QTL identified that influence tuber length-width ratio, degree of flatness, tuber size, and specific gravity in a russet-skinned, tetraploid mapping population.

Potato tuber shape, size, and specific gravity are important agronomic traits in the russet market class of potatoes with an impact on quality, consistency, and product recovery of processed foods such as French fries. Therefore, identifying genetic regions associated with the three traits through quantitative trait locus/loci (QTL) analysis is a crucial process in the subsequent development of marker-assisted selection for use in potato breeding programs. QTL analysis was conducted on a tetraploid mapping population consisting of 190 individuals derived from the cross between two russet-skinned parents, Palisade Russet and the breeding clone ND028673B-2Russ. Field data collected over a 2-year period and used in the QTL analyses included tuber length-width and width-depth ratios that were obtained using a digital caliper. The width-depth ratio provided an assessment of the "flatness" of a tuber, which is of importance in potato processing. To cross-validate the accuracy and differences among tuber shape measurement methods, a trained evaluator also assessed the identical tubers based on 1-5 scale (compressed to long) visual assessment method. Furthermore, the weights of analyzed tubers and specific gravities were also collected during the phenotyping process for each mapping clone. A major tuber shape QTL was consistently observed on chromosome 10 with both the length-width ratio and visual assessments. On chromosome 4, a significant QTL for tuber shape from the visual assessment phenotypic data was also detected. Additionally, a tuber shape-related QTL on chromosome 6 was also detected from the length-width ratio data from 2020. Chromosome 2 was also identified as having a significant QTL for the width-depth ratio, which is of importance in influencing the flatness of a tuber. One significant QTL for tuber weight (i.e., tuber size) was observed on chromosome 5, and a significant QTL for specific gravity was found on chromosome 3. These significant and major QTL should be useful for developing marker-assisted selection for more efficient potato breeding.

Identification of QTL associated with plant vine characteristics and infection response to late blight, early blight, and Verticillium wilt in a tetraploid potato population derived from late blight-resistant Palisade Russet.

Potato late blight (causal agent Phytophthora infestans) is a disease of potatoes with economic importance worldwide. Control is primarily through field monitoring and the application of fungicides. Control of late blight with fungicides and host plant resistance is difficult, with documented cases of such control measures failing with the advent of new pathotypes of P. infestans. To better understand host plant resistance and to develop more durable late blight resistance, Quantitative Trait Locus/Loci (QTL) analysis was conducted on a tetraploid mapping population derived from late blight-resistant potato cultivar Palisade Russet. Additionally, QTL analyses for other traits such as Verticillium wilt and early blight resistance, vine size and maturity were performed to identify a potential relationship between multiple traits and prepare genetic resources for molecular markers useful in breeding programs. For this, one hundred ninety progenies from intercrossing Palisade Russet with a late blight susceptible breeding clone (ND028673B-2Russ) were assessed. Two parents and progenies were evaluated over a two-year period for response to infection by the US-8 genotype of P. infestans in inoculated field screenings in Corvallis, Oregon. In Aberdeen, Idaho, the same mapping population was also evaluated for phenotypic response to early blight and Verticillium wilt, and vine size and maturity in a field over a two-year period. After conducting QTL analyses with those collected phenotype data, it was observed that chromosome 5 has a significant QTL for all five traits. Verticillium wilt and vine maturity QTL were also observed on chromosome 1, and vine size QTL was also found on chromosomes 3 and 10. An early blight QTL was also detected on chromosome 2. The QTL identified in this study have the potential for converting into breeder-friendly molecular markers for marker-assisted selection.

An evaluation based on the analytic hierarchy process and GGEbiplot on French fry potato genotypes in Yunnan, China.

