Update 4 – June 9, 2024

In This Issue:

  • Potato and vegetable disease forecasting updates
  • Onion Botrytis leaf blight risk accumulation
  • Updates and management of Potato leafhopper, Cutworms, Cucumber beetles, Aster leafhopper, Colorado Potato Beetles
  • Colorado potato beetle insecticides



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.

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 6 6 6 3 3 3
Antigo 9 9 9 5 5 5
Plover 9 9 9 2 2 2
Hancock 9 9 9 2 2 2
Grand Marsh 8 8 8 1 1 1
Arlington 5 5 5 3 3 1
Spring Green 9 9 9 4 4 2

Any cumulative values above the preventive action threshold of 18 DSV are highlighted in red.

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 175.9 158.2 133.2 101.3 89.7 47.4
Antigo 176.3 156.9 131.6 101.2 90.6 48.9
Plover 212.8 186.0 154.4 117.5 105.6 57.9
Hancock 214.6 187.3 154.4 116.3 104.6 57.6
Grand Marsh 221.2 192.5 159.0 118.9 107.5 59.1
Arlington 233.5 203.0 166.1 124.2 110.8 61.5
Spring Green 235.6 202.6 165.4 123.8 111.5 60.4

Any cumulative values above the preventive action threshold of 300 P-days are highlighted in red.

Late blight of potato/tomato.  The usablight.org website (https://usablight.org/map/) indicates no reports of late blight from the US so far in 2024.  The site is not comprehensive.  We accumulated few to no Blitecast Disease Severity Values over the past week in WI.

Early blight of potato.  P-Day values will continue to amass (up to ~10 per day) and develop conditions optimum for early blight disease caused by Alternaria solani. Earliest inoculum comes from within field  (small crop residue fragments can harbor the pathogen) 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 growing season.

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.  In using this tool, 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 2024 Commercial Veg. Production in WI Extension Document A3422: https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin


Onion Botrytis leaf blight/leaf spot – Onion Botrytis leaf blight/leaf spot is a fungal disease of alliums caused by Botrytis squamosa. Symptoms first appear as small white spots on the leaf. These spots are oval-shaped, and sometimes are surrounded by a light green or silver halo that often appears water-soaked. Leaf tips will begin to dry and wither as the disease progresses, sometimes until the whole leaf dies back. Progressed infection can stunt bulb growth and reduce yield. Heavily infected fields often appear yellow.

Onion leaf showing Botrytis leaf spot symptoms. Note the small, whitish, oval-shaped spots on the leaf surrounded by a light green or silver halo. Photo credit: Lindsey du Toit, Washington State University, via Bugwood.org

Infection.  Primary infection occurs from B. squamosa spores that overwinter in infected in-field plant debris, cull piles, stored bulbs, volunteer bulbs in-field, and in infected soil. Secondary infection can occur when conidia spores spread from moist, infected leaves. Favorable conditions for disease development include high relative humidity and rainfall, prolonged leaf wetness, and warm temperatures.

Disease Cycle.  Botrytis squamosa overwinters as sclerotia in infected in-field plant debris, cull piles, stored bulbs, volunteer bulbs in the field, and infested soil. These sclerotia produce airborne conidia spores and ascospores (sexual spores) that travel to and infect onion leaves during periods of high moisture and low air movement. These same favorable conditions allow for secondary cycles of infection, where infected leaves produce more conidia, which spread to further infect the same leaf or others. Sclerotia are once again formed at the end of the season, and the disease cycle will continue the following season.

Cultural control.  Cultural control strategies include scouting regularly to identify the presence of the disease early before it has had a chance to spread and cause significant damage. Disease spread can be limited by avoiding working in fields when plants are wet and disinfecting tools and machinery. The following practices can help mitigate the risk of this disease:

  • Maintain proper spacing between plants
  • Destroy cull piles
  • Rogue volunteer plants
  • Distance seed and commercial onion fields
  • Destroy infested plant debris
  • Rotate away from susceptible crops (Alliums) to reduce sclerotia in soil (3 years)

Chemical control.  Use disease forecasting tools to properly time the most effective disease prevention sprays. For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.

Predictive modeling.  To view the predicted onion botrytis risk on any given day, visit the Vegetable Disease and Insect Forecasting Network (VDIFN) website. From the Disease tab, select the “Botrytis leaf blight” model. This BOTCAST model uses a cumulative disease severity index (CDSI) computed from gridded NOAA weather data to calculate the risk of onion botrytis development, which is displayed as a colored map overlay.

