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.
We just did our last digging of the season on Wednesday, and our two N treatments are showing more obvious differences in maturity, with more dead and senesced vines under the 100 N treatment as can be seen in the image below. We can also see that the lower N rate at 100 lb/acre set less tubers, showed lower average tuber weight, and yielded less than the higher N rate at 300 lb/acre. Total weight from the 100 lb N/acre was 4.2 lb for two plants, and total weight from the 300 lb N/acre was 5.9 lb for two plants (these numbers were lower than last digging, and could be not representative of the true yield). For the 100 lb/acre treatment, Portage Russet, the yellow French fry variety, and a full-season French fry variety still had green vines. For other varieties, they were mostly fully dead. Our first vine kill application is scheduled on 9/3.
Amanda Gevens, Chair, Professor & Extension Vegetable Pathologist, UW-Madison, Dept. of Plant Pathology, 608-575-3029, gevens@wisc.edu, Lab Website: https://vegpath.plantpath.wisc.edu/
Current P-Day (Early Blight) and Disease Severity Value (Late Blight) Accumulations will be posted at our website and available in the weekly newsletters. Thanks to Ben Bradford, UW-Madison Entomology for supporting this effort and providing a summary reference table: https://agweather.cals.wisc.edu/thermal-models/potato. 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. Data from the modeling source: https://agweather.cals.wisc.edu/vdifn are used to generate these risk values in the table below. I’ve estimated early, mid-, and late planting dates by region based on communications with stakeholders. These are intended to help in determining optimum times for preventative fungicide applications to limit early/late blight in WI.
|
|
Planting Date | 50% Emergence Date | Disease Severity Values (DSVs)
through 8/31/2024 |
Potato Physiological Days (P-Days)
through 8/31/2024 |
|
| Spring Green | Early | Apr 3 | May 9 | 62 | 957 |
| Mid | Apr 17 | May 12 | 62 | 940 | |
| Late | May 10 | May 25 | 57 | 838 | |
| Arlington | Early | Apr 5 | May 10 | 30 | 956 |
| Mid | Apr 20 | May 15 | 30 | 925 | |
| Late | May 12 | May 25 | 28 | 846 | |
| Grand Marsh | Early | Apr 5 | May 10 | 54 | 927 |
| Mid | Apr 20 | May 15 | 54 | 898 | |
| Late | May 12 | May 25 | 47 | 825 | |
| Hancock | Early | Apr 10 | May 17 | 69 | 884 |
| Mid | Apr 22 | May 21 | 67 | 854 | |
| Late | May 14 | June 2 | 62 | 774 | |
| Plover | Early | Apr 14 | May 18 | 56 | 880 |
| Mid | Apr 24 | May 22 | 52 | 848 | |
| Late | May 19 | June 7 | 48 | 733 | |
| Antigo | Early | May 1 | May 24 | 58 | 781 |
| Mid | May 15 | June 1 | 58 | 740 | |
| Late | June 1 | June 15 | 45 | 641 | |
| Rhinelander | Early | May 7 | May 25 | 31 | 770 |
| Mid | May 18 | June 8 | 30 | 675 | |
| Late | June 2 | June 16 | 30 | 627 | |
Late blight of potato/tomato. Late blight diagnostics continue to be available at no cost to WI growers and gardeners. Dr. Brian Hudelson of our UW Plant Disease Diagnostic Clinic and Dr. Amanda Gevens of UW-Potato & Vegetable Pathology can offer confirmation of the pathogen. Dr. Gevens will also offer strain typing of the pathogen. The usablight.org website (https://usablight.org/map/) indicates reports of late blight from the US so far in 2024 including NY (US-23), MI (US-23), ME (US-23), PA, and TN. Please keep in mind that the site is not comprehensive. Outside of this site, I’m aware of our single WI late blight report on tomato from this past week, 2 Ontario Canada confirmations of potato and tomato late blight (US-23), and a Florida late blight sample from potato (March 2024).
Late season late blight control in potato. Late blight does not appear to be widespread in WI – there have been no other reports in WI since last week’s tomato finding on August 28, 2024. https://vegpath.plantpath.wisc.edu/2024/08/28/late-blight-tomato-dane-co-wi/ It’s important to remain vigilant in managing the potato crop for late blight through senescence and harvest. Dr. Yu Monica Chen and Ms. Rosa Fabian in the Gevens Potato and Vegetable Pathology Lab ran allozymes analyses tests this past week to determine the genotype/clonal lineage/strain type of the tomato late blight found in Dane County. It looked like US-23, but we are re-running the tomato fruit samples as there were other pathogens in the infected tissues which muddied the results. Monica worked over the past few days to generate cleaner cultures for improved analyses early this week. I’ll share an update on Tuesday. US-23 Phytophthora infestans can typically be controlled with a range of fungicides including metalaxyl (ie: Metastar) and mefenoxam (ie: Ridomil). Continued foliar fungicide use can keep the crop protected in this final stage and limit late-season late blight infections that can further develop in a stored crop. Practice good harvest, movement, and storage practices to maintain the health and quality of tubers.
We accumulated few (0-8) Blitecast Disease Severity Values over the past week in WI. All WI locations are above the threshold for late blight disease severity values and should receive preventative fungicide application to reduce the risk of disease. An updated listing of fungicides for WI potato late blight management for 2024 can be found at the link below. Base protectants such as chlorothalonil and mancozeb offer broad-spectrum control of fungal and oomycete (water mold – like late blight) pathogens. https://vegpath.plantpath.wisc.edu/wp-content/uploads/sites/210/2022/07/2024-Potato-Late-Blight-Fungicides.pdf
Early blight of potato. All areas of production have reached the threshold for the application of foliar fungicides to limit early blight. This disease was unusual this year with a typical timeline for onset, but slower progression than most years due to high temperatures in July. A late season ‘flush’ of early blight was noted in many central and southern WI potato fields. I suspect that cooler temperatures with moisture in the late season promoted the uptick of activity. https://vegpath.plantpath.wisc.edu/diseases/potato-early-blight/
Cucurbit Downy Mildew: To date, downy mildew field findings in the US, including WI from this past week (8/29), have been caused by Clade 2 – cucumber and cantaloupe strain type. https://vegpath.plantpath.wisc.edu/2024/08/29/cucumber-downy-mildew-adams-co-wi/ No additional reports have come in on other cucurbit types – just cucumber from a commercial field at this time. No cucurbit downy mildew was noted on our cucurbit sentinel plot at the UW Hancock Agricultural Research Station when I last looked on Aug 28, 2024. The sentinel plot has 7 different cucurbit types out in the open field without fungicides. I observe 1-2X each week for downy mildew symptoms.
We have seen ‘look-alike’ diseases in WI which have primarily been angular leaf spot, Alternaria, or Anthracnose. Phytophthora crown and fruit rot has also been active in Wisconsin fields over the past month in fields with soilborne infestations of the pathogen Phytophthora capsici. This particular Phytophthora on cucurbits, as well as solanaceous crops, is highly favored by hot and wet weather. For more information: https://vegpath.plantpath.wisc.edu/diseases/cucurbit-phytophthora-blight-crown-rot/. Management guidance: https://learningstore.extension.wisc.edu/products/commercial-vegetable-production-in-wisconsin