Report of 2004 Research Activities funded by the Maine Potato Board.

Project title: Are the Necrotic Strains of PVY and Immediate and Manageable Threat to the Northeastern Potato Industry?

Principle Investigator: Stewart Gray, USDA, ARS, Dept. Plant Pathology, Cornell University, Ithaca, NY.

Collaborators: Dr. Keith Perry and Mr. Phillip Baldauf (PhD student), Dept. Plant Pathology, Cornell University, Ithaca, NY.

Executive Summary. The first year of greenhouse and field studies examined the effects of PVY strain on tuber infection, tuber necrosis symptom expression, and yield in multiple potato cultivars. A common strain of PVY (PVY^O), a tuber necrotic strain of PVY (PVY^NTN) and a genetic recombinant containing RNA from both the common and necrotic strains of PVY (PVY^N:O) were identified in the 2003 survey of potatoes grown in the Northeast. The incidence of necrotic and recombinant strains of PVY is low in the Northeast, but has been increasing in other seed potato production areas of the U.S. and Canada. We hypothesized that one reason for the increased incidence was these strains of PVY were more efficient at infecting tubers and being carried over into the subsequent years crop. Greenhouse studies indicated that a greater proportion of tubers were infected if the mother plant was infected with PVY^N:O or PVY^NTN than if the mother plant was infected with PVY^O. Only one (Yukon Gold) of six cultivars tested consistently developed symptoms of tuber necrosis in the greenhouse. Data are not complete from the field study, however, preliminary results do not fully support the results from the greenhouse experiments. Field grown Yukon Gold tubers readily express tuber necrosis when infected with PVYNTN; Red LaSoda and Snowden did not express tuber necrosis symptoms. All cultivars suffered a significantly greater yield loss if grown from infected seed than if infection was current season. While yield reduction was similar in most cultivars for all PVY strains, Yukon Gold yield loss (tuber weight and size) was greater when infected with PVY^NTN.

Project Report

In 2004, the Maine Potato Board allocated $5000 for research addressing whether the tobacco necrosis and potato tuber necrosis strains of PVY can be expected to spread and survive in potato seed stocks similarly to the common strain of PVY. This research is conducted by Mr. Phillip Baldauf, a PhD candidate in the Department of Plant Pathology, Cornell University. He is advised by myself and Dr. Keith Perry. A similar level of funding was requested from the NY IPM program in Feb 2004, but funding was cut to the IPM program and no grants were funded. Funding was also requested from the NPC/ARS Potato Grants program. While the proposal received high marks for scientific quality, virus diseases are not a high priority research topic for the program and no funding was awarded.

The foundation of commercial potato production is the planting of certified seed, a practice that has reduced or eliminated some pathogens, most notably viruses, so they are no longer a limiting factor in potato production. However, in recent years, the incidence of PVY^O has increased, and necrotic strains of PVY (e.g. PVY^N, PVY^NTN), once thought to be excluded from U.S., have been identified in all seed production areas. Our recent experiments funded by the MPB have identified the common strain of PVY (PVY^O) as the predominant strain of PVY infecting potatoes grown in the Northeast, however, the necrotic strains of PVY (PVY^N and PVY^NTN) as well as genetic recombinant strains (e.g. PVY^N:O) can be found at a low incidence. Additionally, we have shown that the impact of PVY on yield, quality and tuber infection levels is significantly influenced by cultivar and time of spread of the virus during the crop development. The 2004 experiments expanded our study to determine if PVY strain influenced the carryover of virus from one crop to the next.

A greenhouse study assessed if the strain of PVY infecting the mother plant would differentially affect the proportion of tubers that would become infected and carry virus over into the next years crop. Six cultivars representing different maturity classes were inoculated 3 wk post emergence with three different PVY strains, PVY^O, PVY^NTN and PVY^N:O. These isolates of PVY were identified in our 2002 and 2003 surveys of field-grown potatoes in NY and ME. Ten plants of each cultivar were inoculated with each PVY isolate. Plants were grown to maturity and tubers harvested. Following 1-2 months of dormancy, tubers were treated with gibberillic acid to induce sprouting. A composite of several sprouts from each tuber were tested by ELISA to determine the proportion of tubers infected with PVY. Results are presented in Figure 1. PVY^N:O infected a significantly greater number of tubers than PVY^O in each cultivar tested. PVY^NTN also infected more tubers than PVY^O except in Red LaSoda, but the differences were not always significant. These results support our hypothesis that recombinant and necrotic strains of PVY may have an advantage over the common strain with respect to tuber infection leading to higher levels of virus in the subsequent crop and a shift in the predominant strain from PVY^O to PVY^N:O or PVY^NTN. PVY^N:O has become the predominant isolate in several Midwestern states and Canadian provinces. PVY^N:O is not distinguished from PVY^O in the current testing schemes used by seed certification agencies and this will also contribute to its increased incidence. A repeat of the greenhouse experiment is underway.

A field study expanded on the greenhouse experiment. Three cultivars were inoculated with each of the three PVY strains. In this experiment, plants were either grown from PVY-infected tubers or were mechanically inoculated at 3 or 6 wk post emergence to simulate early and late season infection. Blocks of 32 plants (four, 8 plant rows) were replicated 3 times for each cultivar and virus isolate. Each block was inoculated with a different virus isolate and blocks were separated with a border of PVY resistant ‘Eva’ to minimize virus movement between blocks. Within each block plants were randomized into four groups; 8 plants were grown from virus-infected seed (generated in the greenhouse experiments and hand planted into the plots), 8 plants were inoculated 3 wk post emergence and 8 plants were inoculated 6 wks post emergence. The remaining plants were not inoculated. Plants were tested by ELISA at various times during the season to determine their infection status. Tubers were harvested by hand from each plant. To date, we have tested one random Rindite-treated tuber per plant to estimate the proportion of infected tubers for each treatment. The preliminary results are presented in Fig. 2. There is no significant difference in proportion of tubers infected among virus strains, but additional tubers that break dormancy naturally need to be analyzed. Per plant yield data were also collected and a preliminary data set is presented in Fig. 3. Yield was significantly reduced in plants grown from infected seed. Yield was also influenced by time of infection, cultivar and virus strain. Yukon Gold yields were significantly impacted by PVY^NTN. Interestingly marketable yield was only based on tuber size and weight and does not take into account tuber necrosis symptoms which were prevalent only in the Yukon Gold-PVY^NTN treatments.

Additional experiments related to the above study included further analysis of recombinant and necrotic isolates of PVY collected in the 2002 and 2003 surveys. We are finding low levels of many different genetic variants of PVY including tuber necrotic isolates. Currently there is little information on which of the North American cultivars are susceptible to tuber necrosis.

We also conducted a survey of potato seed lots imported into ME, WA and OR in 2004. The source of the seed going into all three states was primarily from surrounding areas, i.e. NY, WI and maritime provinces of Canada going into ME, and northwest states and western provinces of Canada going into WA and OR. Although necrotic and recombinant isolates of PVY were identified from all three states, the predominate strain (>80%) was PVY^O. This is in stark contrast to recent studies in the North Central U.S. and Central Canada (e.g. MN, ND, Manitoba) that indicate PVY^N:O is the predominate strain. We are currently orchestrating the national PVY survey of the 2004 seed crop from all seed producing states. This is the first of a planned 3 year study which should provide an accurate assessment of the PVY situation across the US and Canada and allow an effective management plan to be put into place.