  • Threshold 1: (21 ≤ CDSI < 31) Warning threshold of “no spray applied unless rain predicted or overhead irrigation applied”
  • Threshold 2: high risk of rapid disease development, apply initial spray as soon as possible
  • CDSI > 40: extremely elevated risk

The start point should be set to the date of crop emergence. Click any grid point in VDIFN to get more detailed weather and disease progression information for that location.

Currently in the Coloma area, for fields emerging on/around May 20, 16 disease severity values have accumulated.  This brings us close to “Threshold 1” as described above.

Resources.  Commercial Vegetable Production in Wisconsin (A3422) from the UW Extension Learning Store. This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.


Vegetable Insect Update – Russell L. Groves, Professor and Department Chair, 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/

Potato leafhopper – (https://vegento.russell.wisc.edu/pests/potato-leafhopper/). Populations of adult potato leafhopper (PLH) are now increasing in many parts of southern Wisconsin. Recall, these are annual pest of snap beans, hops, clover, alfalfa and potatoes. Both adults and nymphs feed by inserting their mouth parts into the plant’s phloem and extracting sap and thereby injecting saliva containing toxic substances. It is the plant’s response to the saliva (salivary proteins) that results in the observed damage.

Potato leafhopper (Empoasca fabae)

Migratory populations of the PLH have now entered the southern portion of Wisconsin. For this reason, begin regular scouting of these crops now to ensure nymphal populations are not building in number. Leafhopper populations can build over successive weeks before any damage symptoms begin to show, and it is critical to gain control before they display the “hopperburn” symptomology. Leafhoppers tend to move into other crops in early summer after forage alfalfa had hay crops are cut and we are underway with second cutting in many locations of the state. This is a key time to scout for early migrants in vegetable plantings.


Cutworms (https://vegento.russell.wisc.edu/pests/#leps). Damage resulting from flights of cutworms are becoming apparent in select areas of southern Wisconsin. There are a variety cutworm moths and associated larvae that can feed on and damage newly emerged vegetable crops. The caterpillars (worms) are active, nocturnal feeders, clipping many seedlings at or below the soil line in a single night. They prefer crops sown as seed (rather than transplants); susceptible crops include beets, carrots, cucumber, leafy greens, melons, peas, potato, pumpkin, snap beans, squash, and sweet corn. If not controlled, these pests can destroy large areas in short periods of time once they have reached later larval stages.

Common cutworm larvae. Photo: Alabama Cooperative Extension.

Not all cutworms overwinter in Wisconsin and are presumed to migrate into the state from the southern states in early spring. This year populations have been observed near grassy field corners and areas where moths began laying eggs. Newly hatched larvae feed on leaves but are unable to chew entirely through, creating a “window pane” effect. As they mature, large infestations of cutworms can completely kill plants as larvae begin cutting plant stems.

Economic treatment thresholds for black cutworms have been developed for the following crops:

  • Snap bean= 2 larvae/ row foot
  • Potatoes= 4 larvae/row foot
  • Sweet Corn= >5% of plants damaged
  • Leafy greens= <3% of the stand affected


Cucumber beetles. – (https://vegento.russell.wisc.edu/pests/cucumber-beetles/). Striped and spotted cucumber beetles will now begin to infest many of our cucurbit crops planted over the past 3 weeks. Spotted cucumber beetle (aka southern corn rootworm) can cause damage in vine crops, but the striped beetle is more common and damaging in Wisconsin. Feeding from larvae and adults causes direct damage to roots, leaves, flowers, and fruits.

Adult striped cucumber beetle can vector the bacteria, Erwinia tracheiphila. Cucumbers and melons are particularly susceptible to bacterial wilt, and damage from this can be severe. Only the striped cucumber beetle overwinters in Wisconsin. They emerge in mid- to late May and lay eggs in the soil at the base of cucurbits. Spotted beetles migrate to northern locations in early to mid-July. This late arrival generally seldom makes them a serious problem.

Plants infected with bacterial wilt will not recover. Therefore, it is important to control the beetles early in the season to prevent spread of the disease. Scout fields for adult beetles 2-3 times per week early in the season and weekly thereafter. Particular attention is needed in field edges where beetles congregate. The treatment threshold for cucumber beetles is 1 beetle per plant in melons, cucumber, Hubbard and Butternut squash, and younger pumpkins and 5 adults per plant in watermelon, other varieties of squash and older pumpkins. Beetle populations in excess of 20 per plant may transmit the bacterial wilt before insecticides have a chance to control the beetles.

