It would be difficult to find any commercial potato grower in Canada who is not aware that the disease known most commonly as “PVY” is a serious threat to the successful production of an acceptable seed lot. Potato virus Y has a wide host range, and not only does it affect plants in more than nine different families, but the disease can be caused by a great number of different strain types. It is found in most potato-producing countries around the world, where it negatively affects potato crops.
PVY infection of plant material in the field is transmitted by aphids, and the green peach aphid (Myzus persicae) has been found to be most effective as a viral vector. Other vectors include the potato aphid, soybean aphid and bird cherry-oat aphid. When aphids feed on the plant foliage, the virus is transmitted to the tubers. PVY particles get stuck in the aphid’s stylet (its piercing-sucking mouthpart). If the aphid then moves from an infected plant to a healthy plant and begins to feed, the virus particles are transmitted to the healthy plant. The stylet may remain infectious for several hours and the aphid can infect countless healthy plants during this time period.
Infected tubers are also a main source of initial inoculum in an emerging potato crop, and the planting of infected seed tubers in a field eventually becomes a major contributor to overall virus incidence in that field. Infected volunteer plants are also recognized as an important source of inoculum.
Potato plants display a range of symptoms when infected by PVY—but symptoms may vary depending on the strain of the virus, and not all varieties are similarly affected either. According to Khalil Al-Mughrabi, pathologist at the Potato Development Centre, New Brunswick Department of Agriculture, Aquaculture and Fisheries, foliar symptoms of PVY can range from extremely mild mosaic to severe foliar necrosis, to the death of infected plants. He points out some of the symptoms typical of the most common PVY strains:
- PVYN can cause a virtually symptomless current-season infection that is carried to the next generation,
- PVYO can cause dwarfing and premature death, and
- PVYC can cause necrosis, mottling of leaves, yellowing and leaf dropping, and premature death.
Al-Mughrabi says the PVY disease is constantly changing and new strains (e.g. PVYNTN) are emerging. These new strains not only reduce quantity (yield) but also quality (necrosis in the flesh) which make the tubers unmarketable.
PVY: An Evolving Concern
PVY is a complex disease, and one that is becoming of greater concern to potato producers in Canada and the United States. Potato specialists worked together from 2004–2006 to conduct an extensive survey with the aim of identifying the PVY isolates present in these two countries.
The results of this important survey are discussed in an article published in the journal Plant Disease entitled “Potato Virus Y: An Evolving Concern for Potato Crops in the United States and Canada.” The article points to PVY as a disease of extremely serious concern for future potato production in North America.
In reference to the three-year survey, the authors of the report state: “Results of the survey presented in this article clearly show that PVY in the Canadian/U.S. potato industry consists of a complex of strains. While the preponderance of isolates collected and characterized from the survey belonged to the ordinary PVYO strain, a number of variant types that have apparently arisen at some point as a result of recombination and mutation are not uncommon and appear to be spreading within North America. It is likely that mixed infections with recombinant strains resulted in even greater strain diversity.”
In view of the growing threat of PVY, what can growers do to protect their crops? Most growers are aware of basic management techniques, which involve a combination of several approaches including best management practices, as is the case with many other disease prevention programs.
Al-Mughrabi emphasizes the following basic guidelines for PVY control at the grower level:
- Use disease-free seed. Field reading and post-harvest test results may be used as guides to select seed lots with low virus levels.
- Plant resistant cultivars if possible.
- Properly destroy cull piles according to established guidelines.
- Rogue early in the season to remove infected plants from the field.
- Use insecticides to prevent the population of aphids from increasing within a field.
- Disinfect all cutting and planting equipment before it comes into contact with seed.
- Minimize mechanical damage of plants during cultivation and spraying, as well as visitor entry into potato fields.
- Avoid planting seed potatoes downwind from commercial fields.
- Control volunteer potato plants and weeds, such as wild rose, wild mustard and wild radish—they can act as hosts for aphids, and on which large populations can develop.
- Top-kill seed fields early to prevent late-season virus infection.
- Avoid planting susceptible varieties in close proximity to fields with varieties that have poor symptom expression.
- Use mineral oils.
- Plant crop barriers. These consist of non-PVY host crops (such as cereals) planted around small early-generation seed lots to provide buffers between the seed lots and the in flight of aphids. Aphids usually land at the interface between fallow ground and green crop.
Experts recommend that growers spray the crop with a mineral oil. Mineral oils seem to interfere with virus transmission, and while specialists are not certain of the exact reason for this interference, some believe the oils act as a deterrent when aphids feed on the foliage. Earlier this season, the Pest Management Regulatory Agency in Ottawa announced the approval of a minor use label expansion for Bartlett Superior 70 Oil and UAP Superior 70 Oil as a foliar spray for potatoes to reduce the spread of PVY.
One of several research studies to investigate the potential of mineral oils was undertaken from 2006 to 2008 by Debbie McLaren, crop production pathologist at Agriculture and Agri-Food Canada based in Brandon, Man. McLaren and her team used Superior 70 Oil, supplied by N.M. Bartlett Inc., for these studies.
“Mineral oils are an attractive option as they can reduce toxicity to both humans and the environment, can be applied with existing spray equipment, and can potentially reduce costs to the grower,” McLaren says.
The results of this study clearly show that the spread of PVY was reduced with the use of mineral oils. In 2006, PVY spread was reduced in three out of four replicates, and in 2007, the spread of PVY was also reduced with the application of oil, according to study results. The 2008 study included an assessment of the application of three different rates of Superior 70 Oil. The lowest incidences of PVY were observed when oil was applied at the intermediate rate.
A Combined Approach
The use of both mineral oils and crop barriers has been in the spotlight over the past number of years in Canada as a potential practical in-season solution to aphid control.
During a three-year study conducted in New Brunswick on mineral oils and aphid dispersion in potato fields, Gilles Boiteau, a research scientist in insect ecology at AAFC, led a team of researchers who investigated whether the control of PVY in seed potatoes could be improved by combining border crops and mineral oil sprays. They also wanted to determine whether the border crop acts as a physical barrier or a “virus sink.”
Boiteau and his team concluded that mineral oils alone are an effective barrier to PVY, and showed that borders alone act as a PVY sink, and not a physical barrier as such. “Combining the familiar mineral oil and the more recent crop border methods was almost twice as effective in reducing PVY incidence when compared to using one method alone,” Boiteau says.
The combination provided consistently high PVY control compared with the variable, and often a higher level of control than either method on its own.
Based on research findings thus far, the combination approach of oils and field barriers might turn out to be a very practical strategy for growers to more effectively combat PVY in the future. “Combining borders and oils provides producers with a tool to reduce year-to-year variation in the effectiveness of crop borders or oil sprays used separately,” says Boiteau.