Update 21 – August 29, 2021

Amanda Gevens, Chair, Professor & Extension Vegetable Pathologist, UW-Madison, Dept. of Plant Pathology, 608-575-3029, Email: gevens@wisc.edu.

Potato Disease Modelling and Management of Early Blight and Late Blight: 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. 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 comes from weather stations that are placed in potato fields in each of the four locations (substitute data from https://agweather.cals.wisc.edu/vdifn as needed). Data are available in graphical and raw formats for each weather station at: https://vegpath.plantpath.wisc.edu/dsv/

Late blight and Early blight risk scores:

Location Planting Date 50% Emergence Date Disease Severity Values (DSVs)
Potato Physiological Days (P-Days)
Grand Marsh Early April 2 May 10 111 795
Mid April 10 May 15 111 781
Late May 1 May 23 105 715
Hancock Early April 5 May 12 57 800
Mid April 15 May 15 57 791
Late May 5 May 23 53 729
Plover Early April 7 May 12 96 760
Mid April 20 May 20 93 715
Late May 7 May 30 88 653
Antigo Early April 26 May 28 55 743
Mid May 10 June 5 55 705
Late May 20 June 13 55 636

All potato fields of Wisconsin have reached/surpassed the threshold for Disease Severity Values (18) and should continue to be preventatively treated for late blight management. Accumulations over the past week were roughly at one dozen DSVs, indicating moderate risk from disease-promoting weather. Potato late blight (US-23) was confirmed from a commercial potato field in Portage County on 8/25/2021. To date, there are now 3 confirmations of US-23 potato late blight all in Portage County WI (Jul 28, Aug 9, and Aug 25).

Outside of Wisconsin, tomato late blight was confirmed in Cocke County Tennessee on Aug 19 on tomato (US- 23), North Carolina on Aug 16 on tomato (strain typing in process); on tomato in Ontario (Haldimand-Norfolk) Canada (no sample submitted for strain determination on Aug 10); on potato in Aroostook County Maine (US-23 on Aug 9); and on tomato in northeastern Georgia (US-23 on Jul 28) (usablight.org). US-23 is typically sensitive to the fungicides in the phenylamide group (including mefenoxam and metalaxyl). The map below, from usablight.org shows the counties in which late blight has been confirmed in 2021. For more information on this disease: https://vegpath.plantpath.wisc.edu/resources/potato-late-blight/.

The early blight P-Day threshold of 300 has been exceeded in all potato plantings of Wisconsin. A listing of details of currently registered fungicides for early blight management can be found in our 2021 Wisconsin Vegetable Production guide: https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3422-2021.pdf

Early blight has defoliated our non-treated ‘Russet Burbank’ control plantings at the Hancock Agricultural Research Station. Plots have looked quite desiccated for the past 2 weeks. Some of our better foliar fungicide programs are retaining foliar health for at least an additional month which typically results in substantial yield and quality increase. We are planning to vine kill the trial in the coming week or two with harvest to follow during the third week of September. Pictures from this afternoon, below, show one of our non-treated control plots on the left and a successful foliar fungicide program (so far, plot received 9/10 applications) on the right.

Cucurbit Downy Mildew Update: Over the past week, cucurbit downy mildew was confirmed in the following locations: OH (pumpkin), MS (cucumber and cantaloupe), and TN (cucumber, ornamental gourds). There have been no samples of cucurbit downy mildew through our UW Plant Disease Diagnostic Clinic, or my Vegetable Pathology Lab so far this season. No downy mildew has been observed in our sentinel plots in WI.

This season, so far, the disease has been documented in AL, CT, DE, FL, GA, IN, KY, LA, MA, MD, MI, MS, NC, NH, NJ, NY, OH, Ontario Canada, PA, RI, SC, TN, VA, and WV. There is no predicted movement of the pathogen into Wisconsin at this time– as reflected in the recent forecast (for Sunday August 29, 2021) depicted below from https:// cdm.ipmpipe.org/

Please contact me or the UW Plant Pathology Diagnostic Clinic for confirmed diagnoses of cucurbit downy mildew. https://pddc.wisc.edu/

Due to the presence of unique pathogen types (Clade 1 and 2 types with unique host ranges among cucurbits), our improved understanding of the cucurbit downy mildew type that may be in our region can aid in recommending the most appropriate prevention of crop disease and resulting loss.

View up to date disease risk models for the upper Midwest in an interactive map at the Vegetable Disease and Insect Forecasting (VDIFN) website.


View this newsletter as a PDF