DIVERSITY AND HOST RESISTANCE TO NOVEL POTATO VIRUS Y ISOLATES IN COLORADO

The U.S. potato industry and over 330 growers identify PVY as the number one disease problem affecting the seed potato crop (Stewart Grey, personal communication). The commercial crop will be at risk if the tuber necrotic strains become widespread. The emergence of other tuber necrotic viruses such as PMTV and TRV only exacerbates this problem; these are, however, not the focus of the proposal. Controlling tuber necrotic viruses and PVY remains an ongoing challenge, and current methods are not working effectively. Virus diseases cannot be managed using chemical or biological protectants as is often effective for fungal, bacterial, or nematode pathogens. Chemical control of the vectors by insecticides and fungicides is not effective against PVY.Recent data indicates that nearly 40% of the U.S. seed lots had detectable levels of PVY, and more than 20% of all lots exceed the 2% rejection level prescribed by the State National Harmonization Program. Much of the known virus incidence in the seed can be traced to four factors: (1) virus strains that induce mild or transient foliar symptoms, (2) the emergence of novel or "rare" recombinant strains, (3) release of tolerant cultivars that do not express foliar symptoms, and (4) late-season infections that do not manifest symptoms in the foliage, but a high percentage of tubers in some cultivars can still become infected. All these factors lead to low virus incidence observed and reported from summer field inspections conducted by seed certification programs. Still, high tuber infection levels may exist in the seed, requiring breeding programs to grow out plants for an entire season before they can determine if a resistance response is occurring. A post-harvest test of all seed lots is prohibitively expensive and not done in most states. Knowing which cultivars are most at risk for inaccurate visual summer field evaluations and having reliable diagnostics to determine accurate infection levels would allow limited certification resources to be directed where most efficacious. Seed certification agencies have identified the development of training materials and applications that will assist seed certification inspectors and growers in identifying the various foliar and tuber symptoms induced by the various strains of PVY and other viruses predominant in that region as a priority. This need is addressed in this proposal.Virus resistance in potatoes has not been a priority for U.S. breeding programs until recently. Now it is recognized that virus resistance is a desirable trait, and, out of necessity, it has become an important strategy to manage virus incidence and spread. Although resistance genes to PVY have been identified in wild and cultivated potato, only a few provide broad-spectrum resistance to PVY [25; 38]. However, only a few newly released cultivars have been screened against all predominant PVY strains. None have been screened against emerging novel and "rare" recombinant strains to determine their susceptibility or their symptom expression when infected. Growers and breeders need to know which are most susceptible to viruses, emphasizing those susceptible to tuber necrosis and methods to facilitate the development and release of cultivars with levels of resistance useful in virus disease management. These needs are addressed in this proposal.ObjectivesObjective 1. Characterize the diversity, prevalence, and incidence of PVY strains in Colorado.This research-led objective aims to characterize the genetic diversity of PVY in the San Luis Valley.Objective 2. Identify host plant resistance to recombinant PVY strains in the Colorado potato breeding germplasm.The goal of this research-led objective is to screen selections (a combination of early, intermediate, and late generations) in the Colorado Potato Breeding Program (CPBP) for their response to the predominant recombinant strains of PVY in the Valley (identified as part of objective 1)Objective 3. Develop effective methods to provide research-based information regarding PVY detection and disease management to potato growers.The goal of this extension-led objective is to provide resources and training to stakeholders.