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/
Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations. 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 2022. 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 will come from weather stations that are placed in potato fields in each of the four locations, once available. Data from an alternative modeling source: https://agweather.cals.wisc.edu/vdifn will be used to supplement as needed. Data are available for each weather station at: https://vegpath.plantpath.wisc.edu/dsv/.
| Location | Planting Date | 50% Emergence Date | Disease Severity Values (DSVs) 7/16/2022 | Potato Physiological Days (P-Days) 7/16/2022 | |
| Grand Marsh | Early | Apr 5 | May 10 | 34 | 504 |
| Mid | Apr 20 | May 15 | 34 | 463 | |
| Late | May 12 | May 25 | 34 | 405 | |
| Hancock | Early | Apr 7 | May 12 | 24 | 451 |
| Mid | Apr 22 | May 17 | 24 | 417 | |
| Late | May 14 | May 26 | 22 | 373 | |
| Plover | Early | Apr 7 | May 15 | 59 | 443 |
| Mid | Apr 24 | May 20 | 59 | 409 | |
| Late | May 18 | May 27 | 58 | 374 | |
| Antigo | Early | May 1 | Jun 3 | 15 | 330 |
| Mid | May 15 | June 15 | 11 | 256 | |
| Late | June 10 | June 24 | 11 | 186 | |
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 (stations are not situated within potato fields). 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.
We have reached thresholds for preventative fungicide treatment in potatoes to manage early blight in all potato plantings in Grand Marsh, Hancock, Plover, and the earliest planted potatoes in Antigo areas of Wisconsin. Potatoes should be on a preventative fungicide program with effective disease management selections to limit early blight.
Grand Marsh, Hancock, and Plover potato fields have exceeded late blight disease risk thresholds and should be on preventative programs with effective fungicide selections to control late blight. Thresholds have not yet been met in the Antigo area. A fungicide list for potato late blight in Wisconsin was provided in last week’s newsletter and is available here: https://vegpath.plantpath.wisc.edu/2022/07/03/update-10-july-3-2022/
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 2022 Commercial Vegetable Production in Wisconsin Guide, Extension Document A3422, linked here: https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin
According to usablight.org there have not been recent diagnoses of late blight in tomato or potato crops in the US. For this year, there were just 2 reports entered back in March in southern Florida (US-23 clonal lineage/strain type).
Cucurbit Downy Mildew: During this past week, cucurbit downy mildew was confirmed in cucumber in NC, PA and VA. Previously in this growing season, the disease was confirmed in Alabama, Florida, Georgia, South Carolina, North Carolina, Pennsylvania, and New Jersey. No findings of cucurbit downy mildew in our Wisconsin-based sentinel plots in Dane County.
There are no confirmations of downy mildew on cucurbits in our region at this time, nor risk of spread of the pathogen to Wisconsin.
As a reminder, the pathogen is now known to have two ‘strains’ for clade types. The type (Clade 2) which infects cucumber, can also infect melon. Due to fungicide resistance within the downy mildew pathogen population, especially in Clade 2, selection of fungicides is important. Management of cucurbit downy mildew requires preventative fungicide applications as commercial cultivars are generally susceptible to current strains (Clades) of the pathogen. Management information can be sourced here: https://vegpath.plantpath.wisc.edu/2022/07/03/update-10-july-3-2022/