Update 9 – July 16, 2023

In This Issue:

  • Potato and tomato early blight and late blight disease updates
  • Cucurbit downy mildew updates
  • Cucurbit powdery mildew updates
  • Potato Virus Y updates and management
  • Two-spotted spider mites
  • Caterpillar pests


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.  Accumulations of P-days (recall these are influenced by heat) ramped up this past week and on average we saw roughly 60 P-days across the state of Wisconsin.  In all locations with the exception of Antigo (later plantings) and Rhinelander, all potato fields have reached/surpassed threshold and should receive (and continue to receive) preventative fungicide applications for early blight management.  Hotter days generate roughly 10 P-days per day if you are looking ahead to likely accumulations and planned preventative fungicide applications.

Late blight of potato/tomato. Accumulations of Blitecast DSVs have been low this past week, ranging from 0 to 5 added DSVs statewide. The usablight.org website 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. 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/.



Planting Date 50% Emergence Date Disease Severity Values (DSVs) through 7/15/2023 Potato Physiological Days (P-Days) through 7/15/2023
Spring Green* Early Apr 3 May 9 6 517
Mid Apr 17 May 12 6 496
Late May 10 May 23 6 427
Arlington* Early Apr 5 May 10 6 518
Mid Apr 20 May 15 6 481
Late May12 May 25 6 421
Grand Marsh Early Apr 5 May 10 4 488
Mid Apr 20 May 15 4 454
Late May 12 May 25 4 402
Hancock Early Apr 10 May 17 5 453
Mid Apr 22 May 19 5 448
Late May 14 May 28 5 398
Plover Early Apr 14 May 19 7 441
Mid Apr 24 May 20 7 436
Late May 19 May 29 7 387
Antigo Early May 1 May 28 5 374
Mid May 15 June 3 5 329
Late June 7 June 23 5 191
Rhinelander* Early May 7 June 1 2 340
Mid May 18 June 5 2 305
Late June 9 June 24 2 182

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 from the US. Dr. Mary Hausbeck reported the interception of cucurbit downy mildew spores in an air/spore trap in Monroe, Berrien, and Allegan counties in Michigan last week.  Through molecular biological testing, Dr. Hausbeck and her laboratory at Michigan State University characterized the pathogen as the Clade 2 type of the cucurbit downy mildew pathogen which tells us that this type is likely to infect cucumber and melon crops.  In past years, when SW MI had cucurbit downy mildew in production fields, WI did see some movement of the disease into southeastern WI.  For this reason, I am being vigilant in tracking reports of the disease in southern Michigan.  To date, there have been no reports of the disease developing in cucumber fields in MI or WI.  If reports arise, we should be considering preventative treatment of cucumber and melon crops here in southeastern Wisconsin.

The disease has been confirmed on cucumber in Ontario and Quebec, Canada, GA, NC, NJ, NY, SC, and VT.  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.

Counties with confirmed Cucurbit downy mildew, July 16, 2023. Source: https://cdm.ipmpipe.org/

Cucurbit Powdery Mildew:  Cucurbits have been showing signs of powdery mildew in central Wisconsin over the past week.  This disease is typically caused by the fungal pathogen Podosphaera xanthii and we see it appear on most susceptible cucurbits first in mid to late July of most years.  While some cucurbits can tolerate powdery mildew infection, if the disease onsets early and on a highly susceptible variety, control may be necessary.  There are several fungicides with effective control of powdery mildew (list provided below from the A3422 Commercial production guide for vegetables in Wisconsin).  However, cucurbit powdery mildew pathogen populations in Wisconsin have resistance to the strobilurin fungicides which include azoxystrobin, pyraclostrobin, and trifloxystrobin. In our past field trials, Quintec (quinoxyfen) was most effective in alternation and tank-mixed with chlorothalonil.  For multi-pick cucurbits, it’s important to have a look at the allowable days to harvest.  The images below show typical powdery mildew signs (talcum-like white spore production on foliage) and the disease cycle of powdery mildew on cucurbits.

Squash leaf showing Cucurbit powdery mildew symptoms on underside.
Disease cycle of Cucurbit powdery mildew.


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 virus Y (PVY) – (https://vegento.russell.wisc.edu/pests/plant-pathogens/).

A screen shot from VDIFN (July 15, 2023) map focusing upon Antigo, WI. Risk of aphid activity and potential PVY transmission is expected to exceed the estimated 1967 FDD on July 19. Peak risk for transmission is rapidly moving across Wisconsin and the map included indicates an estimated 29 FDD accumulated each day.

The application of foliar insecticides in the growing season to potato has almost no effect on preventing PVY inoculation of plants by non-potato colonizing aphid species that are migrating into and transmitting the virus through susceptible potato. At-plant, systemic insecticides can, however, help to reduce populations of potato-colonizing species, and this can provide some relief from virus spread in the current season. Consult your scouting reports and ensure that no potato colonizing species are present (green peach aphid, potato aphid, buckthorn aphid).

