In recent years, growers have been reporting increased losses in potato and tobacco across the valley, the state, and the nation, due to Potato Virus Y (PVY). PVY is an aphid-transmitted virus that affects many crops in the solanaceous family including potato, tobacco, tomato, and pepper, as well as many solanaceous weeds. PVY can cause 50-80% yield losses in heavily infected potato fields, and also causes reduced storage quality and post-harvest tuber death. PVY has been present throughout the US for decades causing little damage, but has recently re-emerged as a major threat to potato and tobacco production for several reasons including the development of new strains of the virus that cause tuber necrosis, widespread planting of varieties that show little or no PVY symptoms leading to undetected reservoirs of the pathogen, and contamination of seed stocks.
Strains & Symptoms
For decades the PVY strain that was present in the US and Canada, known as PVYO for "ordinary", caused noticeable mosaic symptoms on potato foliage. Symptoms of PVYO on potato and tobacco may include leaf streaking, mottling, or mosaic, or in severe cases could cause leaf death, leaf drop and plant stunting. In tobacco, vein-banding or vein-clearing is common. Symptoms on potato vary by cultivar, with some varieties showing only mild foliar symptoms while others, especially Dark Red Norland and Yukon Gold, are extremely susceptible and show rugose mosaic symptoms (wrinkly deformation of leaves). Since the PVYO strain caused noticeable symptoms on foliage, growers could rogue out infected plants visually, and levels of PVY in seed lots and crop damage and loss remained low. However, new varieties of potato have recently been released which do not show typical stunting and mosaic symptoms, and may exhibit no symptoms at all, but still carry the virus. These “carriers” contribute to the undetected spread of the disease through fields and seed lots. Varieties that serve as PVY carriers include: CalWhite, Gem Russet, GemStar Russet, Russet Norkotah, Shepody, and Silverton Russet.
Furthermore, new strains of PVY have been making their way into the North American potato system which cause different symptoms, or no symptoms at all. PVYN, the N standing for necrotic (dead), causes severe necrosis on tobacco foliage rendering it unmarketable, but only mild leaf mottle and necrosis on potato foliage, and therefore often goes undetected in potato fields and seed lots. There is evidence that the PVYO and PVYN strains have recombined to produce strains with some characteristics of both parents, and these are dubbed PVYN:O. Another newly evolved strain causing a lot of damage to both potato and tobacco is PVYNTN, which stands for a tuber necrotic variant of the PVYN strain. This strain causes severe necrosis on tobacco foliage, mild foliar symptoms on potato, but causes necrotic flecking and ringspots on potato tubers, leading to severe losses in some varieties. Again, symptoms vary by cultivar with some showing mild or no symptoms while others, including Yukon Gold, exhibiting severe tuber necrosis.
Disease Cycle. Infected seed tubers are by far the most important source of PVY. Seed tubers are certified by state departments of agriculture to ensure little to no viruses is present. “Foundation” seed is the best grade and should have less than 0.55% total virus (including viruses other than just PVY) while “certified” seed may have anywhere from 0.56-5.0% total virus. Investing in foundation seed is the best way to keep PVY off your farm. Once the virus is present in a field it is transmitted mechanically or is vectored by aphids. Mechanical transmission occurs by movement of virus particles through plant sap via wounds caused by wind, workers, and equipment moving through the field.
Aphids are much more efficient at transmitting the virus and are considered the most important mode of disease spread. PVY is non-persistently transmitted, meaning that aphids can pick up virus particles on the tips of their mouthparts while probing or feeding in a matter of seconds and can spread the virus just as quickly to healthy plants—the virus does not have to move through the aphid vector at all as in persistently transmitted virus diseases. Since the virus is spread quickly through aphid probing, insecticides are not very useful in reducing spread of the virus by aphids, since some insecticides actually cause aphids to twitch and increase their probing activity. More than 50 species of aphids can spread PVY, including species which are not considered pests of potato or for which potato is not a preferred host. The most important aphid vectors in the Northeastern US are the green peach aphid, the potato aphid, the bird cherry-oat aphid and the soybean aphid.
Other solanaceous crops may harbor the disease without showing symptoms. This includes crops such as tomato and pepper as well as many weed hosts such as hairy nightshade. These asymptomatic carriers serve as reservoirs for PVY and contribute to undetected spread of the disease throughout the season, but luckily, true seed cannot be infested with PVY and so you don’t need to worry about the virus surviving between crops in weed seed.
Transmission of viruses occurs most easily in young plants, and the virus can more easily move throughout the plant when it is young. Furthermore, if young plants are infected the virus has more time to build-up within the plant and cause more severe symptoms or migrate to the tubers. Weather and other environmental conditions also influence the severity of PVY, and the expression of symptoms in different crops and cultivars.
- Use only certified disease free seed tubers. For many years seed certification programs were highly successful in maintaining low levels of PVY is seed stock, but the presence of new strains that show mild or no symptoms, the widespread planting of symptomless “carrier” varieties, it has become harder to produce disease free seeds. However, seed certification programs do post-season testing which reliably detect the virus and classify seed lots as “foundation” or “certified”, meaning they harbor less than 0.55% total virus, or 0.56-50.% total virus, respectively..
- Choose potato varieties carefully.
- Plant resistant varieties: Villeta Rose, Eva, Rio, Grande Russet, and Premier Russet
- Avoid planting symptomless varieties: Shepody, Silverton Russet, and Russet Norkotah
- Reduce areas of bare soil around or within the crop. Aphids find plant tissue based on the color contrast between the foliage and the bare ground, so if there is no bare ground the aphid cannot “see” the crop.
- Plant a barrier crop. Plant a border of non-host crop such as rye, sorghum, or wheat, several yards wide around your potato or tobacco planting. Migrating aphids will be more likely to land on the barrier, and when they probe the barrier crop to see if it is a suitable host their mouthparts will be effectively cleaned of virus particles. Remember not to leave any bare ground between the crop and the barrier.
- Control solanaceous weeds. These include all of the nightshades which can be symptomless carriers of the virus, increasing disease severity and spread.
- Rogue out affected plants. Infected plants will spread virus to their neighbors so walk the field and pull out any plants with signs of leaf mosaic or necrosis. Some of the new strains do not cause mosaic symptoms but some do, as does PVYO, and losses in yield and storability can occur from any PVY strains. Volunteer potato plants that pop up in spring should always be rogued out, as these could be infected with PVY or other diseases such as late blight.
- Aphid control. Since aphids spread PVY non-persistently, insecticides are often ineffective and are not considered a valuable control strategy. However, repellents such as horticultural oils (especially early on when aphid populations are low and plants are young) and newer behavior modifying pesticides may be of use, including: Assail, Belay, Admire Pro, Fulfill, Movento, Platinum.
- Plant early and kill vines early. Aphid populations skyrocket in the late season, so planting early may allow you to get in more growth free of high densities of aphid feeding and potential disease spread. Growers may hesitate to kill vines too early because tuber size may be compromised, but if PVY is present, killing vines will prevent it from spreading to tubers causing total loss. Virus transmission from foliage to tuber takes 14-26 days depending on plant age.
If you have experienced PVY in your fields please get in touch with us so that we can begin to better document the extent of damage being caused in MA and so we can put you on a list of contacts for upcoming outreach events and workshops on PVY. Write us at firstname.lastname@example.org or call 413-577-3976.
--Susan B. Scheufele, 2016