Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Disease Severity Value (DSV) Accumulation as of June 18, 2008 is as follows:
Location: Broad Acres, Zimmerman Farm, Rt. 9, Kent County
Greenrow: April 27
| LATE BLIGHT | EARLY BLIGHT | |||
| Date | Daily DSV | Total DSV | Spray Recs | Accumulated P days* |
| 5/30-6/1 | 3 | 35 | 10-day interval | 252 |
| 6/2-6/4 | 1 | 36 | 10-day interval | 279 |
| 6/4-6/5 | 4 | 40 | 5-days | 279 |
| 6/5-6/6 | 2 | 42 | 5-days | 288 |
| 6/6-6/7 | 1 | 43 | 5-day | 298 |
| 6/7-6/11 | 0 | 43 | 10-day interval | 321 |
| 6/12-6/15 | 1 | 44 | 10-day interval | 350 |
| 6/15-6/18 | 0 | 44 | 10-day interval | 376 |
* P days– We use the predictive model WISDOM to determine the first fungicide application for prevention of early blight as well. The model predicts the first seasonal rise in the number of spores of the early blight fungus based on the accumulation of 300 physiological days (a type of degree-day unit, referred to as P-days) from green row. To date, 350 P-days have accumulated at the site. The 300 P-day threshold has been exceeded so fungicides for early blight should now be applied if not already done.
If pink rot or leak is a concern and no pink rot fungicide was applied at planting consider applying one of the following when potatoes are nickel-sized and repeating 14 days later. Apply in as much water as possible (20-30 gal/A): Mefanoxam/chlorothalonil (Ridomil/Bravo or Flouranil) 2 lb/A, or Ridomil Gold/MZ 2.5 lb/A, or Ridomil Gold/Copper 2 lb/A. If Platinum/ Ridomil Gold was applied at planting the label allows one foliar application of one of those products at tuber initiation if conditions warrant.
Early Blight and Black Dot
Many fields are flowering or have flowered and this is a good time to consider switching to an application or two of Gem, Headline, Quadris, or Evito (no black dot label) for early blight susceptible varieties. This can also be helpful for late season varieties, including russets, if stress makes plants susceptible to black dot later. Make one or two applications at the end of flowering and repeat 14 days later. Apply mancozeb or chlorothalonil 7-days later between the two applications.
For specific fungicide recommendations, see the 2008 Delaware Commercial Vegetable Production Recommendations Book.