Vegetable Insect Update – Russell L. Groves, Professor and Department Chair, UW-Madison, Department of Entomology, (608) 698-2434 (mobile), e-mail rgroves@wisc.edu. Vegetable Entomology Webpage: https://vegento.russell.wisc.edu/
Two-spotted spider mites – (https://vegento.russell.wisc.edu/pests/). Two-spotted spider mites (TSSM) are small arthropods related to insects that are related to spiders, ticks, daddy-longlegs and scorpions. The TSSM has a cosmopolitan distribution and has been recorded on more than 300 species of plants, including all vegetables, fruits and ornamentals. Vegetables that are often affected include cucumbers, snap beans, lettuce, peas, potatoes and tomatoes. Each female mite produces up to 20 eggs per day, and the larvae that hatch from these eggs after 2-5 days develop through 3 immature stages that can result in reproductive adults in as few as 5-8 days during hot, dry weather (Fig 1).
Our forecast temperatures combined with local drought conditions in many portions of Wisconsin, predispose susceptible crops to infestation. The first sign of infestation by TSSM is usually 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.
Insecticidal and miticidal soaps and botanical oils can be effective solutions when paired with conservation biological control strategies to limit developing populations. Maintaining the nutritional (fertility) and hydraulic (water) health of plants is also key to lessen the success of TSSM populations.
Cutworms and armyworms – (https://vegento.russell.wisc.edu/pests/black-cutworm/). 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 cornfields or migrate into cornfields from small grain or forage fields. They may hide in soil crevices and beneath clods by day. At night, they chew corn leaves and weaken plants.
Cutworm larvae feed on newly emerged vegetable crops. The worms are active 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 later plantings in very short periods of time. Careful scouting can reveal the damage and presence of these pests in later planted / fall planted vegetables. If identified early, young ,larvae can be easily controlled with formulations of Bacillus thuringensis subsp. kustaki (e.g. Dipel) B. thuringensis subsp aizawai (XenTari).
European corn borer – (https://vegento.russell.wisc.edu/pests/european-corn-borer/). In most of Wisconsin, two generations of eggs are laid on the undersides of leaves. First generation larvae typically cause damage only to leaves and stalks, unless the corn is already tasseling, in which case the larvae will enter the ear. In Southern Wisconsin, begin checking early sweet corn for egg masses now by August 10-15 (Fig 2). Second generation larvae develop from eggs laid in mid-August and cause heavy infestations in late-planted corn, and corn that does not have a transgenic event.
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/
Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations. 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 2022. 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. Weather data used in these calculations will come from weather stations that are placed in potato fields in each of the four locations, once available. Data from an alternative modeling source: https://agweather.cals.wisc.edu/vdifn will be used to supplement as needed. Data are available for each weather station at: https://vegpath.plantpath.wisc.edu/dsv/.
| Location | Planting Date | 50% Emergence Date | Disease Severity Values (DSVs) 8/6/2022 | Potato Physiological Days (P-Days)
8/6/2022 |
|
| Grand Marsh | Early | Apr 5 | May 10 | 48 | 663 |
| Mid | Apr 20 | May 15 | 48 | 622 | |
| Late | May 12 | May 25 | 48 | 564 | |
| Hancock | Early | Apr 7 | May 12 | 27 | 637 |
| Mid | Apr 22 | May 17 | 27 | 617 | |
| Late | May 14 | May 26 | 25 | 558 | |
| Plover | Early | Apr 7 | May 15 | 79 | 604 |
| Mid | Apr 24 | May 20 | 79 | 570 | |
| Late | May 18 | May 27 | 78 | 535 | |
| Antigo | Early | May 1 | Jun 3 | 29 | 483 |
| Mid | May 15 | June 15 | 25 | 409 | |
| Late | June 10 | June 24 | 25 | 338 | |
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.
We have reached thresholds for preventative fungicide treatment in potatoes to manage early blight in all potato plantings in Wisconsin. Accumulations of P-Days were high over the past week. Potatoes should be on a preventative fungicide program with effective disease management selections to limit early blight.
All monitored Wisconsin locations accumulated very few to no DSVs this past week indicating a low-risk week for promoting late blight in potato plantings in Grand Marsh, Hancock, Plover, and Antigo. Antigo plantings have now reached/exceeded the threshold for receiving a preventative application of fungicide for the management of late blight. A fungicide list for potato late blight in Wisconsin was provided in last week’s newsletter and is available here: https://vegpath.plantpath.wisc.edu/2022/07/03/update-10-july-3-2022/
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 2022 Commercial Vegetable Production in Wisconsin Guide, Extension Document A3422, linked here: https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin
According to usablight.org there have not been recent diagnoses of late blight in tomato or potato crops in the US. For this year, there were just 2 reports entered back in March in southern Florida (US-23 clonal lineage/strain type). Eastern Ontario Canada had 2 reports of tomato late blight over the past 2 weeks. No further reports have surfaced.
Cucurbit Downy Mildew: During this past week, cucurbit downy mildew was confirmed on cucumber and/or cantaloupe in MI, OH, NY, MA, and VA. Previously this growing season the disease was confirmed in: AL, CT, DE, FL, GA, MA, MD, NC, NH, NJ, NY, OH, PA, SC, and VA. No findings of cucurbit downy mildew in our Wisconsin-based sentinel plots in Dane County. Red counties below indicate recent reports (less than 1 week old) of cucurbit downy mildew.
As a reminder, the pathogen is now known to have two ‘strains’ for clade types. The type (Clade 2) which infects cucumber, can also infect melon. Due to fungicide resistance within the downy mildew pathogen population, especially in Clade 2, selection of fungicides is important. Management of cucurbit downy mildew requires preventative fungicide applications as commercial cultivars are generally susceptible to current strains (Clades) of the pathogen. Management information can be sourced here: https://vegpath.plantpath.wisc.edu/2022/07/03/update-10-july-3-2022/
Brian Hudelson, Sue Lueloff, Sarah deVeer, and Ann Joy. UW-Madison/Extension, Plant Disease Diagnostic Clinic (PDDC) Update.
In 2022, the PDDC continues to provide diagnoses through examination of digital photographs, as well as physical samples. Click here for the PDDC’s current submission policy, as well as information on the PDDC’s current fee structure. Digital diagnoses will be included in the Wisconsin Disease Almanac and when a digital diagnosis would normally require a lab confirmation, the disease/disorder will be labeled as “suspected”. The following diseases/disorders have been identified at the PDDC from July 30, 2022 through August 5, 2022.
| PLANT/SAMPLE TYPE | DISEASE/DISORDER | PATHOGEN | COUNTY |
| VEGETABLE CROPS | |||
| Potato | Black Leg | Pectobacterium carotovorum subsp. carotovorum, Pectobacterium parmentieri | Iowa |
| Tomato | Bacterial Canker
Bacterial Speck (Suspected) Fusarium Wilt Herbicide Damage Septoria Leaf Spot (Suspected) |
Clavibacter michiganensis subsp. michiganensis
Pseudomonas syringae pv. tomato Fusarium oxysporum None Septoria lycopersici |
Bayfield
Dane Bayfield Dane Iowa |
| SPECIALTY CROPS | |||
| Ginseng (American) | Alternaria Leaf Blight | Alternaria panax | Walworth |
To learn more about plant diseases and their control, as well as PDDC educational resources and activities, visit the PDDC website at pddc.wisc.edu, follow the clinic on Facebook and Twitter @UWPDDC or email pddc@wisc.edu to subscribe to the PDDC listserv “UWPDDCLearn”.