A total of 33 potato (Solanum tuberosum L.) cultivars and breeding clones imported from the United States and two local cultivars (Yunshu 401 and Cooperation 88, CK) were planted and evaluated. To determine their suitability for processing into French fries at five locations (e1-e5) in Yunnan Province, China, we developed a comprehensive evaluation system using the analytical hierarchy process (AHP). Eleven evaluation indicators for French fry quality, yield, and agronomic characteristics with a relative importance (weight coefficients) of 0.483, 0.301 and 0.216, respectively, were used to analyze the 35 potato genotypes (designated g1-g35).The genotypes were ranked and the results revealed that (1) on the average, the 33 potato genotypes imported from the United States showed a lower performance compared to the local cultivars. Compared with the CK, they were classified as not vigorous (Mean 5.11 vs CK 7.75), matured earlier (Mean 5.79 vs CK 1.70), and had a low resistance to late blight (Mean 3735.59 vs CK 1418.55), requiring the use of fungicides to control the disease at the five trial locations. (2) The US cultivar 'Defender' (g3) ranked in the top six at all five test locations because it had higher yield (29.56 t h m-2), better fry quality (4.64), higher dry matter content (20.41%), better tuber length/width ratio (1.99), longer tubers (13.57cm), stronger plant vigor (7.17) and higher resistance to late blight (AUDPC: 3134.2). (3) By using GGEbiplot analysis, superior genotypes with high and stable yields were g3 and 'Echo Russet' (g33). 'Yunshu 401' (g34) and 'Yukon Gem' (g4) had high but not stable yields. The ideal test environments and hence experimental locations were Luquan (LQ, e2) and Lijiang (LJ, e4) which resulted in the best discrimination between genotypes among the five experimental locations in Yunnan. Overall, the developed evaluation system based on AHP and GGEbiplot analysis including 11 evaluation indicators for French fry quality, yield and agricultural traits can be a model for evaluation and promotion of new French fry cultivars, and evaluating and selecting the test location.

Development of a Droplet Digital PCR Assay for Detection and Quantification of Stubby Root Nematode, Paratrichodorus allius, in Soil.

The stubby root nematode Paratrichodorus allius is an important plant-parasitic nematode species within the Trichodoridae family. It can directly harm the plants by feeding on the roots or indirectly by transmitting Tobacco rattle virus. These nematodes are mostly diagnosed either by traditional microscopic methods or a polymerase chain reaction (PCR)-based method. Droplet digital PCR (ddPCR) is a novel PCR technique which is sensitive and precise in quantifying DNA templates of the test samples. In this study, we developed a ddPCR assay to detect and quantify P. allius in soil. The specificity and sensitivity of the assay was first determined using P. allius nematode DNA or DNA from sterilized soil artificially inoculated with P. allius, and the assay was used to quantify P. allius populations in field soils. The assay did not detect nematodes other than P. allius, thus showing high specificity. It was able to detect P. allius equivalent to a 0.01 and 0.02 portion of a single nematode in soil DNA and nematode DNA extracts, respectively. Highly linear relationships between DNA copy numbers from ddPCR and serial dilutions of known concentrations were observed with DNA from P. allius nematodes (R2 = 0.9842) and from artificially infested soil (R2 = 0.9464). The P. allius populations from field soils determined by ddPCR were highly correlated with traditional microscopic counts (R2 = 0.7963). To our knowledge, this is the first report of applying ddPCR to detect and quantify stubby root nematode in soil. The results of this study support the potentiality of a ddPCR assay as a new research tool in diagnostics of plant-parasitic nematodes.

Linkage and QTL mapping for tuber shape and specific gravity in a tetraploid mapping population of potato representing the russet market class.

BACKGROUND: Tuber shape and specific gravity (dry matter) are important agronomic traits in potato processing and impact production costs, quality, and consistency of the final processed food products such as French fries and potato chips. In this study, linkage and QTL mapping were performed for these two traits to allow for the implementation of marker-assisted selection to facilitate breeding efforts in the russet market class. Two parents, Rio Grande Russet (female) and Premier Russet (male) and their 205 F1 progenies were initially phenotyped for tuber shape and specific gravity in field trials conducted in Idaho and North Carolina in 2010 and 2011, with specific gravity also being measured in Minnesota in 2011. Progenies and parents were previously genotyped using the Illumina SolCAP Infinium 8303 Potato SNP array, with ClusterCall and MAPpoly (R-packages) subsequently used for autotetraploid SNP calling and linkage mapping in this study. The 12 complete linkage groups and phenotypic data were then imported into QTLpoly, an R-package designed for polyploid QTL analyses. RESULTS: Significant QTL for tuber shape were detected on chromosomes 4, 7, and 10, with heritability estimates ranging from 0.09 to 0.36. Significant tuber shape QTL on chromosomes 4 and 7 were specific to Idaho and North Carolina environments, respectively, whereas the QTL on chromosome 10 was significant regardless of growing environment. Single marker analyses identified alleles in the parents associated with QTL on chromosomes 4, 7, and 10 that contributed to significant differences in tuber shape among progenies. Significant QTL were also identified for specific gravity on chromosomes 1 and 5 with heritability ranging from 0.12 to 0.21 and were reflected across environments. CONCLUSION: Fully automated linkage mapping and QTL analysis were conducted to identify significant QTL for tuber shape and dry matter in a tetraploid mapping population representing the russet market class. The findings are important for the development of molecular markers useful to potato breeders for marker-assisted selection for the long tuber shape and acceptable dry matter required by the potato industry within this important market class.

