Update 20 – August 16, 2020

Amanda Gevens, Dept. Chair, Professor & Extension Specialist, UW-Madison Plant Pathology, gevens@wisc.edu, Cell: 608-575-3029. https://vegpath.plantpath.wisc.edu/

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). A P-Day value of ≥300 indicates the threshold for early blight risk and triggers preventative fungicide application. A DSV of ≥18 indicates the threshold for late blight risk and triggers preventative fungicide application. Red text in table indicates threshold has been met/surpassed. TBD indicates that data is To Be Determined as time progresses. Weather data used in these calculations comes from weather stations that are placed in potato fields in each of the four locations. Data are available in graphical and raw data formats for each weather station at: https://vegpath.plantpath.wisc.edu/dsv/

Location Planting Date 50% Emergence Date Disease Severity Values 8/16/20 Potato Physiological Days 8/16/20
Grand Marsh Early Apr 17 May 18 129 695
Mid Apr 25 May 26 126 640
Late May 6 June 1 123 599
Hancock Early Apr 8 May 18 62 690
Mid Apr 20 May 25 60 639
Late May 4 May 30 57 601
Plover Early Apr 10 May 23 105 636
Mid Apr 20 May 30 99 582
Late May 5 June 1 99 570
Antigo Early May 14 June 5 63 565
Mid May 24 June 10 63 527
Late Jun 1 June 17 62 480

Late Blight Management: Our DSVs are reported here from emergence to August 16. Over the past week, we saw very moderate accumulations; Aug 8-10 were big accumulators with highly favorable weather for late blight. Plantings of potatoes in the Grand Marsh, Hancock, Plover, and Antigo areas have exceeded threshold and should receive routine (~weekly) preventative fungicide application for late blight management. For locations near Adams County, consideration for tightened, 5-day fungicide schedules are advisable.

Early Blight Management: PDays are exceeding the threshold of 300 for early planted potatoes in Grand Marsh, Hancock, Plover, and Antigo areas. For more information about fungicide selections, please see the Potato section of the A3422 Commercial Vegetable Production Guide for Wisconsin, 2020. https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3422-2020.pdf

National late blight update: Late blight was reported in Wisconsin on Aug 10 in Adams County on potato. The clonal lineage/strain type was US-23.  In the last week of July, late blight had been reported in the state of Washington, and it was found in British Columbia, western Canada (Delta and Surrey) about 3 weeks ago now. The site: https://usablight.org/map/ includes reports as they are submitted in the US.  Previous reports documented the disease in NC, FL and AL.  Where the late blight pathogen has been tested in the US so far this year, the clonal lineage has been US-23.

National cucurbit downy mildew update: No downy mildew reported from WI at this time. Nearest and new report from LaPorte County Indiana in the northwestern corner (now >7 days ago). Reports to date, have come from: AL, CT, DE, GA, IN, KS, KY, MA, MD, MI, NC, NJ, NY, OH, Ontario & Quebec Canada, PA, SC, TN, VA, and WY. No forecasted movement of the pathogen in our direction, with prevailing air moving eastward. https://cdm.ipmpipe.org/forecasting/

In graphic, below, green counties indicate locations in which cucurbit downy mildew was reported >7 days ago; red counties indicated locations in which the disease was reported <7 days ago.


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