Update 16 – July 19, 2020

Yi Wang, Assistant Professor & Extension Potato and Vegetable Production Specialist, UW- Madison, Dept. of Horticulture, 608-265-4781, Email: wang52@wisc.edu.

This week’s digging data from six Plover Russet plants of each N rate showed that higher N rate resulted in more tubers that were larger than 4 oz. Digging date (July 12th) was 52 days after plant emergence (May 21st). For example, under 40 lb N/acre, 3% were larger than 4 oz, under 180 lb N/acre, the number was 4%, but under 300 lb N/acre, 10% were larger than 4oz.

Like what we found last week, the highest N rate resulted in less tuber set and more tubers that bulked up into the marketable size. There is another 60 units of N scheduled to be applied next week. We are expecting more marketable tubers to be developed.

Also this week we got our groundwater test results back. The water was collected on July 5th from a well that irrigated our research plots at the Hancock Ag Research Station.

Time from start of irrigation system (hr) NITRATE-N (ppm)
0 24.5
0.5 23.5
1.0 25.6
2.0 22.6

Those numbers were consistent with what we saw three weeks ago, which was around 24 ppm, and no difference of nitrate-N level was noticed within the irrigation event. We have several rainfall events that were larger than 1 inch over the past three weeks, suggesting higher nitrate leaching potential from our N treated plots, however, there has not been noted any increase of groundwater nitrate level in the well that was only 20 feet from the soil surface. I will keep updating on our test results for the rest of the growing season.


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

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 7/18/20 Potato Physiological Days 7/18/20
Grand Marsh Early Apr 17 May 18 76 462
Mid Apr 25 May 26 73 407
Late May 6 June 1 70 366
Hancock Early Apr 8 May 18 43 471
Mid Apr 20 May 25 41 420
Late May 4 May 30 38 382
Plover Early Apr 10 May 23 56 414
Mid Apr 20 May 30 50 360
Late May 5 June 1 50 347
Antigo Early May 14 June 5 35 338
Mid May 24 June 10 35 301
Late Jun 1 June 17 34 253

Late Blight Management: Our DSVs are reported here from emergence to July 18. Over the past week, we saw greatest accumulations in the most southern locations (on average 2 DSVs per day, whereas further north about 1 DSV per day). All 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.

Early Blight Management: PDays are exceeding the threhold of 300 for early planted potatoes in Grand Marsh, Hancock, Plover, and Antigo areas. Totals are rapidly accumulating with higher temperatures. 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: Tomato late blight was reported in North Carolina 2 days ago. No new reports of late blight on potato in this past week, as per https://usablight.org/map/ . Previous reports documented the disease in FL and AL.

National cucurbit downy mildew update: No downy mildew reported from WI at this time. Over the past week, no additional county reports in Michigan. Reports to date, have come from: AL, DE, GA, MD, MI, NC, OH, Ontario Canada, NY, NJ, VA, MD, SC. No forecasted movement of the pathogen in our direction, with prevailing air moving eastward.


Langlade County Virtual Field Day – July 23, 2020 1-3PM

The 2020 Field Day will be moving to a virtual platform based on social distancing guidelines. The flyer is attached with a list of presenters and researchers who will be contributing to the event. Join us Thursday July 23rd at 1:00 PM for a Virtual Field Day, zoom link or call in information is listed below.

Join Zoom Meeting https://uwextension.zoom.us/j/96045954238?pwd=TktVOWdYWktWK3Z3TzNyQ3haaEhIZz09

Meeting ID: 960 4595 4238
Password: Potato

One tap mobile: +16465588656,,96045954238#

Dial by your location: +1 312 626 6799
Meeting ID: 960 4595 4238


View this newsletter as a PDF