In This Issue:
- Disease forecasting updates for potato early blight and late blight
- Cucurbit downy mildew updates
- White mold on vegetables
- Seedcorn maggot and Colorado potato beetle risks and management
Amanda Gevens, Chair, Professor & Extension Vegetable Pathologist, UW-Madison, Dept. of Plant Pathology, 608-575-3029, gevens@wisc.edu, Lab Website: https://vegpath.plantpath.wisc.edu/.
Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations will be posted at our website and available in the weekly newsletters. Thanks to Ben Bradford, UW-Madison Entomology for supporting this effort and providing a summary reference table: https://agweather.cals.wisc.edu/thermal-models/potato. A Potato Physiological Day or P-Day value of ≥300 indicates the threshold for early blight risk and triggers preventative fungicide application. A Disease Severity Value or DSV of ≥18 indicates the threshold for late blight risk and triggers preventative fungicide application. Data from the modeling source: https://agweather.cals.wisc.edu/vdifn are used to generate these risk values in the table below. I’ve estimated early, mid-, and late planting dates by region based on communications with stakeholders. These are intended to help in determining optimum times for preventative fungicide applications to limit early and late blight in Wisconsin.
Late blight of potato/tomato
The usablight.org website (https://usablight.org/) indicates no new confirmed reports of late blight on tomato or potato in the US this past week. There was a US-23 late blight strain type confirmation in Collier County FL in 2025 (now several weeks old). The site is not comprehensive. This genotype/clonal lineage is generally still responsive to phenylamide fungicides meaning that Ridomil and Metastar fungicides (mefenoxam and metalaxyl) can still effectively control late blight caused by these strain types.
Cumulative late blight disease severity values (DSV) since date:
| Location | May 10 | May 15 | May 20 | May 25 | Last 14 days | Last 7 days |
|---|---|---|---|---|---|---|
| Rhinelander | 5 | 5 | 4 | 4 | 4 | 4 |
| Antigo | 6 | 6 | 5 | 5 | 5 | 5 |
| Plover | 2 | 1 | 1 | 1 | 0 | 0 |
| Hancock | 0 | 0 | 0 | 0 | 0 | 0 |
| Grand Marsh | 0 | 0 | 0 | 0 | 0 | 0 |
| Arlington | 3 | 3 | 3 | 0 | 0 | 0 |
| Spring Green | 4 | 4 | 4 | 1 | 1 | 1 |
Early blight of potato/tomato
We continue to steadily increase P-Days in potatoes. Accumulations were 33-59 or roughly 7/day over the past week. Values will continue to amass and develop conditions optimum for early blight disease caused by Alternaria solani. Earliest inoculum typically comes from within a field and from nearby fields. Once established, early blight continues to create new infections due to its polycyclic nature – meaning spores create foliar infection and the resulting lesion on the plant can then produce new spores for ongoing new infections in the field and beyond. Early season management of early blight in potato can mitigate the disease for the rest of the season. https://vegpath.plantpath.wisc.edu/diseases/potato-early-blight/
Cumulative early blight potato physiological days (P-days) since date:
| Location | May 10 | May 15 | May 20 | May 25 | Last 14 days | Last 7 days |
|---|---|---|---|---|---|---|
| Rhinelander | 173.0 | 140.3 | 119.6 | 114.2 | 81.7 | 32.9 |
| Antigo | 180.1 | 149.1 | 127.1 | 122.7 | 86.2 | 33.9 |
| Plover | 226.6 | 191.3 | 164.8 | 155.6 | 110.0 | 52.9 |
| Hancock | 229.9 | 193.9 | 167.4 | 158.5 | 112.7 | 54.6 |
| Grand Marsh | 235.3 | 199.2 | 172.0 | 162.0 | 116.3 | 57.4 |
| Arlington | 244.5 | 207.7 | 179.7 | 168.2 | 119.8 | 58.6 |
| Spring Green | 249.4 | 212.6 | 182.1 | 169.2 | 119.8 | 60.1 |
Fungicides can provide good control of early blight in vegetables when applied early on in infection. Multiple applications of fungicide are often necessary to sustain disease management to time of harvest due to the typically high abundance of inoculum and susceptibility of most common cultivars. For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website which is annually updated.
