Amanda Gevens, Chair, Professor & Extension Vegetable Pathologist, UW-Madison, Dept. of Plant Pathology, 608-575-3029, Email: gevens@wisc.edu, Lab Website: https://vegpath.plantpath.wisc.edu/
Early blight of potato/tomato. With the cooler weather of this past week, accumulations of P-days (recall these are influenced by heat) slowed a bit and on average we saw just roughly 40 P-days over the past week across the state of Wisconsin. No locations have yet reached the threshold of 300 for a recommended initiation of a preventative management program for early blight in potato. It is likely that during this coming week, southern Wisconsin earliest planted potato crops will reach threshold and require preventative fungicide treatments for management of early blight.
Late blight of potato/tomato. Accumulations of Blitecast DSVs have been low to non-existent. Of the locations that I highlight in the table, below, only Hancock has just a single DSV (accumulated on 6/13). Overall, the weather has been very dry, with temperatures a bit too hot to promote the pathogen prior to this recent cool period. Temperatures are forecasted to increase this week to nearly 90°F in some parts of the state. By next weekend, thunderstorms may bring precipitation and increased risk. The usablight.org website (https://usablight.org/map/) indicates no reports of late blight in potato or tomato from across the US in 2023. This website continues to provide a very useful mechanism for tracking this potentially destructive crop disease, but it’s not comprehensive. To date, I’ve not heard of reports of late blight in susceptible crops in the upper midwestern states. With limited reports of the disease in 2022 (no reports in WI!), inoculum sources are limited/low and early pressure should be at a minimum. Fungicides for management of late blight in tomato and potato crops are provided: https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin
Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations. Many thanks to Ben Bradford, UW-Madison Entomology; Stephen Jordan, UW-Madison Plant Pathology; and our grower collaborator weather station hosts for supporting this disease management effort again in 2023. 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. Red text in table indicates threshold has been met or surpassed. TBD indicates that data are To Be Determined as time progresses. Weather data used in these calculations is from weather stations that are placed in potato fields in each of the four locations, as available. Data from an alternative modeling source: https://agweather.cals.wisc.edu/vdifn will be used to supplement as needed for missing data points and for additional locations (indicated with *). Data are available in graphical and raw formats for multiple locations at: https://vegpath.plantpath.wisc.edu/dsv/.
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Planting Date | 50% Emergence Date | Disease Severity Values (DSVs) through 6/17/2023 | Potato Physiological Days (P-Days) through 6/17/2023 | |
| Spring Green* | Early | Apr 3 | May 9 | 0 | 277 |
| Mid | Apr 17 | May 12 | 0 | 256 | |
| Late | May 10 | May 23 | 0 | 187 | |
| Arlington* | Early | Apr 5 | May 10 | 0 | 271 |
| Mid | Apr 20 | May 15 | 0 | 233 | |
| Late | May12 | May 25 | 0 | 173 | |
| Grand Marsh | Early | Apr 5 | May 10 | 0 | 247 |
| Mid | Apr 20 | May 15 | 0 | 213 | |
| Late | May 12 | May 25 | 0 | 161 | |
| Hancock | Early | Apr 10 | May 17 | 1 | 209 |
| Mid | Apr 22 | May 19 | 1 | 203 | |
| Late | May 14 | May 28 | 1 | 154 | |
| Plover | Early | Apr 14 | May 19 | 0 | 199 |
| Mid | Apr 24 | May 20 | 0 | 194 | |
| Late | May 19 | May 29 | 0 | 147 | |
| Antigo | Early | May 1 | May 28 | 0 | 137 |
| Mid | May 15 | June 3 | 0 | 94 | |
| Late | June 7 | TBD | TBD | TBD | |
| Rhinelander* | Early | May 7 | June 1 | 0 | 107 |
| Mid | May 18 | June 5 | 0 | 72 | |
| Late | June 9 | TBD | TBD | TBD | |
In addition to the potato field weather stations, we have the UW Vegetable Disease and Insect Forecasting Network tool to explore P-Days and DSVs across the state (https://agweather.cals.wisc.edu/vdifn). This tool utilizes NOAA weather data (stations are not situated within potato fields). 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 2023 Commercial Vegetable Production in Wisconsin Guide, Extension Document A3422, linked here: https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin
Cucurbit Downy Mildew. The Cucurbit Downy Mildew forecasting webpage (https://cdm.ipmpipe.org/) is not forecasting the movement of the pathogen, but the group is offering reporting of findings of cucurbit downy mildew. To date, the disease has been confirmed on cucumber in NJ; butternut squash and cucumber in SC; and watermelon, acorn/yellow summer squash, and cucumber in Georgia. These data suggest that there are both strain types of the pathogen active along the east coast. We should be watchful of all cucurbit crops. In past recent years, we have predominantly seen the cucumber strain types impacting cucurbits in Wisconsin.
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/). The potato leafhopper (PLH) is a serious annual pest of snap beans, hops, clover, alfalfa and potatoes. Damage caused by leafhoppers includes stunted plants, brown leaves and reduced plant vigor. Both adults and nymphs feed by inserting their mouth parts into the plant’s vascular tissue and extracting sap. Damage results when the insect injects saliva containing toxic substances and creates physical damage during feeding, plugging the vascular tissue and permanently reducing the plant’s photosynthetic efficiency.
Migratory populations of the PLH were slow to enter the state, but have now arrived in many portions of southern and central Wisconsin. Significant damage is attributed to feeding by the nymphs more so than the adults. For this reason, regularly scout potatoes, beans, hops and alfalfa now to ensure nymphal populations are not building in number. Leafhopper populations can build over successive weeks before any overt symptoms begin to show, and it is critical to gain control before they display the “hopperburn” symptomology. Snap beans and potatoes should be scouted regularly for PLH activity. Leafhoppers tend to migrate into other crops in early summer after alfalfa is cut and we are approaching the second cutting in many locations of the state. This is a key time to scout for early migrants in vegetable plantings.
