Introduction

Potato virus Y (PVY) is an aphid-transmitted virus that causes disease in numerous solanaceous crops including tomato, pepper, tobacco, and potato. In potato, PVY can be a yield-limiting pathogen that can cause yield loss in heavily infected commercial lots and in selected, susceptible varieties. The virus may also cause post-harvest losses due to tuber necrosis and reduced storage quality. PVY has been managed in Wisconsin for decades, but in recent years it has re-emerged as a potentially serious disease problem. The emergence of new genetic recombinant strains of PVY that can cause mild disease symptoms, the over-wintering of potato-colonizing aphid species (green peach aphid, potato aphid), and the widespread adoption of potato varieties that express mild symptoms of PVY infection are all thought to contribute to the re-emergence of PVY in Wisconsin.

Both commercial and seed potato growers are at risk of direct yield loss due to PVY infection when levels exceed established tolerances. Commercial growers of fresh-market, processing, and chip potatoes are at risk of reduced yield and tuber storage quality if PVY-infected seed pieces are planted. Seed growers are at risk of having their lots downgraded or even rejected from certification due to PVY infection. (allowable tolerances for PVY infection in Wisconsin seed lots are 0.5 percent infected seed pieces for ‘Foundation’ class and 5.0 percent for ‘Certified’ class: Wisconsin ATCP 156).

The production season of 2024 has brought new challenges to our seed production area in north-central Wisconsin. First among these challenges is the amount of local inoculum we have replanted in the region that resulted from a challenging virus management season in 2023. This inoculum (many lots) will serve as a source for spread to other fields within the 2024 production season, and it is important to limit this movement of the virus. Another challenge for 2024 results from the presence and establishment of green peach aphid populations in the fields already. It will be critically important for producers to make efforts to limit potato-colonizing species from increasing in potato and spreading from these infested fields. The weather to date has brought significant rain which can limit access to fields for timely application of crop protectants.

Risk modeling

For most aphid chemical management tools, timing of application occurs with the appearance of the first, small colonies of potato-colonizing aphids. Spraying for colonizing aphids can reduce the spread of PVY within the field. Spray only when scouting indicates aphid populations have become established and scouts can identify small colonies of apterous (wingless) aphids. Critical factors affecting the efficacy of these spray applications include timing, application conditions and coverage. The green peach aphid, potato aphid, and (to a lesser extent) buckthorn aphid will colonize and reproduce on potato and are efficient vectors of PVY. Systemic insecticides applied at planting are critical components of early generation seed potato protection, however, they lose efficacy by mid-season.

Remember – most vectors of PVY do not colonize potato and their ability to transmit PVY to the plant will not be affected by systemic insecticides. In these instances, non-colonizing aphids (several species) will only intermittently move in and through potato over short durations. In these specific instances, we use anti-feedant compounds to include the paraffinic oils. Several species of non-colonizing aphids (e.g soybean aphid, corn leaf aphid, bird cherry-oat aphid, corn leaf aphid, English grain aphid and pea aphid) can be present moving through the seed crop and at predictable intervals through the season. The timing of these flights have been modeled as a collective, or cumulative risk model and for Wisconsin and the upper Midwest and these map-based projections are available daily at the Wisconsin Vegetable Disease and Insect Forecasting Network site. Once the model predicts the risk of non-colonizing aphids is moderate to high, producers are alerted to apply paraffinic oils on a 5-7 day reapplication interval.

The Upper Midwest Aphid Suction Trap Network, developed and maintained out of the University of Illinois at Urbana-Champaign, has been in operation since 2005 (WI locations listed). Originally conceived to detect soybean aphid flights and provide timely information to growers, the wealth of data now available after 14 years of operation has enabled us to model flight patterns for a number of different species captured by these traps. These predictive models can also be aligned to degree-days rather than calendar days to control for site-to-site and annual climatic variations.

We have taken these species-specific models and incorporated published PVY transmission efficiency values to compute risk-adjusted counts for each species. These counts are then added, and a single model is generated from this aggregate risk value that indicates the PVY risk window in Wisconsin. The model output is a curve where the primary rising segment indicates the start, midpoint, and end of the flight event (for an individual species) or the risk window for the aggregate PVY risk model. This model incorporates all potential PVY aphid vectors and so is more useful than flight models generated for any one species.