Assessment of Factors Associated with Molecular Quantification of Stubby Root Nematode Paratrichodorus allius from Field Soil DNA.

Factors relating to SYBR Green-based quantitative real-time PCR (qPCR) quantification of stubby root nematode Paratrichodorus allius using soil DNA were evaluated in this study. Soils used were loamy sand from potato fields in North Dakota and Idaho. Results showed that the largest nematode individuals (body length >720 µm) produced significant lower Cq values than the smallest individuals (<359 µm), indicating more total DNA amount in the largest nematodes. Soil pre-treatments showed that autoclaved field soil had significantly reduced DNA amount and quality. The air- or oven-dried soil yielded a lower amount of DNA with similar purity, compared with natural field soil. PCR inhibitors were detected in soil DNA substrates targeting pBluescript II SK(+)-plasmid DNA. Al(NH4)(SO4)2 treatment during DNA preparation significantly reduced the inhibitors compared with post-treatment of soil DNA with polyvinylpolypyrrolidone column. The effect of PCR inhibitors on qPCR was suppressed by bovine serum albumin. Quantification results did not significantly change when increasing the number of DNA extractions from three to six per soil sample when soil grinding and grid sampling strategies were used. Two standard curves, generated from serial dilutions of plasmid DNA containing P. allius ITS1 rDNA and soil DNA containing known nematode numbers, produced similar correlations between Cq values and amount of targets. The targets in soil DNA quantified by qPCR using either standard curve correlated well with microscopic observations using both artificially and naturally infested field soils. This is the first study for assessing various factors that may affect qPCR quantification of stubby root nematodes. Results will be useful during the setup or optimization of qPCR-based quantification of plant-parasitic nematodes from soil DNA.

Current Status of Potato Cyst Nematodes in North America.

The potato cyst nematodes (PCNs) Globodera rostochiensis and Globodera pallida are internationally recognized quarantine pests. Although not widely distributed in either the United States or Canada, both are present and are regulated by the national plant protection organizations (NPPOs) of each country. G. rostochiensis was first discovered in New York in the 1940s, and G. pallida was first detected in a limited area of Idaho in 2006. In Canada, G. rostochiensis and G. pallida were first detected in Newfoundland in 1962 and 1977, respectively, and further detections of G. rostochiensis occurred in British Columbia and Québec, most recently in 2006. Adherence to a stringent NPPO-agreed-upon phytosanitary program has prevented the spread of PCNs to other potato-growing areas in both countries. The successful research and regulatory PCN programs in both countries rely on a network of state, federal, university, and private industry cooperatorspursuing a common goal of containment, management/eradication, and regulation. The regulatory and research efforts of these collaborative groups spanning from the 1940s to the present are highlighted in this review.

Developing a One-Step Multiplex PCR Assay for Rapid Detection of Four Stubby-Root Nematode Species, Paratrichodorus allius, P. minor, P. porosus, and Trichodorus obtusus.

