217-A Method to Control Monilinia Blight

Fact Sheet No. 217, UMaine Extension No. 2255

Original text prepared by Rick Delbridge, Nova Scotia Department of Agriculture and Marketing, Kentville, Nova Scotia, and Dr. Paul Hildebrand, Agriculture Canada, Kentville, Nova Scotia, and modified by David Yarborough, Extension Blueberry Specialist, The University of Maine, Orono, ME 04469. Original April 1992. Revised February 1998.

There are several factors that will help blueberry growers make better management decisions to control Monilinia blight:

  • Following application the fungicide moves rapidly into the plant, within one hour. Therefore, a fungicide application may be applied one hour before an expected rain with good results.
  • The fungicides have excellent post-infection activity. They may be applied up to 72 hours after an infection period has started with good results.
  • The recommended fungicides will protect buds from infection for about four to ten days after application, depending on the infection conditions. If conditions favor infection, then a shorter spray interval is needed to get protection.
  • Research indicates that rain or fog is necessary to initiate an infection period. Moisture from dew usually is not enough to initiate infection.
  • Field frost for even one hour dramatically increases the susceptibility of buds to Monilinia. The increased bud susceptibility lasts for approximately four days after the frost. Infections do occur without frost, but levels are much lower, and longer wetness durations and higher temperatures are required.
  • Spray conditions are extremely important, gusty winds and/or changes in wind direction reduce deposition of the spray material on the buds and therefore both protectant and post-infection activity of fungicide will be severely curtailed.

These findings mean that once blueberry buds reach a susceptible stage, you can wait until after an infection period has occurred (as long as the wet period is not over 72 hours long!) to apply a fungicide. To determine when you need to apply the first spray, ask yourself the following questions:

  1. Is my field prone to Moniliniablight, that is, did I ever have blight? If the answer is NO, then a spray is not necessary. If YES, move to question 2.
  2. Are 40 percent to 50 percent of the leaf buds at a susceptible stage, i.e., leaf buds showing 0.2 inches of a green tip? If the answer is NO, a spray is not necessary at this time. If YES, go to question 3.
  3. Have the temperature and wetness duration requirements occurred (within four days of a frost) for a moderate to high infection? Refer to table below. If YES, apply the fungicide within 72 hours from the start of the wet period. If NO, wait for the next wet period.
Severity Rating of Monilinia Infection Periods
Mean Temperature (°F) during Infection Period
Wetness
Duration (Hours)		 36° 43° 50° 57° 65°
2 NONE NONE NONE NONE NONE
4 NONE NONE NONE LOW MOD
6 NONE LOW LOW HIGH HIGH
8 NONE MOD HIGH HIGH HIGH
10 MOD HIGH HIGH HIGH HIGH
15 MOD HIGH HIGH HIGH HIGH
24 HIGH HIGH HIGH HIGH HIGH

IMPORTANT NOTE: If a frost has not occurred, the risk is greatly reduced. Spray only when the rating is HIGH.

The second application should be applied at least seven days later when there is a MODERATE to HIGH severity rating after a frost or a HIGH rating when no frost occurs.

Refer to Wild Blueberry Fact Sheets 211, Blueberry Diseases 1, and 219, Disease Control Guide for Wild Blueberries, for details on identification of blight and disease control treatments.

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

© 1992, 1998

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