Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Late Blight Advisory
Location: Art and Keith Wicks Farm, Rt 9, Little Creek, Kent County.
Greenrow: May 3
| Date | DSV | Total DSV | Spray Interval Recommendation |
| 5/3–5/12 | 0 | 0 | none |
| 5/13-5/15 | 11 | 11 | none |
| 5/15-5/16 | 3 | 14 | none |
| 5/16–5/19 | 19 | 33 | 5-days |
| 5/19-5/21 | 5 | 38 | 5-days |
| 5/21-5/23 | 12 | 50 | 5-days |
| 5/23-5/24 | 2 | 52 | 5-days |
| 5/24-5/25 | 0 | 52 | 5-days |
The threshold of 18 DSVs has been exceeded. The weather since last Friday has been very favorable for late blight if the fungus was present. Fifty-two (52) DSVs have accumulated so far for any potatoes that established green row (approximately 50% emergence) prior to and since May 3. Spraying as soon as possible with a protectant fungicide would be advised.
The summer-like weather we are having now with the high temperatures will reduce the risk and slow any development of late blight for the duration of the hot weather. Be sure to scout fields after that very conducive period for infection.
Next week I will start including P-day values for predicting early blight. We have only accumulated 160 so far. The early blight threshold is 300 P-days which indicates when conditions have become favorable for early blight infections to occur.