Update 22 – Sep 11, 2022

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 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, as 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)
Potato Physiological Days (P-Days)
Grand Marsh Early Apr 5 May 10 77 914
Mid Apr 20 May 15 77 873
Late May 12 May 25 77 815
Hancock Early Apr 7 May 12 48 908
Mid Apr 22 May 17 48 888
Late May 14 May 26 48 829
Plover Early Apr 7 May 15 119 857
Mid Apr 24 May 20 119 823
Late May 18 May 27 118 788
Antigo Early May 1 Jun 3 54 733
Mid May 15 June 15 50 659
Late June 10 June 24 50 574

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.

Accumulations of P-Days were ~40 in most locations over the past week. Potatoes should continue to be on a preventative fungicide program with effective disease management selections to limit early blight in long-season potatoes.

All monitored Wisconsin locations accumulated ~5 DSVs this past week indicating a relatively low risk week for promoting late blight in potato, however, the DSVs came primarily in the past 3 days with the precipitation which provides can promote late blight. A fungicide list for potato late blight in Wisconsin was provided in a past newsletter and is available here: https://vegpath.plantpath.wisc.edu/2022/07/03/update-10-july-3-2022/

To my knowledge, there have been no reports of late blight in Wisconsin on potato or tomato so far this season. Michigan reported potato late blight in a few fields over this past week. The genotype has not yet been reported. According to usablight.org there were no other diagnoses of late blight in the US in the past week. Previous diagnoses in the US this season included those in NC, FL, CA, TN, and Ontario Canada. These have been primarily on tomato, with only the FL report on potato in early spring.

Cucurbit Downy Mildew: During this past week, cucurbit downy mildew was confirmed in WV and SC. Previously this growing season, the disease was confirmed in AL, CT, DE, FL, GA, KY, MA, MD, ME, MI, NC, NH, NJ, NY, OH, PA, SC, VA, and WI. The first report in WI had come from Waushara County (UW Hancock ARS) on Aug 15; no subsequent reports have followed from WI. Red counties, on the figure 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/

Onion Downy Mildew was confirmed on commercial onion leaves from central WI this past week. Onion downy mildew can be very problematic in onion fields. This foliar disease is caused by a fungus-like pathogen called Peronospora destructor. Infection is favored by temperatures less than 72°F and high humidity and leaf wetness. The pathogen can overwinter in volunteer onion, culls, and wild Allium weed species if the pathogen was present in your location in previous years. Symptoms include pale or white elongated patches on leaves that start off small and can elongate and produce a purple-gray sporulation which appears “downy.” Leaves can bend over and eventually die due to severe downy mildew infection. Please refer to picture below. This disease can impact bulb size, quality, and storability.

Management recommendations include practicing a 3+ year rotation to non-hosts such as small grains and corn, eliminating culls and volunteers, avoiding dense planting, avoiding excess N and overhead irrigation, and orienting rows parallel to prevailing wind to avoid prolonged leaf wetness.

Effective fungicides for onion downy mildew control include: ametoctradin+dimethomorph (Zampro) azoxystrobin (Quadris, Amistar, others) azoxystrobin + propiconazole (Quilt Excel) copper hydroxide (Kocide, Champ, others) cymoxanil + chlorothalonil (Ariston) cymoxanil + famoxadone (Tanos) dimethomorph (Forum) fenamidone (Reason) fluazinam (Omega) fosetyl-aluminum (Aliette) mancozeb (Dithane, Manzate, others) mandipropamid (Revus) mefenoxam (Ridomil Gold) oxathiapiprolin+chlorothalonil (Orondis Opti) oxathiapiprolin+mandipropamid (Orondis Ultra) pyraclostrobin (Cabrio) pyraclostrobin & boscalid (Pristine) zoxamide+chlorothalonil (Zing!) zoxamide+mancozeb (Gavel)

Although labeled for onion downy mildew, coppers and chlorothalonil are not very effective for downy mildew control, and coppers can be phytotoxic to onions. Please see the 2022 Wisconsin Vegetable Production Guide A3422 for further details on application rates and specifications. If you suspect you have Downy mildew in your onions, please get a sample and contact your county agent, our disease diagnostic clinic, or myself for confirmation. The 2022 A3422 Commercial Vegetable Production in Wisconsin guide is available for purchase through the UW Extension Learning Store: https://learningstore.extension.wisc.edu/products/commercial- vegetable-production-in-wisconsin