Four trichodorid species, Paratrichodorus allius, P. minor, P. porosus, and Trichodorus obtusus, were found in multiple states in the United States. Traditional diagnosis based on morphology and morphometrics is laborious and requires an experienced taxonomist. Additionally, end-point diagnosis using PCR was only available for P. allius. To increase diagnostic efficiency and reduce costs, a one-step multiplex PCR assay was developed to simultaneously identify these four species using one PCR reaction. Available sequences of 18S ribosomal DNA and internal transcribed spacer 1 (ITS1) region of these species were aligned and five primers were designed. The conserved forward primer located in the 18S region, in combination with the species-specific antisense primer in the ITS1 region, amplified a single distinctive PCR fragment for each species (421/425 bp for P. allius, 190 bp for P. minor, 513 bp for P. porosus, and 353 bp for T. obtusus). In silico analysis with 10 other trichodorid species and experimental analysis using samples with these four species, 20 other plant-parasitic and three non-plant-parasitic nematodes demonstrated high specificity with the primers designed. The multiplex PCR amplified desirable fragments using a set of artificially mixed templates containing one, two, three, or four targeted species. The reliability of multiplex PCR results was demonstrated by using nematode populations isolated from infested fields from diverse geographic regions in eight states. The multiplex PCR-based tool developed in this study for the first time provides a simple, rapid, and cost-friendly assay for accurate diagnosis of the four major trichodorid nematodes in the United States.

Molecular Characterization and Identification of Stubby Root Nematode Species From Multiple States in the United States.

Stubby root nematodes (SRN) are important plant parasites infecting many crops and widely distributed in many regions of the United States. SRN transmit Tobacco rattle virus, which causes potato corky ringspot disease, thereby having a significant economic impact on the potato industry. In 2015 to 2017, 184 soil samples and 16 nematode suspensions from North Dakota, Minnesota, Idaho, Oregon, Washington, South Carolina, North Carolina, and Florida were assayed for the presence of SRN. SRN were found in 106 soil samples with population densities of 10 to 320 SRN per 200 g of soil and in eight of the nematode suspensions. Sequencing of ribosomal DNA (rDNA) or species-specific polymerase chain reaction assays revealed the presence of four SRN species, including Paratrichodorus allius, P. minor, P. porosus, and Trichodorus obtusus. Accordingly, their rDNA sequences were characterized by analyzing D2-D3 of 28S rDNA, 18S rDNA, and internal transcribed spacer (ITS) rDNA obtained in this study and retrieved from GenBank. Both intra- and interspecies variations were higher in ITS rDNA than 18S rDNA and D2-D3 of 28S rDNA. Based on phylogenetic analysis, the four SRN species formed a monophyletic group, with P. allius more closely related to P. porosus than P. minor and T. obtusus. Indel variation of ITS2 rDNA was present in P. allius populations from the same geographic regions. This study documented the occurrence of SRN species across multiple states. The intra- and interspecies genetic diversity of rDNA in this study will provide more information for understanding the evolutionary relationships of SRN and will be valuable for future studies of SRN species identification and management.

Resistance of Potato Breeding Clones and Cultivars to Three Species of Potato Cyst Nematode.

In the United States, potato cyst nematodes Globodera rostochiensis and G. pallida are quarantined pests. A new cyst nematode species, Globodera ellingtonae, discovered in Oregon and Idaho, reproduces well on potato but is not currently a quarantine pest. Identifying resistance to all three Globodera spp. would provide a valuable management tool. Thirteen breeding clones and nine cultivars were evaluated in Oregon, Idaho, and New York laboratories where the nematode populations are maintained. Minitubers or tissue culture plants were planted into pots and inoculated with eggs in replicated experiments. Results indicated that five entries were partially resistant or resistant to all three species, while another five were resistant or partially resistant to G. rostochiensis and G. ellingtonae. Resistance to G. rostochiensis pathotypes Ro1 and Ro4 is controlled by the H1 gene and this study suggests that H1 may confer resistance to G. ellingtonae as well. Observed resistance to G. pallida was lower relative to the levels of resistance observed for G. rostochiensis and G. ellingtonae. Germplasm with G. pallida or G. ellingtonae resistance will be used in hybridizations to develop russet-skinned cultivars with long tubers which represent the predominant market class in western U.S. production, and to further explore the basis of potato resistance to Globodera spp.

Assessing Potato Cultivar Sensitivity to Tuber Necrosis Caused by Potato mop-top virus.

