Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Late blight Advisory
We are using the E-WEATHER SERVICE from SkyBit, Inc as we have in the past. The service determines specific requested weather parameters (temperature, relative humidity and rainfall) based on calculations of data from the nearest National Weather Service stations. This weather data is used in the WISDOM software program for predicting late blight and early blight and making spray recommendations.
Location: Art and Keith Wicks Farm, Rt 9, Little Creek, Kent County
Green row: May 6
| Date | DSV | Total DSV | Spray Recommendation |
| 5/1 – 5/10 | 0 | 0 | none |
| 5/11 – 5/12 | 3 | 3 | none |
| 5/12 – 5/13 | 1 | 4 | none |
| 5/14 – 5/17 | 0 | 4 | none |
| 5/17 – 5/19 | 7 | 11 | none |
| 5/20 | 0 | 11 | none |
| 5/21 – 5/23 | 0 | 11 | none |
| 5/23 – 5/25 | 18 | 29 | 5-7 days |
Planting was delayed due to the wet weather this spring so we are about a week behind last year when comparing greenrow on early planted potatoes. Disease severity values have been accumulating very slowly this year especially compared to last season. The threat of late blight from seed infection is low, but there was some in Maine last season. Be vigilant anyway, given this recent weather pattern. The first late blight fungicide application is recommended once 18 Disease Severity Values (DSVs) accumulate from green row. Green row occurred approximately on May 6, 2010. Please be vigilant and keep a look out for suspect infections on young plants coming from infected seed pieces! Growers opting not to use the forecast system should put the first late blight fungicide application on when the plants are 6 inches tall, and repeat every 7 days. There are numerous fungicides now labeled for late blight control; however, use of mancozeb (Manzate, Penncozeb, or Dithane), chlorothalonil or Polyram are still very effective early season protective fungicides to use.
The recent wet weather from Sunday through late Tuesday was responsible for the accumulation of 18 DSVs. This was enough to trigger the first spray recommendation of the season. Fungicide sprays should be initiated if not done so already.