Update 14 – Aug 20, 2023

In This Issue:

  • Potato crop production updates
  • Snap bean production updates
  • Potato and tomato early blight and late blight disease updates
  • Cucurbit downy mildew updates


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

Despite the dry conditions, so far, we have had a good growing season with warm days and cool nights (fig. below), resulting in higher than average (even close to record-breaking) yield and really good quality as reported by several folks. And this has been noted not only in Wisconsin but also in other neighboring Upper Midwestern states like Minnesota. Harvested varieties such as Colomba, Plover Russet and Caribou Russet have shown very little hollow heart and not-too-bad scab issues. The 1.25’’ of rainfall occurring on August 14th made it unneeded to irrigate for about a week.

At the Hancock Ag Research Station, we have irrigated our full-season potatoes with 16.85’’ of water in the C field (Figure 2) and 17.35’’ of water in the K field (Figure 3). Therefore, as of August 19th, we have received about 40 lb/acre of extra nitrogen from the irrigation water in the C field (10 ppm of nitrate-N), and about 100 lb/acre of extra nitrogen from the groundwater in the K field (25 ppm of nitrate-N). Even with the additional nitrogen from the irrigation water, we can still see apparent canopy color difference between the control treatment (only 40 lb/acre of starter, highlighted plots in the figures) and the higher N rates (250 and 350 lb/acre of seasonal total N, the rest of the plots).

I dug some Snowden (left) and Plover Russet (right) tubers under 360 lb N/acre on August 19th, and their size profiles are shown below. Definitely more yield potential could be achieved before vine kill in early September.

Also I dug some Dark Red Norland (DRN) tubers under two different N rates on August 19th. It is very interesting to observe that tubers under 150 lb N/acre have way more uniform size profile than those under the high N program at 300 lb N/acre. This could suggest that unnecessary N fertilization could lead to not ideal tuber sizes. In general, very minimum scab problems were noticed on the red potatoes.

On July 28th we harvested our snap bean nitrogen trial at HARS, and the yield data is reported in the figure on the left here. We only got significant yield difference between the control treatment (20 of starter N/acre) and the highest N rate (150 lb/acre) for both DM88 and Huntington. Overall, the yield was very good this year, even the plots under the control treatment. One thing to point out is that we irrigated 13.2’’ of water to the snap beans this season, and thus we have received about 76 lb/acre of extra N from the irrigation water..


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

Early blight of potato/tomato.  Accumulations of P-days this past week were between 39-50 across the state of Wisconsin.  In all locations and all planting dates, potato fields have surpassed the threshold and should receive (and continue to receive) preventative fungicide applications for early blight management.  Hotter days generate roughly 10 P-days per day if you are looking ahead to likely accumulations and preventative fungicides. Early blight in our UW Hancock ARS plots has looked a bit different this year, with fewer smaller, newer lesions overall making for less early blight.  Because of the hotter, dryer weather, Potato Early Dying seems more visually pronounced in our Russet Burbank plots.  We’re anxious to have a look at our yields in these plots which have remained quite healthy and green for a longer duration than typical.

Late blight of potato/tomato.  Accumulations of Blitecast DSVs were low this past week in the 7 sites detailed in our table, below, ranging from 3 to 4.  The usablight.org website (https://usablight.org/map/) indicates no new late blight reports in the past week.  The most recent was a report of tomato late blight from Henderson County North Carolina on 8/9/23 (genotype not yet known).  No new reports from NY or Canada.  So far, all characterizations of the late blight pathogen identified in North America this growing season have resulted in the US-23 type.  Fungicides for the management of late blight in tomato and potato crops are provided:  https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin.  A specific list of fungicides for potato late blight in Wisconsin was also offered in a special report shared via email on July 28.  https://vegpath.plantpath.wisc.edu/wp-content/uploads/sites/210/2023/08/2023-Potato-Late-Blight-Fungicides.pdf

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 again in 2023.  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.  TBD indicates that data are To Be Determined as time progresses.  Weather data used in these calculations is from weather stations that are placed in potato fields in each of the four locations, as available.  Data from an alternative modeling source: https://agweather.cals.wisc.edu/vdifn will be used to supplement as needed for missing data points and for additional locations (indicated with *).  Data are available in graphical and raw formats for multiple locations at:  https://vegpath.plantpath.wisc.edu/dsv/.



Planting Date 50% Emergence Date Disease Severity Values (DSVs) through 8/19/2023 Potato Physiological Days (P-Days) through 8/19/2023
Spring Green* Early Apr 3 May 9 12 802
Mid Apr 17 May 12 12 780
Late May 10 May 23 12 711
Arlington* Early Apr 5 May 10 11 811
Mid Apr 20 May 15 11 773
Late May12 May 25 11 713
Grand Marsh Early Apr 5 May 10 10 769
Mid Apr 20 May 15 10 735
Late May 12 May 25 10 683
Hancock Early Apr 10 May 17 11 736
Mid Apr 22 May 19 11 730
Late May 14 May 28 11 681
Plover Early Apr 14 May 19 15 723
Mid Apr 24 May 20 15 717
Late May 19 May 29 15 668
Antigo Early May 1 May 28 11 635
Mid May 15 June 3 10 590
Late June 7 June 23 10 454
Rhinelander* Early May 7 June 1 7 603
Mid May 18 June 5 7 568
Late June 9 June 24 7 445

In addition to the potato field weather stations, we have the UW Vegetable Disease and Insect Forecasting Network tool to explore P-Days and DSVs across the state (https://agweather.cals.wisc.edu/vdifn).  This tool utilizes NOAA weather data.  In using this tool, be sure to enter your model selections and parameters, then hit the blue submit button at the bottom of the parameter boxes.  Once thresholds are met for risk of early blight and/or late blight, fungicides are recommended for optimum disease control.  Fungicide details can be found in the 2023 Commercial Vegetable Production in Wisconsin Guide, Extension Document A3422. https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin

Cucurbit Downy Mildew.  The Cucurbit Downy Mildew forecasting webpage (https://cdm.ipmpipe.org/) is not forecasting the movement of the pathogen, but the group is offering reporting of findings of cucurbit downy mildew from the US (see current map below showing red counties with new reports from  PA and MD). Dr. Mary Hausbeck reported cucumber downy mildew in commercial fields in 11 Michigan Counties as of 8/16/2023.

To date, there have been no reports of downy mildew here in WI.  We should be considering preventative treatment of cucumber and melon crops here due to the likelihood of the disease resulting from clade 2 downy mildew. A list of best-performing conventional fungicides is provided below.

  • Elumin + chlorothalonil or mancozeb
  • Omega (Orbus) + chlorothalonil or mancozeb
  • Orondis Opti (Chlorothalonil is part of the premix, additional chlorothalonil is suggested. See label for maximum chlorothalonil rates.)
  • Previcur Flex + chlorothalonil or mancozeb
  • Ranman + chlorothalonil or mancozeb
  • Zampro + chlorothalonil or mancozeb


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