Potato mop-top virus (PMTV) causes mop top disease in potato. This disease can result in a decline in tuber quality causing economic losses to growers due to the production of necrotic lesions and discolored tissue in infected tubers. Due to the soilborne nature of PMTV, identifying and developing host resistance against the virus is considered the best disease management option. Very little is known about the sensitivity of U.S. potato cultivars to PMTV-induced tuber necrosis. The current study is aimed at investigating the sensitivity of a large number of potato cultivars to PMTV-induced tuber necrosis. Sixty-three cultivars representing all market-types were evaluated in North Dakota over a 2-year period for virus-induced tuber necrosis incidence and severity. PMTV-induced tuber necrosis (P < 0.0001) and severity (P < 0.0001) were significantly different among cultivars. Cultivars were categorized into sensitive, insensitive, and moderately sensitive/insensitive groups based on the virus-tuber induced necrosis data from both years. Based on data from ND trials, six cultivars (Red Endeavor, Viking, Dakota Jewel, Dark Red Norland, Nicolet, and Modoc) were rated as sensitive and 43 were rated as insensitive to PMTV-induced tuber necrosis. Four cultivars, including Bannock Russet, Gemstar Russet, Lelah, and Waneta showed zero PMTV incidence over 2 years. These results will help growers in making individual or coordinated decisions for the management of PMTV-induced tuber necrosis under field and storage conditions.

Assessing Potato Cultivar Sensitivity to Tuber Necrosis Caused by Tobacco rattle virus.

Tobacco rattle virus (TRV) causes the economically important corky ring spot disease in potato. Chemical control is difficult due to the soilborne nature of the TRV-transmitting nematode vector, and identifying natural host resistance against TRV is considered to be the optimal control measure. The present study investigated the sensitivity of 63 cultivars representing all market types (evaluated at North Dakota and Washington over 2 years) for the incidence of TRV-induced tuber necrosis and severity. This article also investigates the cultivar-location interaction (using a mixed-effects model) for TRV-induced necrosis. TRV-induced tuber necrosis (P < 0.0001) and severity (P < 0.0001) were significantly different among cultivars evaluated separately in North Dakota and Washington trials. Mixed-effects model results of pooled data (North Dakota and Washington) demonstrated that the interaction of cultivar and location had a significant effect (P = 0.03) on TRV-induced necrosis. Based on the virus-induced tuber necrosis data from both years and locations, cultivars were categorized into sensitive, moderately sensitive, insensitive, and moderately insensitive groups. Based on data from North Dakota, 10 cultivars, including Bintje, Centennial Russet, Ciklamen, Gala, Lelah, Oneida Gold, POR06V12-3, Rio Colorado, Russian Banana, and Superior, were rated as insensitive to TRV-induced tuber necrosis. Similar trials assessing TRV sensitivity among cultivars conducted in Washington resulted in a number of differences in sensitivity rankings compared with North Dakota trials. A substantial shift in sensitivity of some potato cultivars to TRV-induced tuber necrosis was observed between the two locations. Four cultivars (Centennial Russet, Oneida Gold, Russian Banana, and Superior) ranked as insensitive for North Dakota trials were ranked as sensitive for Washington trials. These results can assist the potato industry in making cultivar choices to reduce the economic impact of TRV-induced tuber necrosis.

Responses of Aphid Vectors of Potato leaf roll virus to Potato Varieties.

Potato leaf roll virus (PLRV) can reduce tuber yield and quality in potato. Green peach aphid (Myzus persicae [Sulzer]) and potato aphid (Macrosiphum euphorbiae [Thomas]) are the two most important potato-colonizing PLRV vectors in the Pacific Northwest. We compared My. persicae and Ma. euphorbiae densities and PLRV incidences among potato varieties in the field to clarify the relationships between aphid abundance and PLRV incidence in plants. Aphids were sampled weekly over three years in the potato varieties Russet Burbank, Ranger Russet, and Russet Norkotah in a replicated field trial. In all years, My. persicae was more abundant than Ma. euphorbiae, representing at least 97% of samples. My. persicae densities did not differ among potato varieties across years; very low numbers of Ma. euphorbiae precluded such statistical comparisons for this species. PLRV infection did not differ significantly among potato varieties, although the percent of PLRV-infected plants differed among years when all varieties were combined (46% in 2013, 29% in 2011, 13% in 2012). For Ranger Russet and Russet Norkotah, PLRV incidence was positively correlated with aphid abundance as well as proportion of PLRV-positive aphids. In Russet Burbank, only aphid abundance was positively correlated with PLRV infection. Our results suggest that the three most commonly grown potato varieties in our region do not differ in their susceptibility to PLRV infection, and that aphid density was a consistent indicator of the risk of infection by this virus across varieties. Both of these findings can be used to hone PLRV monitoring and modeling efforts.