Colonizing species of aphids: (https://blogs.cornell.edu/potatovirus/pvy/aphid-vectors-of-pvy/)

Buckthorn aphid (Aphis nasturtii)
Description: Small-sized insects. Body color of apterous yellow to greenish Distribution: Worldwide, except Australasia.
Host biology: Highly polyphagous and feeds on herbaceous plants that belong to 28 different families in summer, and overwinters as an egg on buckthorn, Rhamnus cathartica.
Photo credit: David Voegtlin, University of Illinois

Green peach aphid (Myzus persicae)Most efficient PVY vector. Description: Body color varies from yellow to all shades of green, to pink, red, or black with a dorsal black patch at the base of abdomen. Distribution: Worldwide, present in all areas of North America.
Host biology: Able to feed on many plant species. Exhibits host alternation, overwintering on woody (Prunus spp.) and infesting hundreds of plant species (including potato) in summer.
Photo credit: David Voegtlin, University of Illinois

Potato aphid (Macrosiphum euphorbiae)
Description: Large insects with pear-shaped bodies and distinct red eyes. Body color varies from solid pink to green and pink mottled to light green.
Distribution: Native of North America, worldwide except India.
Host biology: Potato aphid attacks over 200 plant species including vegetable and ornamental crops, as well as weeds.
Photo credit: David Voegtlin, University of Illinois


Two-spotted spider mites – (https://vegento.russell.wisc.edu/pests/).

Two-spotted spider mites (TSSM) are small arthropods which are more related to spiders, ticks, daddy-longlegs and scorpions. The TSSM has a worldwide distribution and can infest 100’s of plant species including vegetables, fruits and ornamentals. Vegetables that are often affected include cucumbers, snap beans, lettuce, peas, potatoes and tomatoes. Populations can increase rapidly since each female mite produces up to 20 eggs per day, and the resulting immature stages will develop to reproductive adults in as few as 5-8 days during hot, dry weather.

Forecast temperatures combined with ongoing local drought conditions in many portions of Wisconsin, predispose susceptible crops to infestation. Scouting agencies revealed the first sign of infestation by TSSM in early July and populations continue to progress. Drought stressed crops help to create ideal conditions for TSSM increase. Early symptoms of infestation include a chlorotic, stippled appearance on the leaves, as feeding mites remove leaf cell contents, including the chlorophyll that gives leaves their green color. Without the chlorophyll, those empty cells appear whitish or bronze. Heavily infested leaves turn completely pale, dry up, and fall off. Maintaining the nutritional (fertility) and hydraulic (water) health of plants is key to lessen the success of TSSM populations.

Insecticidal and miticidal soaps and botanical oils can be effective solutions for organic producers when paired with conservation biological control strategies to limit developing populations. Conventional producers often rely upon a set of abamectin (Agri-Mek), bifenazate (Acramite), bifenthrin (Brigade), dimethoate or fenpyroximate (Portal) applications spaced 5-7 days apart to control feeding stages.


Caterpillar pests (sweet corn, dry beans, tomatoes, peppers, snap beans) – (https://vegento.russell.wisc.edu/pests/corn-earworm/).

Pest Survey specialists at Wisconsin Department of Agriculture, Trade and Consumer Protection (WI DATCP) collect and manage data on plant pests that threaten agricultural production (Wisconsin Home Pest Survey). This very important program focuses on surveillance and early detection of economically significant pests, including insects relevant to Wisconsin agriculture. Data is collected through field-based sampling and from networks of cooperators across Wisconsin. Species-specific pheromone trapping is conducted for relevant pests in the state, and special reference should be paid to counts obtained for both corn earworm and true armyworm over the next 1-2 weeks in many portions of the state, with special emphasis to the southern half where trapping is concentrated.

The best technique for monitoring earworms is through the use of pheromone traps. These traps use a special scent to attract male moths. Knowing when moths are present helps to determine when to treat fields. Weekly pheromone trap captures of adult corn earworm (Helicoverpa zea) are available through the Wisconsin Home Pest Survey and this past week (ending July 13) represents initial counts for the state.

For producers interested to have very site-specific data for their field locations, it is best to place a trap 4 to 6 feet above the ground on the south or west side of fields when corn is in the green silk stage. Pheromones should be changed every 2 weeks with the unused lures kept frozen until needed. Hercon pheromone lures (as only one example) have been very effective at attracting earworm moths. For accurate counts, be sure to remove used lures from the trap area.

Another technique for monitoring earworms uses a black light to lure night-flying insects. Data from statewide blacklight traps are also available through the Wisconsin Home Pest Survey,  and these are However, black light traps are more expensive, less effective, and more difficult than pheromone traps to monitor. Counts in blacklight traps are consistently lower than those in pheromone traps in adjacent fields.

Pheromone trap catches of 5 to 10 moths or blacklight trap captures of 3 to 5 moths per night for three consecutive nights indicate that moths are probably laying enough eggs to warrant treatment of fields that are in the vulnerable stage between brush and silk browning to add precision to your scouting, check silks for the small, spherical corn earworm eggs before beginning a spray program.

Recent capture data in portions of southern Wisconsin do indicate elevated captures of the 1st full generation of corn earworm that have now developed in the state. With early silking sweet corn, it is important to scout the silking crop and apply insecticides on vulnerable silks when eggs hatch. The first application is the most critical and tank-mixes containing an ovicide (an egg killer) and synthetic pyrethroid are recommended. On sweet corn, treat every 3 to 4 days beginning when silks first appear and continuing until they brown. On hybrid dent seed corn, make 1 to 2 applications during this period. Discontinue treatments 10 days before harvesting fresh market sweet corn as additional sprays will not improve product quality.


Read this newsletter as a PDF