Non-chemical control is possible in small plantings by covering the plants with floating row covers. Be sure to uncover flowering plants to allow bees to enter and pollinate the plants. Rotating crops with grain, tomatoes, or a cover crop or using perimeter trap crops can delay infestations. If a trap crop is used, exercise care that the trap crop will not act as a reservoir for bacterial wilt. If bacterial wilt infections have already occurred, remove the diseased plants immediately to prevent the spread of the disease while insects are present.


Aster leafhopper (https://vegento.russell.wisc.edu/pests/aster-leafhopper/). The aster leafhopper is a serious pest of many crops in the upper Midwest because of its ability to spread aster yellows disease. Aster yellows is untreatable and the only solution is to remove infected plants and treat for the leafhopper if numbers are too great. Both the insect and the disease can attack a broad range of plants, including vegetables, field crops, flowers, and weeds. Infected flowers, particularly those in the aster family (Compositae), are severely disfigured by the disease, destroying both their visual appeal and their economic value.

Aster leafhopper adult. Photo: North Dakota State University

Leafhoppers prefer lettuce, carrots, celery, and small grains for feeding and breeding, while other crops such as potatoes and onions provide a temporary source of food and refuge. Only adults use these temporary sites; immature leafhoppers fail to develop on these plants. All plants are susceptible to aster yellows infection.

Migratory populations of the aster leafhopper have arrived in Wisconsin over the past 3 weeks and, similar to other migratory pests mentioned (e.g. cutworm, potato leafhopper) are associated with the recent storm events that have brought southern winds into the state in advance of the fronts. Adult leafhoppers that migrate into the state can be infected with the bacterium that causes the disease.

Begin scouting for aster leafhoppers in early spring when plants are newly sprouted and continue scouting weekly throughout the end of July. Yellow sticky cards may be placed in the field to determine when the first migrants arrive. Once leafhoppers are observed, you’ll need to estimate the size of the population using an insect net to sweep the area. Take 100 sweeps per sample site and sample at least four areas. Established thresholds for treatment vary by crop susceptibility, but are generally: i) susceptible carrot varieties: 20 adult ALH/100 sweeps; ii) resistant carrot varieties: 40 adult ALH/100 sweeps; iii) onion, 15 adult ALH/100 sweeps; iv) celery: 10 adult ALH/100 sweeps. Consult A3422 (Commercial Vegetable Production in Wisconsin), to get a listing of carrot varieties considered susceptible and resistant.


Colorado potato beetle (CPB) – (https://vegento.russell.wisc.edu/pests/colorado-potato-beetle/). Continue to scout populations of CPB adults especially as nearly all potato plants have emerged in central Wisconsin. In southern Wisconsin, adult colonization is slowing in many fields and less than 25% of egg masses remain unhatched. Later larval stages (2nd and 3rd instar) are now common in southern locations, whereas mostly early instar larvae are present in portions of the state north of Hwy 23. In each instance, the choice of insect control product can vary widely. Northern production areas can still use perimeter treatments (e.g., indoxacarb) and insect growth regulators (e.g., novaluron), whereas central and southern locations will not benefit as much from these treatments at this time. Recall, there can be considerable variability in the predominant lifestages present, and this often results from planting date (later dates have younger larvae) and proximity to previous year potato (larger larvae in fields close to previous year potato).

Egg masses and early instar larvae of the Colorado potato beetle.

For most CPB chemical management tools, timing application occurs with the appearance of first instar larvae in the field. Early instar larvae are the most susceptible life stage for chemical management, and applications should be timed with the midpoint of egg hatch. The first application should be followed up in 7-10 days later with a second application of the same compound depending on the formulation and label restrictions. Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.

Application timing of reduced-risk (RR) modes of action (MoA) for the control of CPB.

Applications of ledprona (Calantha), novaluron (Rimon) tolfenpyrad (Torac), spinetoram (Radiant, Delegate), or abamectin (Agri-Mek) should be applied when nearly 50-75% of egg masses have hatched, and a few 2nd instar larvae are present from the earliest hatched egg masses.  This milestone will be reached in the coming week in many fields in central Wisconsin, with several egg masses continuing to be being deposited as overwintered adults continue to be active in many fields in the Central Sands. These 1st generation larvicides often require 2-3 subsequent re-applications spaced on a 7-10 day interval to achieve sufficient control. In northern Wisconsin, CPB adults are still colonizing fields, and mating and egg laying are just underway along field perimeters. With warm and dry daytime high and low temperatures forecast for the coming week, populations will move fast so don’t delay! Careful scouting will reveal the exact timing! Recommended products for control are listed below.

Wisconsin CPB management options 2024


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