Update 15 – August 15, 2024 – UW Vegetable Pathology

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/

Colonizing aphids – (https://vegento.russell.wisc.edu/pests/aphids/)

Through the late summer we continue to observe increasing aphid populations in a variety of vegetables, field crops and ornamentals. Aphids that colonize potatoes are not subsiding and continue to increase rapidly in many portions of the state, and particularly northern Wisconsin. Once scouts reveal these aphids in susceptible crops, it is essential to consider application of an appropriate aphicide to limit the development of these populations. Listed below, there are several aphid specific compounds registered for use in potatoes (and other vegetables) and are all considered very good for the control of colonizing species.

Trade name	Chemical name	Mode of Action Class	Max labeled rate (single application)
Admire Pro	imidacloprid	Group 4A	1.3 fl oz/ac
Actara 25WG	thiamethoxam	Group 4A	3.0 oz/ac
Assail 30SG	acetamiprid	Group 4A	4.0 oz/ac
Belay	clothianadin	Group 4A	3.0 fl oz/ac
Beleaf 50SG	flonicamid	Group 29	2.8 oz/ac
Exirel 10SL	cyantraniliprole	Group 28	13.5 fl oz/ac
Fulfill 50WG	pymetrozine	Group 9B	5.5 oz/ac
Movento HL	spirotetramat	Group 23	2.5 fl oz/ac
PQZ	pyrifluquinizon	Group 9B	3.2 fl oz/ac
Sefina Inscalis	afidopyropen	Group 9D	6.0 fl oz/ac
Sivanto HL	flupyradifurone	Group 4D	7.0 fl oz/ac
Torac	tolfenpyrad	Group 21	21.0 fl oz/ac
Transform 50WG	sulfoxaflor	Group 4C	1.5 oz/ac
Venom 70SG	dinotefuran	Group 4A	1.5 oz/ac

**Corn leaf aphid.** Photo credit: NBAIR

The corn leaf aphid is another example of an aphid that has been steadily increasing in both field and sweet corn. The corn leaf aphid shows a preference for barley, sorghum, and (sweet) corn, and infests many other wild and cultivated grasses. An occasional pest of winter wheat, it is an important vector of Barley Yellow Dwarf virus (BYDV). BYDV symptoms include reddening or yellowing of leaves (starting from the tip and leaf edges), reduced root biomass, and decreased stem height, ultimately affecting yield and grain quality. Once infected, there is no remedy for the affected plants, however, a combination of cultural and chemical approaches can help minimize the risk of BYDV spreading. Managing weeds and volunteers can eliminate BYDV and aphid reservoirs. Late fall planting provides an opportunity for the young seedlings to evade infestation by aphids that will be on the move from these corn reservoirs. Seed treatment with systemic insecticides combined with late planting are expected to further minimize the risk of BYDV infection in the fall. Any infections that occur in the spring are not expected to result in significant losses since plants often outgrow BYDV infection resulting from spring inoculations.

**Green peach aphid (Myzus persicae).** This is perhaps the most efficient vector of PVY in potato. Body color varies from yellow to all shades of green, to pink, red, or black. Small groups of wingless aphids suggest colonization of potato is underway. Photo credit: David Voegtlin, University of Illinois.

Green peach aphids (GPA) are increasing in a variety of nightshades including potato, pepper, eggplant and tomato. We may also expect this species to infest late-planted greens in both the field and the hoophouse. Check late planted greens often and consider an application once populations are detected in these crops. Many crops are attacked including greenhouse transplants of pepper, tomato, and cabbage, along with beet, carrot, broccoli, Brussels sprouts, lettuce, eggplant. GPA is often considered a pest of cold-weather crops and are inherently difficult to kill with contact insecticides because they are often under the leaves or on new, sheltered growth. Cooler weather (less than about 20°C) exacerbates the problem because there is less volatilization by contact insecticides (e.g., tolfenpyrad, Torac). Even systemic insecticides (many listed above), which will kill GPA feeding under the leaf where the insecticide was applied, are much less mobile and thus less effective at cool temperatures. The upcoming warm temperatures will not only drive rapid increase in aphid populations but may also increase the efficacy of our insecticides. The average generation time for several aphid species occurs over just a few days and is temperature dependent.

Corn earworm – (https://vegento.russell.wisc.edu/pests/corn-earworm/)

Captures of adult male corn earworm moths are back on the rise signaling the emergence of the second generation. Pheromone traps (see capture data included) are the best method to document the timing of adult emergence, but the magnitude of these captures do not always predict risk of injury within a field. If corn is pollinating or silking during scouting, it is also a good idea to inspect the tassel and silk for early instar larvae. Adult moths remain active in terms of mating and egg laying for as much as 15-20 days after their initial emergence. The risk for continued infestations and colonization of fields will continue through the last week of August in southern and central Wisconsin and will continue throughout early to mid-September in central and northern portions of the state.