Potato virus Y Transmission Efficiency from Potato Infected with Single or Multiple Virus Strains.

There has been a recent shift in the prevalence of Potato virus Y (PVY) strains affecting potato with the ordinary strain PVYO declining and the recombinant strains PVYNTN and PVYN:O emerging in the United States. Multiple PVY strains are commonly found in potato fields and even in individual plants. Factors contributing to the emergence of the recombinant strains are not well defined but differential aphid transmission of strains from single and mixed infections may play a role. We found that the transmission efficiencies by Myzus persicae, the green peach aphid, of PVYNTN, PVYN:O, and PVYO varied depending on the potato cultivar serving as the virus source. Overall transmission efficiency was highest from sources infected with three virus strains, whereas transmission from sources infected with one or two virus strains was not significantly different. Two strains were concomitantly transmitted by individual aphids from many of the mixed-source combinations, especially if PVYO was present. Triple-strain infections were not transmitted by any single aphid. PVYO was transmitted most efficiently from mixed-strain infection sources. The data do not support the hypothesis that differential transmission of PVY strains by M. persicae is a major contributing factor in the emergence of recombinant PVY strains in the U.S. potato crop.

Interactions among potato genotypes, growth stages, virus strains, and inoculation methods in the potato virus Y and green peach aphid pathosystem.

Potato virus Y (PVY) is an economically important and reemerging potato pathogen in North America. PVY infection reduces yield, and some necrotic and recombinant strains render tubers unmarketable. Although PVY(O) is the most prevalent strain in the United States, the necrotic and recombinant strains PVY(NTN) and PVY(N:O) are becoming more widespread. Infection rates in aphid-inoculated (Myzus persicae (Sulzer)) and mechanically inoculated plants were compared across two potato genotypes ('Yukon Gold' and A98345-1), three PVY strains (PVY(O), PVY(N:O), and PVY(NTN)), and two growth stages at inoculation (pre- and postflowering). Susceptibility of genotypes was measured as infection rate using a double-antibody sandwich-enzyme-linked immunosorbent assay; virus titer and tuber mass also were recorded from the infected plants. Yukon Gold generally was more susceptible than A98345-1 to all three PVY strains, especially following mechanical inoculation. Within genotypes, Yukon Gold was most susceptible to PVY(O) and A98345-1 was most susceptible to PVY(N:O). Plants exhibited age-based resistance, with both genotypes showing higher susceptibility at the pre- than postflowering stage. The overall ranking pattern of virus titer in infected plants was PVY(O) > PVY(NTN) > PVY(N:O); across all three strains, infected Yukon Gold had higher titer than infected A98345-1 plants. Yukon Gold plants had lower tuber mass than A98345-1 when infected, and there were differences between the two inoculation methods in regard to tuber mass for the three stains. The results showed differences in infection response between inoculation methods and as a function of genotype, strain, inoculation stage, and their interactions. These factors should be considered when screening genotypes for resistance.

Strain specificity and simultaneous transmission of closely related strains of a Potyvirus by Myzus persicae.

