Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Disease Severity Value (DSV) Accumulation as of June 10, 2009 is as follows:
Location: Shadybrook Farms, Little Creek, DE in Kent County.
Greenrow: May 1
| LATE BLIGHT | EARLY BLIGHT | |||
| Date | Daily DSV | Total DSV | Spray Recs | Accumulated P- days* |
| 6/1 | 0 | 58 | 5-day interval | 251 |
| 6/2 | 1 | 59 | 7-day interval | 259 |
| 6/3 | 0 | 59 | 10-day interval | 269 |
| 6/4- 6/6 | 19 | 78 | 5-day interval | 304 |
| 6/7- 6/8 | 4 | 82 | 7-day interval | 313 |
| 6/9 | 2 | 84 | 7-day interval | 323 |
| 6/10 | 0 | 84 | 7-day interval | 332 |
Maintain the recommended spray interval, the 300 P-day threshold has been reached for early blight and conditions are favorable for late blight.
Early this week late blight was diagnosed on commercial potatoes near Pocomoke City, MD. This is the closest report to date of late blight on potatoes.
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. Another product that is labeled for early blight is Revus Top from Syngenta. Revus Top is a combination product of Revus (mandipropamid, Group 40) plus difenoconazole which is triazole fungicide (Group 3). Difenconazole has been used in Europe very successfully for early blight control. Since it is a different mode of action than the strobilurin fungicides (Group 11) it would be a good alternative if resistance is an issue if Gem, Quadris, or Headline are not controlling early blight.
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. With all the rain we have had this application would be strongly advised if nothing was applied at planting. Apply in as much water as possible (20-30 gal/A): Mefanoxam/chlorothalonil (Ridomil/Bravo) 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.
For specific fungicide recommendations, see the Delaware Commercial Vegetable Production Recommendations.