 2003, stem colonization explained 57.5% of the variability of yield loss, whereas disease severity explained 1.7% of the variability of yield loss. Correlations comparing clone ranks from repeated pathogen measurements indicated that culturing sap from individual stems or bulked stems generated more repeatable clone rankings than culturing dried stems. Clone rankings were more repeatable between years if pathogen measurements were made earlier in the growing season. The results indicate a need to characterize the effect of the environment on the relationship among pathogen population sizes in planta, disease symptoms, and yield loss.

Colonization Dynamics and Spatial Progression of Verticillium dahliae in Individual Stems of Two Potato Cultivars with Differing Responses to Potato Early Dying.

Potato early dying (PED), caused by Verticillium dahliae, is a chronic yield-limiting disease of potato (Solanum tuberosum). In this study, we describe the colonization dynamics of V. dahliae in two potato cultivars with varying responses to PED. We utilized a quantitative real-time polymerase chain reaction (Q-PCR) assay to assess the colonization and spatial progression of V. dahliae in cvs. Ranger Russet (moderately resistant) and Russet Norkotah (highly susceptible). Ninety plants per cultivar were inoculated with a conidial suspension in the greenhouse. Every 2 weeks until week 10, we collected basal samples from 15 plants, and repeatedly sampled the growing apices of another 15 plants. The mean infection coefficient (IC) values in the basal and apical samples were significantly lower in cv. Ranger Russet at all five sampling dates. The pathogen was detected in basal samples of both cultivars by week 2, and in apical samples of cv. Russet Norkotah at week 4 and of cv. Ranger Russet at week 6. Colonization of cv. Russet Norkotah consistently increased in apical and basal samples during the 10 weeks, while it plateaued after week 6 in cv. Ranger Russet. Differences in response to PED appear associated with the speed of colonization and the establishment of a higher population density by V. dahliae in the plant.

Multiple Disease Resistance in Interspecific Hybrids of Potato.

Wild species of Solanum are excellent sources of disease resistance genes that may be incorporated into S. tuberosum through breeding. This study was initiated to determine whether multiple forms of disease resistance could be identified in interspecific Solanum hybrids. Thirty-two clones were evaluated for resistance to soft rot, common scab, black scurf, Verticillium wilt, and early blight. Most of the clones originated from populations that were not initially selected for disease resistance traits. Comparisons with the cultivars Atlantic, Russet Norkotah, and Russet Burbank indicated that all clones were more resistant than at least one cultivar for at least one disease resistance trait. Clone C545, which exhibited improved resistance to soft rot, scab, pitted scab, early dying disease, and early blight, appears to be an especially valuable source of disease resistance. The use of interspecific hybridization at the diploid level, combined with sexual polyploidization to return to the tetraploid level, provides a method to introduce multiple forms of disease resistance into advanced clones.

The effects of boiling and leaching on the content of potassium and other minerals in potatoes.

The white potato (Solanum tuberosum L.) is a valuable source of potassium in the human diet. While most consumers benefit from high levels of potassium in potato tubers, individuals with compromised kidney function must minimize their potassium intake. This study was undertaken to determine the effects of leaching and boiling on levels of potassium and other minerals in potato tubers. Leaching alone did not significantly reduce levels of potassium or other minerals in tubers. Boiling tuber cubes and shredded tubers decreased potassium levels by 50% and 75%, respectively. Reductions in mineral amounts following boiling were observed for phosphorus, magnesium, sulfur, zinc, manganese, and iron. There was no difference between the leaching and boiling treatment and the boiling treatment. In addition, mineral levels in tubers of 6 North American potato cultivars are reported. Significant differences in mineral levels were detected among cultivars, but they were too small to be nutritionally important. Individuals wishing to maximize the mineral nutrition benefits of consuming potatoes should boil them whole or bake, roast, or microwave them. Those who must reduce potassium uptake should boil small pieces before consuming them.