Potato virus Y (PVY) is one of the most damaging potato viruses and a serious threat to the successful production of an acceptable seed lot around the world. It can cause mosaic on potato leaves, and affect yield and quality. Yield losses range from 10 to 80 per cent. Affected tubers are unusable for propagation. Late season virus transmission by aphids is difficult to detect in field inspections as it seldom produces recognizable symptoms.
Six different PVY strains affect the potato crop. PVYO, PVYC and PVYN are the historical strains. They were gradually displaced in Europe and North America by the strains PVYNTN, PVYN:O and PVYN-Wi which are new recombinants of the historical strains. These strains not only reduce quantity (yield) but also quality (necrosis in the flesh) making the tubers unmarketable.
Symptoms in potatoes vary widely with the virus strain and the cultivar, ranging from extremely mild mosaic to severe foliar necrosis to death of infected plants. The main sources of infection from the virus are infected seed and infected volunteer plants.
PVY is also spread in potato fields by winged aphids flying from an infected source plant to a healthy one. When aphids feed on the potato plant foliage, the virus is then transmitted to the tubers. Green peach aphids and other aphid species are vectors to the virus. Spread of PVY by aphids is in a non-persistent manner (stylet-borne). If aphids’ populations are large, PVY spread can be very extensive.
Younger plants are more easily infected by PVY than older ones. Time required for virus translocation to the tuber is shorter in younger plants. Time required for aphids to acquire and transmit PVY is very short (seconds or minutes), so insecticides cannot act rapidly enough to kill the aphid before the virus is transmitted. Since acquisition and inoculation occur very quickly, aphids do not need to colonize potatoes to transmit PVY. They can come from other crops, shrubs or weeds, etc.
The breaking of the tuber’s dormancy activates transport mechanisms through the phloem of the young tissues of the growing potato sprouts, which are actively draining sap out of the mother tuber. In PVY-infected tubers, this sap is loaded with virus particles that will concentrate at the rose end of the tuber where most of the sprouts are located. Roots will grow horizontally from the base of the sprout while its apex will grow vertically, emerge from the soil, and give rise to a stem with branches and leaves. It is unknown whether all the stems growing from the same tuber will be infected through an upward systemic movement of PVY particles from the mother tuber. The descending flow of sap from the stems to the progeny tubers, which allows the translocation of the virus, is well documented in literature.
The use of mineral oils and a combined approach of mineral oils and crop barriers have been under the spotlight the past number of years here in Canada and around the world as potential practical in-season solutions to aphid control. These methods have been researched by experts and applied by seed growers as well. Mineral oils are an attractive option as they can reduce toxicity to both humans and the environment; they can be applied with existing spray equipment and potentially reduce PVY spread and consequently increase profit to the grower.
Recent studies have also shown that the combination of mineral oils and insecticides act to minimize on-farm PVY spread on seed potato farms. Frequent mineral oil spraying that starts early and continues all season long, supplemented often with insecticides in a simultaneous spray, have shown the greatest potential.
From 2010 to 2014, extensive on-farm research was undertaken in New Brunswick to determine the major factors involved in PVY spread. In collaboration with potato producers, trials were conducted in separate commercial seed, processing and table stock fields representing various potato varieties commonly grown in New Brunswick. The overall study objective was to quantify the importance of virus inoculum, vector, and the numbers, timing and types of insecticide and mineral oil sprays used.
Over the five seasons under study, on-farm PVY spread has declined substantially from 14 per cent in 2010 down to 1.2 per cent in 2014. The average PVY level in seed lots tested as part of the provincial post-harvest seed testing program decreased from 3.34 per cent in 2012 to 0.42 per cent in 2016, an eight-fold reduction. Number of seed lots testing clean (0% PVY) rose from 24 per cent in 2012, to 71 per cent in 2016. Each one per cent decrease in virus level in seed will lead to a 150 lb. increase in yield per acre. Factors correlated with decreased spread included low PVY inoculum planted in the field, numbers of foliar mineral oil and insecticide sprays, later crop planting and earlier first spraying dates.
PVY is a complex disease and one that is becoming of greater concern than ever to potato producers in Canada and the U.S. In view of this growing threat of the PVY disease, what can growers do to protect their crops? Most growers are aware of the basic management techniques which, as is the case with many other disease prevention programmes, are a combination of several approaches, including best management practices.
The following are 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 contact with seed.
- Minimize mechanical damage of plants during cultivation and spraying. Minimize visitor entry into potato fields.
- Avoid planting seed potatoes downwind from commercial fields.
- Control volunteer potato plants and weeds (wild rose, wild mustard, wild radish are hosts for aphids 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. Consist of a non-PVY host crop planted around small early-generation seed lots to provide a buffer between the seed lot and the in-flight of aphids (e.g. cereals). Aphids usually land at the interface between fallow ground and green crop.