Based on the model illustrated below, the PVY risk window begins around 1838 degree-days (base 39°F, max 86°F), peaks around 2515 degree-days, and ends around 3266 degree-days. However, the risk of PVY transmission should be considered to extend through vine kill regardless of degree-day accumulation. Any method for obtaining degree-day data for your location will work (such as a personal weather station), if the min and max parameters are entered correctly.

At the VDIFN site, we offer a web-based model that illustrates the risk for any given date within the production season. The graphic below (July 19, 2024) illustrates where the risk is greatest in the medium to very high categories where the map colors range from orange to red.

Management guidelines

The following management guidelines were developed specifically for the 2024 season.

Limit PVY Introductions

• Do NOT replant seed potatoes with any measurable incidence of PVY. This is the absolute best defense. Relocate all lots with => 0.25% PVY off the farm. Locate these at least 3-5 miles from the farm and not on the windward side of the farm (e.g., S, SW, W borders)
• Rely only on laboratory testing for the estimates of disease incidence in lots from the Starks Farm – NOT visual.

Pre-Plant Considerations

• Sanitize all cutting and planting equipment between seed lots.
• Properly destroy/devitalize all cull potatoes below the ground surface.
• Ensure NO volunteer potatoes emerge in previously planted fields (scout these fields in 2024!!)
• Ensure no other local/neighboring problems with respect to volunteers or local virus sources (e.g., weedy nightshades in rotation years).

Planting Configurations

Arrange lots to be planted in long, parallel strips to facilitate spraying. Strips should be no wider than the 2X boom width.  Ensure the boom reaches and has proper overlap.

Locate highly susceptible varieties in the northern and eastern reaches of a field.  Locate less susceptible varieties in the southern and western reaches of a field.  Note:  On average, aphids migrate and fly into fields from the southwest.

Ensure all tractor spray alleys / strips are planted with a vigorous grass species.  Spray grasses in spray alleys each time an insecticide/oil is applied to the main potato crop.

Consider placement of border crops (e.g., rye grass) around all lots.  This may not be possible in all cases but should be considered.  Spray grasses in spray alleys each time an insecticide/oil is applied to the main potato crop. Source seed for border crops that contain an at-plant seed treatments containing a neonicotinoid insecticide (e.g., Cruiser, Gaucho, Poncho, etc.).

In rotation years without potato, plant crops that contain at-plant seed treatments containing a neonicotinoid insecticide (e.g., Cruiser, Gaucho, Poncho, etc.). Source seed for these rotation crops that contain either:

• thiamethoxam: CruiserMaxx Advanced (Soybean), CruiserMaxx Vibrance Pulses (Pea, Bean, Cowpea), CruiserMaxx Cereals (oat, barley, rye, wheat)
• imidacloprid: Gaucho 600 (many crops)
• clothianidin: Ponch0 600 (many crops and pearl millet!!)

In-Season Field Scouting

Scout a sub-sample of all potato fields weekly during the production season for colonizing aphids.  Twice monthly, scout all non-potato rotation crops for colonizing aphids.

1. Aphid populations are often aggregated in a field.  Anticipate where to look for “hot spots” of aphid activity.
2. Migrating aphids often land./aggregate along southern, western, tree lines bordering fields. They often alight along field edges in more open fields.  They often alight near drive rows within fields.  Wind eddies are often created along edges and aphids will ‘fall out’ of the moving air in these eddies.

All aphids are soft-bodied and pear-shaped with a pair of cornicles, or little horns, projecting from the rear end of their abdomens. Adult aphids may or may not be winged. Visit the following site (https://blogs.cornell.edu/potatovirus/pvy/aphid-vectors-of-pvy/#vectors) to see images of many aphids: some will colonize potato (green peach aphid, potato aphid, buckthorn/melon aphid). Most aphids will not colonize potatoes. Another gallery of aphid images is available at Wisconsin Vegetable Entomology.

Because of the spotty nature of infestations, look for aphids on a number of plants in several areas of the field. Each week, examine a whole leaf (not just leaflets) from at least 25 consecutive plants. Take this leaf from the mid-canopy of 25 separate plants. Look carefully at both the top and bottom surface of leaves. Repeat this process at 10 locations within fields weekly.

