APS Symposium on PVY
Potato Virus Y—An Old Virus and a New Problem in Potato
Come to this special session at the American Phytopathological Society Annual Meeting.
August 4-8, 2012 in Providence, Rhode Island, exact date to be determined.
PVY as an emerging potato problem in North America. S. Gray, USDA-ARS, Cornell Univ, Ithaca, NY
Breeding potato for PVY resistance. S. Jansky, USDA-ARS, Madison, WI
PVY vector biology and control. R. Groves, University of Wisconsin, Madison, WI
PVY and Canadian experience. M. Singh, POTATOES NB, Centreville, NB, Canada
Classification of PVY strains and new recombinants. A. Karasev, University of Idaho, Moscow, ID
Potato seed certification and PVY. P. Nolte, University of Idaho, Idaho Falls, ID
In the past decade, PVY has re-emerged as a significant disease affecting seed potato production and is threatening to become a tuber quality issue. The resurgence of PVY is due, in part, to widespread acceptance of potato varieties that express mild or no foliar PVY symptoms, to displacement of the ordinary PVY strain by recombinant strains that often induce mild foliar symptoms and tuber necrosis, and to increased late season spread of the virus due to changes in aphid populations. These late season infections often are not manifested in foliar symptoms, but result in infected tubers. Each of these changes hampers the detection and removal of PVY-infected plants during field inspections and rouging operations, and leads to more virus being present in seed stocks. A national PVY project, outlined in this talk, has partnered with seed certification and regulatory agencies, and grower organizations across the U.S. to address the needs of the potato industry. Our immediate goal is to work with seed certification agencies to reduce the level of virus in seed stocks below economic thresholds and to eliminate seed lots harboring the tuber necrotic strain. The longer term goals are to improve on-farm Best Management Plans to reduce the impacts of PVY on farm profitability and product quality. Project outcomes are translated to stakeholders via www.potatovirus.com that includes an open discussion forum on any aspect of PVY or other potato virus diseases.
In recent years, PVY has emerged as a serious disease problem in potato production areas throughout North America. One explanation for this increase in incidence is that some widely planted cultivars express mild or no symptoms, preventing effective rogueing of infected plants. Furthermore, recent investigations have revealed a significant increase in the proportion of PVY recombinants (PVYN:O) in major production areas. Effective strategies to address this emerging problem are lacking. The development of cultivars with improved levels of PVY resistance would provide one such strategy. We have identified a new source of extreme resistance to PVYO and PVYN:O in the diploid wild potato relative Solanum chacoense. All plants in accession 275138 were resistant to PVY, while variability for resistance was found in accession 320285. Segregation ratios in the F1 and backcross populations indicate that PVY resistance is conferred by a single dominant gene. The 8,303 SNP array developed by the SolCAP initiative is being used to map the gene in a diploid hybrid population created by crossing a heterozygous resistant clone to a susceptible wild species clone (S. tarijense, PI558129). In addition, a diploid adapted fertile clone has been selected from a cross between a homozygous resistant S. chacoense clone and a susceptible cultivated S. tuberosum clone. It expresses genes for the production of numerically unreduced pollen and eggs, so when crossed to tetraploid cultivars, it produces tetraploid offspring carrying the resistance gene. In addition, it carries the dominant allele of a gene that overcomes the gametophytic self-incompatibility system, so it can be self-pollinated to create clones homozygous for the PVY resistance gene.
The US seed potato industry has identified PVY as its most serious disease problem and is looking for ways to reduce overall disease incidence below acceptable levels. In Wisconsin, seed potato crops are threatened by infection of PVY, vectored by several species of potato colonizing and non-colonizing species. The dispersal and movement of PVY vectors are influenced by processes operating across different temporal and spatial scales (e.g. climate, weather, or host plant condition). We describe a modeling framework that uses a generalized linear mixed modeling approach to partition the variance associated with aphid capture data. These approaches were applied to long term, multi-site and multi-year, aphid suction trap data sets from the North Central Region, Aphid Suction Trap Network. Variance estimates of aphid abundance among years significantly exceeded similar estimates among locations (regions). This suggests that factors such as season better explain variation in aphid abundance than factors driving differences among locations (e.g. surrounding habitat characteristics). These findings are consistent with the hypothesis that selected aphid species are migrating into fields from moderate to long distances. Taken together, this approach improves our understanding of the patterns of variation in aphid abundance, thereby increasing our knowledge of when the aphid vectors move into susceptible seed potato fields and when they spread the virus to highly susceptible varieties. A more accurate assessment of the phenology of the principle aphid vector flights provides an opportunity to deploy behavioral modifying crop protectants at critical periods to limit the transmission of PVY to potato.