For custom values, please explore the UW Vegetable Disease and Insect Forecasting Network tool for P-Days and DSVs across the state (https://agweather.cals.wisc.edu/vdifn). This tool utilizes NOAA weather data. Be sure to enter your model selections and parameters, then hit the blue submit button at the bottom of the parameter boxes. Once thresholds are met for risk of early blight and/or late blight, fungicides are recommended for optimum disease control. Fungicide details can be found in the 2025 Commercial Veg. Production in WI Extension Document A3422: https://cropsandsoils.extension.wisc.edu/articles/2025-commercial-vegetable-production-in-wisconsin-a3422/
Cucurbit Downy Mildew
This national cucurbit downy mildew information helps us understand the potential timing of arrival of the pathogen, Pseudoperonospora cubensis, in our region, as well as the strain type which can give us information about likely cucurbit hosts in WI – as well as best management strategies. Clade 1 downy mildew strains infect watermelon and Clade 2 strains infect cucumber. I am hosting a cucurbit (and basil) downy mildew sentinel plot at the UW Hancock Agricultural Research Station this summer. This ‘sentinel plot’ is a non-fungicide-treated collection of cucurbit plants which are observed weekly for disease symptoms. I will report the presence/absence of downy mildew from this plot in this newsletter throughout the growing season. Additionally, I keep an eye on the downy mildew spore trapping work of Dr. Mary Hausbeck at Michigan State University and include this information as relevant to WI https://veggies.msu.edu/downy-mildew-news/. On May 27, Clade 2 downy mildew spores were confirmed in an Allegan Co. During June 2 – 6, Clade 2 spores were also confirmed in Muskegon, Ingham, and Bay Co. traps. Downy mildew disease was confirmed in Allegan, Muskegon, and Bay County commercial cucumber fields during 5/29-6/3 (map below with red coloration indicating counties with confirmed reports, from Hausbeck, MSU).
For more information: Cucurbit Downy Mildew – UW Vegetable Pathology – UW–Madison
White Mold
Sometimes called Sclerotinia stem rot, white mold is a soilborne fungal disease caused by Sclerotinia sclerotiorum that impacts potatoes and many other broad-leaved crops (>400 plant species). The severity of the disease, and resulting yield/quality losses, can vary greatly and depend upon the quantity of inoculum in soils, environmental conditions, and planting factors including cultivar, crop rotational history, and plant spacing. Symptoms have been showing up in high tunnels in the past week in Wisconsin vegetables. The pathogen may have been introduced in infested seed or soil prior to establishment of the high tunnel, or could have been introduced while growing specialty crops in the tunnel. The high tunnel/greenhouse status can limit fungicide selections, but we have seen effective management with careful and diligent use of the biological fungicide Contans (included on fungicide list below) when incorporated into soil of high tunnels for high value crop production.
Symptoms on Potato (similar to other vegetable crops). Symptoms develop first in the lower leaves and stems of the plant, typically ~2 weeks after row closure. Water-soaked lesions typically form at the stem branch points or where stems are in contact with the soil. In potato, floral infections can occur and lead to stem infections either from movement through the base of the flower, or through the senescing flower dropping onto other lower plant parts and spreading infected tissues. Based on my field observations over the past few years in Wisconsin, most infections initiate on stem branch points and in stem contact with the soil. Lesions are often covered in white, cottony fungal growth. Lesions can expand and girdle stems resulting in wilting of sections of plants or entire plants leading to plant death. Eventually, lesions turn light brown and nearly white in color once they’ve dried out. At this time, you can often crack open the stems and find the black, hard fungal structures referred to as sclerotia. During the progress of infection, any additional contact with other plant parts can result in the spread of infection.
Disease cycle. The pathogen overwinters as sclerotia in the soil or in infested crop residue. Sclerotia can be moved in soil within a field during cultivation, in moving water, soilborne sclerotia form a mushroom structure under plant canopies, can move relatively short distances from where they’re discharged (roughly 1 mile). The apothecial cups form earlier in the summer/late spring from the sclerotia in the top 2 inches of soil when we have cool temperatures (50-70°F), high relatively humidity (95-100%) and several days of moist soil. These conditions are typically met after canopies have closed and soil surfaces are shaded (and there is low air circulation). In many potato cultivars this aligns with 70-100% bloom. The movement is typically from the apothecial cup/mushroom to the plants immediately above/surrounding it. The soilborne sclerotia can also be moved to previously non-infested fields in soil and debris on contaminated equipment. There is little or no plant-to-plant spread of white mold during the growing season, with infections initiated from the overwintered sclerotia. The sclerotia can remain viable in the soil for roughly 5 years.
