Report of 2005 Research Activities funded by the Maine Potato Board.

Project title: Are the Necrotic Strains of PVY an 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 second 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. The ordinary strain of PVY (PVY^O), a tuber necrotic strain of PVY (PVY^NTN) and a genetic recombinant containing RNA from both the ordinary and necrotic strains of PVY (PVY^N:O) were used in the study. These viruses were identified in the 2003 survey of potatoes grown in the Northeast and represent three strains of PVY that are found throughout the North American seed potato production areas. The incidence of necrotic and recombinant strains of PVY remains low in the seed potato production areas of the U.S. and Canada, but they have the potential to become more prevalent. We hypothesized that one reason for the emergence of these strains of PVY in recent years was that they were more efficient at infecting tubers and being carried over into the subsequent year’s crop. Greenhouse experiments 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. This trend was not always evident in field studies and varied depending on cultivar. Only one (Yukon Gold) of six cultivars tested consistently developed symptoms of tuber necrosis in the greenhouse. Field grown Yukon Gold tubers readily express tuber necrosis when infected with PVY^NTN; Red LaSoda and Snowden did not express tuber necrosis symptoms. All three field grown 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. Another notable finding in both years of the study was that many late season inoculations with any PVY isolate are not easily detected in the foliage of the plant (symptoms or ELISA), yet a high proportion of tubers generated by late season inoculated plants do contain high levels of virus and generate infected plants when used as seed. Therefore, in-field visual inspections of PVY incidence may not be an accurate indicator of winter test results.

Project Report

In 2005, the Maine Potato Board allocated $8000 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 ordinary strain of PVY (PVY^O). This research is conducted by Mr. Phillip Baldauf, a PhD candidate in the Department of Plant Pathology, Cornell University. He is advised by Drs. Stewart Gray and Keith Perry. Our PVY research program was also supported by a grant from the National Potato Council and APHIS that funded a national survey of all late generation seed potato lots from 15 seed producing states. Funding was also available for in-depth characterization of a representative sample of the virus isolates found in the survey. The purpose of the survey is to determine the genetic makeup of PVY isolates infecting the crop and to provide information on the symptoms, detection and spread of the various PVY strains. Of direct importance to the MPB, this research allows us to continue to monitor the PVY strain incidence in Maine and relate the PVY population structure in Maine to other seed production areas.

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. Experiments partially funded by the MPB prior to 2004 identified that the ordinary strain of PVY (PVY^O) was 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) were found at a low incidence. A new strain of PVY, PVY^O5, was also discovered. This strain is most closely related to PVY^O, but it contains a mutation in the coat protein gene that allows the virus to be detected by a monoclonal antibody specific for tobacco necrotic strains of PVY. Additionally, we discovered that the impact of PVY on yield, quality and tuber infection levels was significantly influenced by cultivar and time of virus infection during the crop development. Experiments partially funded by the MPB in 2004 and 2005 expanded our study to determine if PVY strain influenced the carryover of virus from one crop to the next. Our 2004 report to the MPB did not contain all the final data from the 2004 field experiments since tubers were still being analyzed. Here we report on the completed 2004 field experiments and 2005 greenhouse experiments and provide preliminary data from the 2005 field experiments.

The 2005 greenhouse experiments focused on the reproducibility of necrotic symptoms on tubers from plants inoculated with various PVY strains and grown to maturity under greenhouse conditions. PVY^O isolates did not produce any reproducible symptoms on any Yukon Gold tubers, whereas five different necrotic isolates produced symptoms on Yukon Gold tubers. However, symptom type varied and symptom expression was not always consistent in every experiment for each of the necrotic isolates. PVY^NTN isolate FL40D consistently produced deep necrotic pits on Yukon Gold tubers in 7 of 8 experiments (Fig. 1A). PVY^NTN isolates PB312 and PB313 consistently produced more typical raised ring symptoms on Yukon Gold tubers in 7 of 8 and 3 of 4 experiments, respectively (Fig. 1B). PVY^N:O isolate AL1 and PVY^O5 isolate PB343 occasionally produced atypical symptoms on Yukon Gold tubers (Fig. 1C), and virus was often not recovered from these tubers or from plants sprouted from these tubers.

The final field disease incidence data from the 2004 field experiments (preliminary data were described in last years report) appears to support several of our hypotheses 1) older plants tend to be more resistant to PVY infection, 2) differences exist among cultivars, and 3) there is a tendency for the necrotic strains to infect more plants than the ordinary strain (Fig. 2). Also, notice that we did observe a fair amount of natural infection by aphids based on the number of uninoculated plants that were infected (Fig. 2). When we analyzed the tubers from those plants, a very different picture emerged. Although a relatively small proportion of the late season inoculated plants tested positive at the end of the season, a large proportion of the tubers produced by those plants gave rise to infected plants (Fig. 3). In addition, a very high proportion of what we thought was our healthy control actually were inoculated late in the season by aphids and did generate infected tubers (Fig. 3). These data indicate that although the foliar portion of the plant appears to be immune to virus infection late in the season, this is not the case. The virus does infect the plant and moves rapidly from the inoculated foliage to the developing and bulking tubers. These tubers will produce virus-infect sprouts in the subsequent season.

A second year of field experiments essentially repeated the 2004 experiments with a slight change in block arrangement aimed at reducing interplot and intraplot movement of the virus. Three cultivars were inoculated with each of three PVY strains, PVY^O, PVY^N:O or PVY^NTN. In this experiment, plants were either grown from PVY-infected tubers or were mechanically inoculated at 5 wk post emergence to simulate late season infection. Blocks of 10 plants were replicated 3 times for each cultivar, virus isolate and inoculation time (i.e. seed inoculation, late season inoculation or uninoculated control). Blocks were separated with a border of PVY resistant ‘Eva’ to minimize virus movement between blocks. Plants were tested by ELISA at various times during the season to determine their infection status. Tubers were harvested by hand from each plant in 2005.

The preliminary data from 2005 supports the 2004 data, i.e. analysis of leaf tissue at the end of the season indicates that many of the late season inoculated plants did not contain much if any virus (Fig. 4), but when tubers were analyzed a high proportion tested positive for PVY (data not shown). As might be expected, the late season infections did not affect yield, although yield was significantly impacted when plants were grown from infected tubers (Fig. 5). 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. So, the actual marketable yield of Yukon Gold infected with PVY^NTN would have been considerably less than shown in Fig. 5 due to surface blemishes.

The national survey of late generation seed potato lots indicate that the average overall PVY incidence is relatively low, <3%, but that there is considerable variation in lots which ranged from 0% to over 80% incidence. High incidence tends to be associated with cultivars that do not consistently express symptoms of PVY infection in the foliage, e.g. Shepody, Russet Norkotah, Silverton. The vast majority of the PVY isolates belong to the PVY^O strain, but we found low levels of many different genetic variants of PVY, including necrotic isolates, in most seed production areas. There are some regional differences in strain distribution, but we do not know at this time if the incidence of the necrotic strains is increasing or is more likely to be associated with certain geographical regions or potato cultivars. Results from the 2005 survey and hopefully a 2006 survey should provide the answers to these and other questions regarding the real threat of necrotic strains of PVY to the U.S. seed potato industry.