Onion Stemphylium This disease has become more common and problematic in commercial onion fields of Ontario, Michigan, and now Wisconsin in recent years. Stemphylium infections were significant in several commercial onion fields in past recent years. This disease is caused by the fungus Stemphylium vesicarium. Symptoms begin as small yellow-tan, watersoaked lesions that elongate into lesions that turn dark olive brown to black due to spore production. Leaves can become completely diseased and necrotic when lesions coalesce. See pictures, below, of symptoms of Stemphylium. Symptoms are not easily distinguished until the spore production phase occurs. Stemphylium, as well as purple blotch and Alternaria, can prematurely defoliate the crop causing reduced bulb quality and increased susceptibility to secondary bacterial diseases that may cause storage rots.

Stemphylium most often infects dead/dying onion leaves when temps are warm (64 – 77°F) and humid, with periods of leaf wetness (16+ hours). Since the pathogen infects onions that have been physically damaged or infected by other diseases, it is important to maintain healthy plant stands and control other common foliar diseases such as purple blotch, downy mildew and Botrytis. Herbicide, hail, and blowing sand damage seem to be the primary initial causes for onion foliar damage which then leads to Stemphylium infection; without the damage, infection is much less likely.

The same cultural methods of control that are used to manage other foliar onion disease should be employed for Stemphylium. Fungicides registered for the control of purple blotch can be effective on Stemphylium leaf blight; however, it seems to be harder to control Stemphylium with fungicides that are highly effective on purple blotch. In our trials on onions in Wisconsin in 2015, most programs looked similar in disease control performance with the exception of programs with Quadris Top and Luna Tranquility. Note that Luna Tranquility contains two active ingredients. One is Scala and the second is a fungicide in FRAC group 7. Endura is also a group 7 fungicide with registration on onion in WI. Please see results of our fungicide trial from 2015, below (Program #6 with Luna Tranquility provided best Stemphylium control).

Watermelon/Cucurbit Anthracnose has been a severe foliar disease concern on cucurbits in central Wisconsin this late summer. Anthracnose caused by the fungus Colletotrichum orbiculare can infect all above-ground plant parts on all cucurbits and symptoms can vary in appearance based on cucurbit type. Irregular brown leaf spots form on squash, melon and cucumber. The center of the leaf spot may drop out resulting in a ragged appearance (very common cucumber). Cucumber leaf spots often have a yellow halo. Watermelon leaf spots are smaller and dark brown to black. Sunken elongated stem infections can occur on cucumber and melon but are not common on other cucurbits. Infections on melon often exude a reddish gum. Fruit infections are sunken black spots 1/4 -1/2 inch across and 1/4 inch deep. Spots may have fluffy white cotton-like mycelia and sticky salmon-colored spores during wet weather.

This pathogen survives in infected plant debris and on seed. Spores form on infected leaves and fruit and are readily spread by splashing rain and/or irrigation water, and on hands and tools. The pathogen is favored by warm, moist conditions and typically appears first in mid-to late summer after canopy closure.

To manage this disease, varietal resistance is available and seed should be purchased from a clean and reputable source. Do not save seed from infected plants. Crop rotation away from cucurbits on a 3-year schedule can limit inoculum and subsequent disease. Irrigate cautiously to limit foliar moisture as possible (drip irrigation is best). Limit work in the field when plants are wet and consider possible practices of removal or destruction of infected vines (cultivation) at the end of the season to reduce residual inoculum.

Several commercial, conventional fungicides are registered for control of anthracnose on cucurbits including: benzovindiflupyr, boscalid, chlorothalonil, cyprodinil, difenoconazole, fluopyram, fluoxastrobin, fluxapyroxad, mancozeb, pyraclostrobin, tebuconazole, thiophanate methyl, and trifloxystrobin. Page 107 of the A3422 Commercial production guide for Wisconsin addresses further details: https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3422-2022.pdf

For more information on this disease: https://edis.ifas.ufl.edu/pdf%5Carchived%5CPP%5CPP266%5CPP266-1026374.pdf


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