Potato Disease Advisory #7 – June 4, 2009

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Disease Severity Value (DSV) Accumulation as of June 4, 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*

5/27

3

56

7- day interval

 

5/28

0

56

 

217

5/29-5/30

2

58

5-day interval

235

5/31

0

58

5-day interval

244

6/1

0

58

5-day interval

251

6/2

1

59

7-day interval

259

6/3

0

59

10-day interval

269

Maintain the recommended spray interval. At 300 P-days fungicide sprays will be needed to control early blight. 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.

* 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. This should happen by next week. To date, 269 P-days have accumulated at the site. Once 300 P-days have accumulated, the first fungicide for early blight control should be applied. This usually occurs when rows are touching.

The Spray Recs column in the table is also generated by the WISDOM software program. This recommendation combines the DSV accumulation for late blight as well as the P-day accumulations for early blight and computes a spray recommendation. This is presented as a guide only. Spray decisions should be made with local conditions in mind and this information can help to determine if disease conditions are favorable.

For specific fungicide recommendations, see the Delaware Commercial Vegetable Production Recommendations.

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) 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.

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