Amanda Gevens, Dept. Chair, Associate 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. TBD indicates that data is To Be Determined as time progresses. 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 graphical and raw data formats for each weather station at: https://vegpath.plantpath.wisc.edu/dsv/
| Location | Planting Date | 50% Emergence Date | Disease Severity Values 6/14/20 | Potato Physiological Days 6/14/20 | |
| Grand Marsh | Early | Apr 17 | May 18 | 13 | 197 |
| Mid | Apr 25 | May 26 | 10 | 142 | |
| Late | May 6 | June 1 | 7 | 101 | |
| Hancock | Early | Apr 8 | May 18 | 11 | 208 |
| Mid | Apr 20 | May 25 | 9 | 157 | |
| Late | May 4 | May 30 | 6 | 119 | |
| Plover | Early | Apr 10 | May 23 | 11 | 169 |
| Mid | Apr 20 | May 30 | 5 | 115 | |
| Late | May 5 | June 1 | 5 | 103 | |
| Antigo Station set up at airport 5/29 |
Early | May 14 | June 5 | 2 | 70.83 |
| Mid | May 24 | June 10 | 2 | 33.63 | |
| Late | Jun 1 | TBD | TBD | TBD | |
National Late Blight Reports: (https://usablight.org/map/) Late blight has been confirmed on tomato (FL) and potato (FL and AL) during the growing season of 2020. Dr. Yu (Monica) Chen from my lab at UW-Madison Plant Pathology processed a potato late blight sample from Putnam County Florida at the end of March and it was clonal lineage US-23. The other samples do not yet have a clonal lineage determination as of yet. I will continue to monitor this national database and offer updates here in the newsletter. This information is useful to consider, especially when clonal lineages are determined, as we can use these data to better understand the likelihood of particular clonal lineages here in Wisconsin. This information gives us indication of likely sensitivity to the phenylamide fungicides, such as Ridomil Gold.
Late blight character in WI over past decade: I provide a few helpful summaries of late blight character from WI, below. We routinely collect and charactize isolates of the pathogen, Phytophthora infestans, to best understand the epidemiology and management. As we are nearing the DSV threshold of 18 in our early plantings of potatoes in southern WI, it is time to consider preparing for early season preventative fungicides that best control the disease. Our 2020 Commercial Vegetable Production Guide for Wisconsin is a useful resourc in identifying fungicides. Page 220 begins listing of fungicides for late blight control. While potato cultivars can vary in susceptibility to late blight, most are quite susceptible and require fungicides for management of this aggressive disease. https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3422-2020.pdf