Potato virus Y (PVY), a Potyvirus, is transmitted by aphids in a nonpersistent manner. PVY severely affects potato production worldwide. Single and mixed infections of PVY strains, namely PVY(O), PVY(NTN), and PVY(N:O) are a common occurrence in potato systems. However, information available on the ability of aphids to simultaneously transmit multiple PVY strains, specificity associated with simultaneous transmission, and factors affecting specificity are limited. Aphid-mediated transmission experiments were conducted to test the ability of individual aphids to transmit multiple strains using a PVY indicator host. Preliminary results revealed that aphids can transmit at least two viral strains simultaneously. Subsequently, aphid-mediated transmission of three dual-strain combinations was tested using potato plants. Individual aphids transmitted two viral strains simultaneously for all three dual-strain combinations. In all aphid-mediated dual-strain infections involving PVY(NTN), the rate of PVY(NTN) infection was greater than the infection rates of the second strain and dual-strain combinations, indicating specificity associated with transmission of PVY strains. Results of aphid-mediated transmission experiments were compared with results obtained through mechanical transmission. In general, PVY infection rates from aphid-mediated transmission were lower than the rates obtained through mechanical transmission. Unlike aphid-mediated transmission, component strains in dual-strain inoculations were not eliminated during mechanical transmission. These results suggest that there may also be interference associated with aphid-mediated transmission of closely related PVY strains. Perhaps, the observed specificity and/or interference may explain the increase in the incidence of PVY(NTN) and other necrotic strains in recent years.

The primary in vivo steroidal alkaloid glucosyltransferase from potato.

To provide tools for breeders to control the steroidal glycoalkaloid (SGA) pathway in potato, we have investigated the steroidal alkaloid glycosyltransferase (Sgt) gene family. The committed step in the SGA pathway is the glycosylation of solanidine by either UDP-glucose or UDP-galactose leading to alpha-chaconine or alpha-solanine, respectively. The Sgt2 gene was identified by deduced protein sequence homology to the previously identified Sgt1 gene. SGT1 has glucosyltransferase activity in vitro, but in vivo serves as the UDP-galactose:solanidine galactosyltransferase. Two alleles of the Sgt2 gene were isolated and its function was established with antisense transgenic lines and in vitro assays of recombinant protein. In tubers of transgenic potato (Solanum tuberosum) cvs. Lenape and Desirée expressing an antisense Sgt2 gene construct, accumulation of alpha-solanine was increased and alpha-chaconine was reduced. Studies with recombinant SGT2 protein purified from yeast show that SGT2 glycosylation activity is highly specific for UDP-glucose as a sugar donor. This data establishes the function of the gene product (SGT2), as the primary UDP-glucose:solanidine glucosyltransferase in vivo.

Effect of Potato virus Y on Yield of Three Potato Cultivars Grown Under Different Nitrogen Levels.

Previous studies have shown that Potato virus Y (PVY) reduces yield in many cultivars. Typical foliar symptoms can include veinal necrosis, leaf drop, and a mosaic pattern sometimes accompanied by leaf roughness. Infection by PVY in Russet Burbank produces identifiable PVY symptoms, whereas cv. Russet Norkotah expresses mild, almost latent symptoms. Yield also is influenced by nitrogen fertilizer levels. This research was conducted to determine whether increased nitrogen mitigates yield reduction caused by PVY. Russet Norkotah, CO80011-5, and Russet Burbank were used in replicated plots of non-PVY-infected and PVY-infected plants at three nitrogen levels in 1995 and 1996. There was a significant yield reduction between PVY-negative and PVY-positive plots in all cultivars, at most nitrogen levels. PVY yield reduction was similar (approximately 38%) in the mild symptom expression clones of Russet Norkotah and CO80011-5, whereas the yield reduction in Russet Burbank, which exhibits typical symptom expression, was 63.5%. We conclude that increased nitrogen can influence total yield, but does not significantly mitigate the yield reduction due to PVY infection.

Effect of Seedborne Potato virus Y on Performance of Russet Burbank, Russet Norkotah, and Shepody Potato.

Potato virus Y (PVY) is one of the most important of the potato viruses, but little is known about the impact on yield of seedborne infection levels below 100%. Blending infected and healthy seed from different seed lots introduces the variable of performance differences between the seed lots, which may obscure the effect of virus alone. Seed lots containing various levels of seedborne PVY (0, 2, 10, 20, and 50% incidence of infected tubers) were created by combining in different proportions seed pieces from healthy and infected tubers from the same seed source. These seed lots were planted in replicated field plots at the University of Idaho Parma R & E Center in Parma, ID from 1995 to 1997. Regression analyses on data from the three consecutive seasons indicate that seedborne PVY has virtually the same negative impact on yield for all three cultivars.