Look carefully at the aphids found in potato and determine if they are winged (alate) or wingless (apterous). At the Wisconsin Vegetable Entomology site, scroll to the bottom of the aphid page and look at the images for corn leaf aphid (wingless) and green peach aphid (winged). If small groups (e.g., 3-6) of wingless aphids are observed in potato, then you undoubtedly have a highly problematic, potato colonizing species getting established in potato.

Examine your rotation crops in a comparable way for aphids. Do not be surprised to find aphids in these crops. As noted previously, attempt to source all rotation crops to contain an at-plant seed treatment containing a neonicotinoid insecticide (e.g., Cruiser, Gaucho, Poncho, etc.). If aphid numbers are observed to be increasing in any local rotation crop, it may be appropriate to spray. Do NOT spray an inexpensive pyrethroid insecticide on these rotation crops if you decide to spray. Apply a foliar neonicotinoid (e.g., Actara, Admire Pro, Assail, Belay, Scorpion, Venom).

Initiate applications of paraffinic oils 2 weeks after full emergence from hilling, or just prior to the beginning of risk illustrated on the VDIFN site. Use an approved paraffinic oil at labeled rates weekly through the production season. Apply oil (and any insecticides) only after an irrigation event – not in advance. If possible, apply oils/insecticides during late evening to limit the potential for phytotoxicity.

When you begin applications of paraffinic oils, also consider weekly applications of insecticides over the entire crop if you are finding colonizing aphids present. Following is a suggested list of insecticides (Mode of Action and maximum application rate) to accompany paraffinic oil applications and these can all be reviewed in the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.

Trade name	Chemical name	Mode of Action Class	Max labeled rate (single application)
Admire Pro	imidacloprid	Group 4A	1.3 fl oz/ac
Actara 25WG	thiamethoxam	Group 4A	3.0 oz/ac
Assail 30SG	acetamiprid	Group 4A	4.0 oz/ac
Belay	clothianadin	Group 4A	3.0 fl oz/ac
Beleaf 50SG	flonicamid	Group 29	2.8 oz/ac
Exirel 10SL	cyantraniliprole	Group 28	13.5 fl oz/ac
Fulfill 50WG	pymetrozine	Group 9B	5.5 oz/ac
Movento HL	spirotetramat	Group 23	2.5 fl oz/ac
PQZ	pyrifluquinizon	Group 9B	3.2 fl oz/ac
Sefina Inscalis	afidopyropen	Group 9D	6.0 fl oz/ac
Sivanto HL	flupyradifurone	Group 4D	7.0 fl oz/ac
Torac	tolfenpyrad	Group 21	21.0 fl oz/ac
Transform 50WG	sulfoxaflor	Group 4C	1.5 oz/ac
Venom 70SG	dinotefuran	Group 4A	1.5 oz/ac

Initiate applications after the appearance of colonizing aphids have become established in the crop. An initial foliar application should be applied to the entire field and followed by a second foliar application one week later. Only two successive applications of any compound should be implemented as a foliar option per crop season for control of colonizing aphids before rotation to a new mode-of-action. Continue to scout field throughout the time interval of the application series and consider rotation to another effective aphicide mode of action if colonizing aphids persist in the crop.

Should be applied with mild, penetrating surfactant and ensure the tank pH is not alkaline (pH < 7.0). Ground-application advised. Only two successive applications of Admire Pro should be implemented as a foliar option per crop season for control of colonizing aphids before rotation to a new mode-of-action.

All insecticide and paraffinic oil combinations should be delivered using extended range, air induction flat fans delivering a minimum of 25 gpa.

Always remember to introduce the products into the tank in the following order: (1) water soluble packets (WSP) (2) wettable powders (WP) (3) water dispersable granules (WDG) (4) flowable liquids (F, L) (5) emulsifiable concentrates (EC) and (6) adjuvants and/or oils. Always allow each product to fully disperse before adding the next product.

Each of the above can be mixed with common fungicides. Care not to add any foliar fertilizers or micronutrients as these compounds are often untested in terms of compatibility.