Management. An integrated program of cultural practices and fungicide applications is necessary to manage white mold in potato and other vegetables. Currently, the application of fungicides is a primary management approach for susceptible potato cultivars. The choice of fungicide, application method, and timing of application are important. Fungicide treatments should be initiated when plants reach the full bloom stage or at row closure, to help prevent the flower petals and stem junctions from becoming infected by ascospores. This timing also enhances coverage in the lower canopy to manage infections caused by limbs touching the soil and sclerotia directly.
A listing of fungicides registered for white mold management in potato in WI is provided below (from Commercial Vegetable Production in Wisconsin, A3422):
Several fungicides are labeled for the control of white mold on potato. Fluopyram, in the “Luna” fungicide series, is a systemic fungicide to protect buds, blooms, and new tissues. Luna Pro combines fluopyram with prothioconazole (FRAC 3). Luna Tranquility combines fluopyram (FRAC 7) with pyrimethanil (FRAC 9) for preventative and curative activity. Other fungicides recommended for controlling white mold include products containing the active ingredients boscalid (Endura), fludioxonil (ie: Miravis Prime with pydiflumetofen), fluazinam (ie: Omega), iprodione (ie: Rovral), penthiopyrad (Vertisan), and thiophanate-methyl (ie: Topsin).
It is likely that over the next few weeks we will begin to see white mold in open field settings. Awareness of field and disease history as well as susceptibility of your vegetable cultivars will greatly aid in best management of this destructive disease.
Vegetable Insect Update – Russell L. Groves, Professor and Department Chairperson, UW-Madison, Department of Entomology, 608-262-3229 (office), (608) 698-2434 (cell), e-mail: rgroves@wisc.edu
Vegetable Entomology Webpage: https://vegento.russell.wisc.edu/
Seed corn maggot – (https://vegento.russell.wisc.edu/pests/ssedcorn-maggot/)
Seedcorn maggot (SCM) second generation is situated squarely over central Wisconsin (inset below). This 2nd generation represents the risk period when eggs have been laid and have hatched into the larval stages that can damage new transplants and recently seeded crops. The developmental minimum temperature for this insect is quite low (39oF) and thus the risk for infestation differs depending upon how long seeds/transplants remain in vulnerable stages. From Wisconsin’s Environmental Weather Mesonet (Wisconet) and the growing degree day dashboard, the accumulated degree days (assuming a development base temperature of 39oF) are lagging behind last season, but remain ahead of the 30-year average. Cooler daytime and nighttime temperatures through the past week have increased the risk for damage due to SCM. Planting of susceptible crops (seeds or transplants) with little or no at-plant protection (eg. insecticide seed treatments) should be delayed by at least 7-10 days to avoid infestation by these damaging larvae (maggots). Documenting peak flights can help to forecast these damaging generations. Remember, adult flies are attracted to volatiles of decaying organic matter and manure, so it is important to limit incorporation of these materials at the time to planting when adult fly peaks are encroaching.
Colorado potato beetle – (https://vegento.russell.wisc.edu/pests/colorado-potato-beetle/)
Adult Colorado potato beetle (CPB) continue to colonize fields over central Wisconsin this past week and egg masses are very abundant in this region. Initial egg hatch has already occurred throughout southern Wisconsin and many early larvae are present in field perimeters and into the middle of fields where adults have been active.
In central portions of Wisconsin, the weather conditions were conducive to slower development of populations. At this same calendar date in 2024, many fields were experiencing an abundance of 1st and 2nd instar larvae. At the present time, we are only just beginning to see 1st instars around field perimeters and still many egg masses have yet to hatch. With forecast daytime temperatures in the low 80’s in the coming days, many eggs will hatch and early larvae will become abundant in many central Wisconsin fields by mid-week.
From the website, overwintered adults are abundant in central Wisconsin and they will continue to emerge and colonize potato in the Antigo and Rice Lake areas in the coming 7-10 days. Early perimeter treatments for adults and egg masses should be implemented at this time and with a special focus on areas of the field where initial colonization is underway. Please consult the attached CPB pest management listing for more details on control options.
Populations of the damaging larvae will hatch from these eggs and will begin feeding in approximately one week after the appearance of first eggs. Producers should be planning to implement applications of larvicides this week and 2nd applications of larvicides in the following week. Active ingredients that successfully target these 1st and 2nd instars that are actively feeding include abamectin, chlorantraniliprole, cyantraniliprole, cyclaniliprole, spinosad, spinetoram and tolfenpyrad. Note, there are many generics that can be successfully used that contain similar active ingredients.
