Update 18 – Sep 17, 2023

In This Issue:

  • 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 Plant and Agroecosystem Sciences, 608-265-4781, Email: wang52@wisc.edu.

Overall, Wisconsin and the Upper Midwest were hot and dry through most of August. However, growers were able to catch up with good irrigation management, and spotty rainfall events could keep potato crops in good conditions. For example, the >1.2’’ of rainfall event on August 14th and 15th supplied a decent amount of moisture to the soils. Warm days and cool nights in August greatly helped the tubers to achieve their bulking potential as much as possible.

Looking back, the state has been dry and hot most of this growing season. With good irrigation management practices, the soil moisture was kept at good levels, and didn’t cause any substantial leaching risk. On the flip side, more irrigation water could mean higher nitrogen credits from irrigation water, if groundwater nitrate-N levels were high. Our calculation so far is that our full season potatoes in the K field (33.3 ppm of nitrate-N in the well) at Hancock have received 22.65’’ of irrigation water and a total of 173 lb/acre of extra N just from the groundwater. This high amount of extra N could cause confounding effects on our nitrogen trial this season. I will keep everyone posted once we harvested those research plots.

For commercial production, this season’s potato crop is in good to excellent condition, which should be associated with above average yields with a larger size profile, similar to 2022. Average yield is estimated to be 430 cwt/acre. In particular, more yellows and less reds were planted this year, yields of early-season yellows have been above average, and those of reds have been close to or slightly below average. For early-season russets such as Caribou, yields and size profile were reported to be ideal. Full-season harvests are ongoing, besides the hot conditions that shut down most of the operations in early September, conditions were almost ideal and potatoes look good going into storage.

According to USDA, growers are expected to harvest 500 more acres in 2023, which is estimated to be 870,000 cwt more potatoes than last season. If ideal conditions continues, we should see a 3.1% increase of total production compared to 2022. Happy harvest!


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

Early blight of potato/tomato. Accumulations of P-days this past week were between 35-66 across the state of Wisconsin. In all locations and all planting dates, potato fields have surpassed the threshold and should continue to receive fungicide applications for early blight management depending upon the time-to-harvest of the field.

Late blight of potato/tomato. Accumulations of Blitecast DSVs were low with a range of 1-6 this past week in the 7 sites detailed in our table, below. The Antigo location did surpass the threshold of 18 DSVs this week with a total of 20 as of 9/16. Prevention of late blight should still be considered even this late in the season since airborne sporangia can move into fields (despite senescing foliage) and make their way down to tubers in the soil to create tuber infection. Late season application of mancozeb can be helpful in limiting tuber blight. 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. The usablight.org website (https://usablight.org/map/) indicates no new late blight reports in the past week. So far, all characterizations of the late blight pathogen identified in North America this growing season have resulted in the US-23 type.

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. 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 9/16/2023 Potato Physiological Days (P-Days) through 9/16/2023
Spring Green* Early Apr 3 May 9 13 1015
Mid Apr 17 May 12 13 994
Late May 10 May 23 13 925
Arlington* Early Apr 5 May 10 12 1027
Mid Apr 20 May 15 12 989
Late May12 May 25 12 929
Grand Marsh Early Apr 5 May 10 14 975
Mid Apr 20 May 15 14 941
Late May 12 May 25 14 889
Hancock Early Apr 10 May 17 14 944
Mid Apr 22 May 19 14 938
Late May 14 May 28 14 889
Plover Early Apr 14 May 19 16 934
Mid Apr 24 May 20 16 929
Late May 19 May 29 16 879
Antigo Early May 1 May 28 20 830
Mid May 15 June 3 20 785
Late June 7 June 23 20 649
Rhinelander* Early May 7 June 1 12 798
Mid May 18 June 5 12 763
Late June 9 June 24 12 640

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. 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. https://vegpath.plantpath.wisc.edu/2023/08/28/update-15-aug-27-2023/


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