If you have a pheromone trap, place the 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. 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. 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.

Vegetable Disease and Insect Forecasting Network (VDIFN) map of risk for infestation by Corn earworm, (CEW), https://agweather.cals.wisc.edu/vdifn (sourced 08/25/2024). Notice areas within the ‘orange or red shaded’ zones indicate high risk zone for adult moth oviposition, and these zones bisect the center of the state, but extend into northern portions of the state. Sweet corn producers with susceptible crop stages (silking corn,) should be scouting for these mobile insects.

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/late blight in WI.

	Planting Date		50% Emergence Date	Disease Severity Values (DSVs) *through 8/24/2024*	Potato Physiological Days (P-Days) *through 8/24/2024*
Spring Green	Early	Apr 3	May 9	58	902
	Mid	Apr 17	May 12	58	884
	Late	May 10	May 25	53	783
Arlington	Early	Apr 5	May 10	30	898
	Mid	Apr 20	May 15	30	867
	Late	May 12	May 25	28	788
Grand Marsh	Early	Apr 5	May 10	51	868
	Mid	Apr 20	May 15	51	840
	Late	May 12	May 25	44	766
Hancock	Early	Apr 10	May 17	61	823
	Mid	Apr 22	May 21	59	793
	Late	May 14	June 2	54	713
Plover	Early	Apr 14	May 18	50	819
	Mid	Apr 24	May 22	46	787
	Late	May 19	June 7	42	672
Antigo	Early	May 1	May 24	50	720
	Mid	May 15	June 1	50	684
	Late	June 1	June 15	37	580
Rhinelander	Early	May 7	May 25	26	706
	Mid	May 18	June 8	25	614
	Late	June 2	June 16	25	565

Late blight of potato/tomato. Late blight diagnostics continue to be available at no cost to WI growers and gardeners. Dr. Brian Hudelson of our UW Plant Disease Diagnostic Clinic and Dr. Amanda Gevens of UW-Potato & Vegetable Pathology can offer confirmation of the pathogen. Dr. Gevens will also offer strain typing of the pathogen. The usablight.org website (https://usablight.org/map/) indicates reports of late blight from the US so far in 2024 including NY (US-23), MI (US-23), ME (US-23), PA, and TN. Please keep in mind that the site is not comprehensive. Outside of this site, I’m aware of 2 Ontario Canada confirmations of potato and tomato late blight (US-23), and a Florida late blight sample from potato (March 2024).

Late season late blight control in potato. While I’m not aware of late blight in potato or tomato in Wisconsin this season so far, it’s important to remain vigilant in managing the potato crop for this disease through senescence and harvest. Any green and succulent tissue in the lower plant canopies can still become infected by the late blight pathogen and should be protected. Further, late-season showers can ‘wash out’ spores from the air and conveniently deliver them to the lower stems and tubers as the suspension percolates down through the soil. Continued foliar fungicide use can keep the crop protected in this final stage and limit late-season late blight infections that can further develop in a stored crop. Practice good harvest, movement, and storage practices to maintain the health and quality of tubers.

We accumulated few to no (0-1) Blitecast Disease Severity Values over the past week in WI. All WI locations are above the threshold for late blight disease severity values and should receive preventative fungicide application to reduce the risk of disease. An updated listing of fungicides for WI potato late blight management for 2024 can be found at the link below. Base protectants such as chlorothalonil and mancozeb offer broad-spectrum control of fungal and oomycete (water mold – like late blight) pathogens. https://vegpath.plantpath.wisc.edu/wp-content/uploads/sites/210/2022/07/2024-Potato-Late-Blight-Fungicides.pdf

Early blight of potato. All areas of production have reached the threshold for the application of foliar fungicides to limit early blight. This disease was unusual this year with a typical timeline for onset, but slower progression than most years due to high temperatures in July. With cooler temperatures at night and the seasonal daily temperatures last week, the accumulation of P-Days is slowing down. https://vegpath.plantpath.wisc.edu/diseases/potato-early-blight/

Cucurbit Downy Mildew: To date, downy mildew field findings in the US have been caused by Clade 2 – cucumber and cantaloupe strain type. Five states reported new disease cases in the past week (map below), including MA, NJ, PA, NC, and KY. We have seen ‘look-alike’ diseases in WI which have primarily been angular leaf spot or Anthracnose (see pictures, below). No field disease confirmations were made in Wisconsin.

Cucurbit downy mildew look-alike diseases: