Update 22 – August 30, 2020

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

On August 10th (82 days after emergence), we conducted our first harvest of an early season fresh market red/yellow variety trial that had two N rates at 150 and 300 lb N/acre. The seed were planted on May 1st, emergence around May 20th, canopy closure around June 20th.

We are only reporting the size A% and size B% here because some varieties had poor seed emergence issues. Based on our statistical analysis, there is not any significant difference between the two N rates with regard to their effects on size A% and size B%. However, we did see some significant difference of those traits for different varieties. Tables below show the traits of each variety across the two N rates.

Values followed by different letters are significant different at p<0.05.

Variety % of A size tubers % of B size tubers
Colomba 68% A 23% E
Dark Red Norland 67% A 28% E
Soraya 66% A 30% DE
Red Prairie 53% B 40% CD
W13103-2Y 48% B 42% BC
Malou 42% B 49% BC
Agata 42% B 52% AB
W15270-12R/Y 21% C 61% A

Our results indicate that the top three varieties that produced high % A size tubers are Colomba, Dark Red Norland and Soraya, and therefore they produced low percentage of B size tubers. The unnamed variety W15270-12R/Y performed poorly for % A size. I will report our second harvest of the same varieties on August 25th (97 dates after emergence) in my next week’s newsletter.

For groundwater nitrate testing of the Hancock Ag Research Station well, it stayed around 22 ppm for the two dates of testing in August. Within each irrigation event, there is no variation of nitrate-N level.

Time from start of irrigation (hr) DATE Nitrate-N (ppm)
0 8/3/20 21.9
0.5 8/3/20 22.0
1 8/3/20 22.1
2 8/3/20 22.1
0 8/13/20 22.2
0.5 8/13/20 22.2
1 8/13/20 22.0
2 8/13/20 22.2


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. 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 multiple formats for each station at: https://vegpath.plantpath.wisc.edu/dsv/

Location Planting Date 50% Emergence


Disease Severity

Values 8/26/20

Potato Physiological

Days 8/26/20

Grand Marsh Early Apr 17 May 18 150 780
Mid Apr 25 May 26 147 724
Late May 6 June 1 144 684
Hancock Early Apr 8 May 18 72 762
Mid Apr 20 May 25 70 711
Late May 4 May 30 67 674
Plover Early Apr 10 May 23 126 721
Mid Apr 20 May 30 120 667
Late May 5 June 1 120 655
Antigo Early May 14 June 5 73 642
Mid May 24 June 10 73 605
Late Jun 1 June 17 71 557

Late Blight Management: Our DSVs are reported here from emergence to August 26. Over the past week, we saw moderate to high accumulations. 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 and Pierce Counties, continued 5 to 7-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: No new reports of late blight in Wisconsin, or elsewhere in the US, this past week. So far this season, there have been just two confirmations of late blight from tomato (Pierce Co.) and potato (Adams Co. US-23) this season. No widespread movement from these sites as far as I’m aware. Earlier this season, late blight had been reported in the state of Washington, and it was found in British Columbia, western Canada (Delta and Surrey) overa a month 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. Reports to date, have come from: AL, CT, DE, GA, IL, IN, KS, KY, MA, MD, ME, MI, MS, NC, NH, NJ, NY, OH,

Ontario & Quebec Canada, PA, SC, TN, VA, and WV. No forecasted movement of the pathogen in our direction, with prevailing air moving eastward. https://cdm.ipmpipe.org/forecasting/


View this newsletter as a PDF