True armyworm (https://vegento.russell.wisc.edu/pests/#leps). Armyworms are dark caterpillars measuring up to 2 inches long. They have a dark stripe running lengthwise on the side with a yellow stripe beneath. Dark and light stripes alternate along their back. Armyworms move up from grassy weeds within corn fields or migrate into corn fields from small grain or forage fields. They may hide in soil crevices and beneath clods by day.
Few armyworms overwinter in Wisconsin and are presumed to migrate into the state from the southern states in early spring. This year, significant populations have been observed statewide in grass pastures and areas where oviposition was highest. Upon arrival, armyworm moths are active during the evening, feeding on nectar, mating, and searching for oviposition sites. After completing six instars, larvae pupate just below the soil surface. Adults emerge in 1 to 2 weeks. A second generation will occur in late June or early July but the likelihood that these will cause significant defoliation in well-established sweet corn is low.
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, adults continue to colonize much of the field and nearly all egg masses have hatched into 1st instar larval stages. Later larval stages (3rd and 4th instar) are now common in southern locations, whereas mostly early instar larvae are present in portions of the state north of Hwy 10. 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 if they have just begun treatments. Recall, there can be considerable variability in the predominant life stages 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).
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, applications should be timed with the midpoint of egg hatch. The first application should be followed up in 5-7 days 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.
Applications of 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 has been reached in many fields in central Wisconsin, with few egg masses being deposited given that many overwintered adults are disappearing due to mortality. These 1st generation larvicides often require 2-3 subsequent re-applications spaced on a 7-10 day interval to achieve sufficient control of this damaging generation. In northern Wisconsin, CPB adults are still colonizing fields, and mating and egg laying is 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.
Flea beetles. – (https://vegento.russell.wisc.edu/pests/flea-beetles/). The damage associated with feeding flea beetles is increasing, and these are commonly found on all members of the cole crop group (kale, brussels sprouts, cabbage, etc.), potato, and spinach, beets, and early planted eggplant. There are several different species of flea beetles that can feed and damage crops in the early season. The crucifer flea beetle attacks cole crops and mustards while the eggplant flea beetle is commonly associated with eggplant. Similarly, the potato flea beetle most often attacks potato, but can be found on other nightshade plants (pepper, tomato, solanaceous weeds). Common Wisconsin flea beetles include the crucifer, eggplant, horseradish, pale-striped, potato, spinach, and striped flea beetles.
Management options are recommended when flea beetle populations exceed established threshold levels, particularly early in the season. When choosing to control flea beetles, take care not to disrupt early populations of natural enemies that are also emerging currently. Synthetic pyrethroids comprise the majority of options currently, as well as pyrethrum (e.g., Pyganic, Azera, etc.) for organics. Spinosad-containing compounds (organics and conventional) can also work to limit some species of flea beetles to include potato, eggplant and crucifer flea beetles. Floating row covers can prevent adults from feeding on leaves and laying eggs on the crop. If used, row covers should be set up just before the crop emerges. Heavy rains and overhead watering can deter adult flea beetle feeding, and this is partially why damage has been accumulating (no rain).
Cole crop pests: Cabbage looper, Diamondback moth and Imported cabbageworm – (https://vegento.russell.wisc.edu/pests/caterpillar-pests-of-cole-crops/). Imported cabbageworms (also known as cabbage whites or small whites), cabbage loopers and diamondback moths are the three most significant caterpillar pests of Wisconsin cole crops, with the imported cabbage worm being the most significant. The first generation of the imported cabbageworm has already been surpassed and now second generation moths are emerging in southern Wisconsin. Adult populations will continue to develop throughout the remainder of the state over the coming week, and egg deposition on cole crops will continue. Imported cabbageworm adults, commonly referred to as the white cabbage butterfly, are white butterflies whereas the larvae appear as velvety green worms up to 1 inch long with a faint yellow stripe running down the back.
Diamondback moths (DBM) have also completed a full generation in most portions of the state, and these typically arrive on infested transplants from southern locations. Diamondback moths overwinter as adults and can therefore be an early season pest if they successfully overwinter. Cold winters increase mortality; however, our past Wisconsin winter was very mild for many insects and survivorship of DBM was quite high. In early spring, females lay eggs on weeds in the mustard family, and later they move into susceptible cole crops. After completing four larval stages they spin white silken cocoons on the underside of leaves in the lower portion of the plant. There are typically 3 to 5 generations of diamondback moths per year in Wisconsin.
Cabbage loopers are now increasing in southern portions of Wisconsin as they enter the state from southern locations where overwintering success was also quite high. The cabbage looper often feeds between veins on the underside of lower leaves. Large loopers will make larger holes in the foliage and can burrow through 3 to 6 layers of tightly wrapped head leaves in cabbage. A good indicator of the presence of loopers and imported cabbageworms is fresh frass (droppings) on leaves. Scout fields weekly throughout the remainder of the season for damage. Check plants carefully, even if no feeding damage is apparent, to look for eggs that will hatch into small caterpillars in just a few days. Examine the lower leaves of the plant for the larvae of each pest. Although feeding damage and fecal material are signs of activity, it is better to rely on larvae counts to determine the level of infestation. Caterpillars cause varying amounts of damage depending on the plant’s maturity, so the need for treatment changes as the crop grows.
Peak flight activity for 1st generation of cabbage looper in the upper Midwest. First generation peak (and subsequent risk) is illustrated across central and southern Wisconsin. (Source: https://agweather.cals.wisc.edu/vdifn).