Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Late Blight Advisory
Disease Severity Value (DSV) Accumulation as of May 15, 2008 is as follows:
Location: Broad Acres, Zimmerman Farm, Rt. 9, Kent County
Greenrow: April 27
Remember that 18 DSVs is the threshold to begin a spray program.
| Date | Total DSV | Spray Recommendation |
| 4/27- 5/6 | 7 | None |
| 5/8- 5/10 | 16 | None |
| 5/11- 5/12 | 21 | 5-day spray interval |
| 5/12- 5/14 | 21 | 5-day spray interval |
The rain events of last Friday and this Monday have produced favorable conditions for late blight if present on seed. The DSV threshold of 18 was exceeded and sprays should be initiated. Remember that these values are for potatoes that would have had about 50% emergence and made a row that you can see on or before April 27.
Growers who do not want to rely only on the DSV calculations for scheduling fungicide applications should apply at least 1-2 sprays of mancozeb (Dithane, Manzate, Pencozeb, Manex II) or Bravo (chlorothalonil) before plants canopy down the row. Late blight has not been a problem here in Delaware for many years and unless you have seed from an unknown source the risk of late blight is low.
For specific fungicide recommendations, see the 2008 Delaware Commercial Vegetable Production Recommendations Book.