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.pdfView this newsletter as a PDF
