Update 21 – August 23, 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 russet variety trial that had two N rates at 150 and 300 lb N/acre. The seeds were planted on May 1st, emergence around May 20th, canopy closure around June 20th.

Since we had poor seed emergence issue in the spring, the total yield data were not meaningful, and we are only reporting the marketable yield % and % of large tubers here. Based on our statistical analysis, there is not any significant difference between the two N rates with regard to their effects on marketable yield percentage (yield of tubers larger than 6oz / total yield) and percentage of tubers that are larger than 10oz. But 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.

Marketable yield %
Caribou R 63 A
W14904-13rus 59 A
W13008-1rus 54 AB
W14002-2rus 51 ABC
GoldRush 50 ABC
Silverton 42 BC
Reveille R 40 BC
W13027-46rus 38 C


% of >10oz tuber yield
Caribou R 22 A
W13008-1rus 19 AB
W14904-13rus 17 AB
GoldRush 14 ABC
W14002-2rus 12 ABC
Silverton 9 ABC
Reveille R 6 BC
W13027-46rus 4 C

Our results indicated that Caribou Russet had potential of producing large marketable tubers, whereas W13027-46rus, an unnamed cultivar, only produced small size tubers. Reveille Russet also showed poor bulking potential in this study. Performance of the common varieties GoldRush and Silverton were not among the top candidates in the lists.

We will conduct our second harvest of the same varieties on August 25th (97 dates after emergence).

Change of topic, table below shows our groundwater testing results. Overall, nitrate-N in the Hancock Ag Research Station well reduced from about 26 ppm in June to around 20 ppm in later July. A nearby potato field had around 14 ppm, and a nearby field corn field had about 16 ppm. Within each irrigation event, there is no variation of nitrate-N level.

Site Time from start of irrigation (hr) DATE Nitrate-N (ppm)
0 7/21/20 20.4
0.5 7/21/20 20.5
1 7/21/20 20.8
Hancock 2 7/21/20 20.8
0 7/27/20 20.3
0.5 7/27/20 20.6
1 7/27/20 20.4
2 7/27/20 20.2
2 7/23/20 14.1
Commercial potato field 4 7/23/20 14
6 7/23/20 14.1
2 7/23/20 16.3
Commercial field corn field 4 7/23/20 16.4
6 7/23/20 16
10 7/23/20 16


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 Date Disease Severity Values 8/22/20 Potato Physiological Days 8/22/20
Grand Marsh Early Apr 17 May 18 129 745
Mid Apr 25 May 26 126 689
Late May 6 June 1 123 649
Hancock Early Apr 8 May 18 62 734
Mid Apr 20 May 25 60 683
Late May 4 May 30 57 645
Plover Early Apr 10 May 23 105 686
Mid Apr 20 May 30 99 632
Late May 5 June 1 99 619
Antigo Early May 14 June 5 63 612
Mid May 24 June 10 63 575
Late Jun 1 June 17 62 527

Late Blight Management: Our DSVs are reported here from emergence to August 22. Over the past week, we saw very modest accumulations. Generally, weather has been hot and dry. 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: Late blight was reported in Wisconsin on Aug 20 on tomato in Pierce County. This has been just the second confirmation of late blight in WI this season. The previous confirmation was Aug 10 in Adams County on potato (US-23). 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) about 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, IN, KS, KY, MA, MD, ME, MI, 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/


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