Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Late Blight Advisory
Disease Severity Value (DSV) Accumulation as of May 26, 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 |
| 5/16 – 5/17 | 27 | 7-day spray interval |
| 5/18 – 5/21 | 32 | 5-day spray interval |
| 5/22 – 5/26 | 32 | 7-day spray interval |
The current weather pattern has not been favorable for late blight. The DSV threshold of 18 is exceeded and sprays should be initiated if not already done. 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. At this point weekly fungicide applications would be suggested. For specific fungicide recommendations, see the 2008 Delaware Commercial Vegetable Production Recommendations Book.