Potato virus Y is an evolving problem throughout the world and Canada is no exception. PVY made headlines in Canada in 1990’s when PVYN was discovered in Eastern Canada, which resulted in the loss of export markets. Since then significant progress has been made in PVY research. PVY is managed in seed potatoes by visual inspection during growing season followed by post-harvest test. A national survey conducted in 2004-2006 revealed that all strains of PVY were present but PVYO was the predominant strain, which was further confirmed by a survey conducted in New Brunswick in 2009. The responses of 14 potato cultivars to five isolates belonging to four PVY strains varied significantly, ranging from local lesions to systemic necrosis and tuber symptoms depending on potato cultivar and virus strains. Field experiments on the current season PVY spread and the transmission risk by aphid species unveiled the presence of few potato colonizing aphids but many non-colonizing species, and among them, several were found to be PVY-positive by RT-PCR. Current season spread of PVY in the fields increased as the crop season progressed, and reached up to 40% at the time of harvest in some fields. PVY incidence determined from developing tubers exhibited a significant positive correlation with that from tubers after harvest using real-time RT-PCR. Application of mineral oil spray along with insecticide was found to be a very effective tool for the management of PVY in a growing crop.
Currently, strains or strain groups of PVY are classified based on two distinct approaches. The genetic classification groups PVY strains according to their ability to induce hypersensitive resistance (HR) responses in a set of potato indicators carrying three different N genes, hence five strains of PVY are defined based on this system, PVYO, PVYC, PVYZ, PVYN, and PVYE. The molecular classification groups PVY strains according to the sequence of their genomes which are often recombinant, built of segments of parental genomes, most often of PVYO and PVYN parents. The two most prominent recombinant strain groups are PVYN-Wi and PVYNTN, the latter associated with the potato tuber necrotic ringspot disease (PTNRD). Up until recently, the two classifications were not merged, due to unavailability of the PVYZ and PVYE sequences and lack of genetic typing of recombinant strain groups on potato indicators. In the past few years, both PVYZ and PVYE recombinant genomes were sequenced, and preliminary genetic typing of PVYNTN was completed. This allowed development of a modified classification of PVY strains which combines genetic and molecular characteristics of recombinant and non-recombinant strain groups, and simplifies typing of new PVY isolates. Application of this modified classification to typing field isolates of PVY and search of the PTNRD genetic determinant will be discussed.
New strains of PVY are adding to the challenges faced by seed potato certification officials across North America. The use of the traditional visual inspection to detect these new PVY strains has not been effective. This lack of effectiveness occurs because the new strains, which often cause milder symptoms in common North American varieties than the original PVY strains, are simply more difficult or even impossible to see. Poor visual detection has resulted in the certification of seed lots with high levels of PVY and has been contributing to a general increase in PVY throughout the continent. The history of PVY in the Idaho Certification system will be discussed as well as some of the reasons the virus has been on the increase over the last two decades. In response to steadily increasing levels of PVY in Idaho seed, the industry recently abandoned the increasingly ineffective visual inspections for PVY in favor of the laboratory-based ELISA test for determination of PVY during the winter seed grow out. This change has been responsible for a dramatic decrease in PVY in the Idaho Seed System and could prove to be a template for the entire North American seed potato industry.
Organizers: Alexander Karasev, University of Idaho, Moscow, ID, U.S.A. and Stewart Gray, USDA-ARS, Cornell University, Ithaca, NY, U.S.A.