Monilinia Blight Post #4 May 5

 

We are now in the thick of the Monilinia Blight season with apothecia emerging and active (Figure 1) and many blueberry varieties having buds in the F2 stage of development (Figure 2). We had conditions for infection periods during the rain we received Friday and Sunday last week, and there is potential for another infection period this Thursday with the forecasted rain and warm temperatures.  

Many growers sprayed last week ahead of the precipitation received on Friday and Sunday.  Growers are urged to reapply either before Thursday's rain using any registered fungicide for Monilinia Blight in highbush blueberry, or directly after using a registered group 3 fungicide, which can provide back action up to 72-hours after an infection period. At this point the weather for the weekend is unsettled, but there is potential for rain, and applications made for the infection period Thursday will provide residual control over the weekend.   

Figure 1. Apothecia observed May 4, 2026. 

Figure 2. Blueberry buds at the F2 development stage observed May 4, 2026. 

Monilinia Blight Post #3 April 28

From monitoring early this week mature apothecia cups have been observed in the Annapolis Valley, but most blueberry varieties are still in the majority bud scale separation stage with a few buds entering the F2 stage. However, with the warm weather development may advance rapidly, and we will likely have our first infection period later this week, if we get the forecasted rain. 

Monilinia requires blueberry plants to be wet from rain or fog to cause infection. The risk of infection increases with the longer the plant is wet and with warmer. Monilinia infection period severity ratings for specific wetness durations and temperatures can be found below  (Table 1 ).  

Table 1. Severity rating of Monilinia Blight infections based on the length 

Most growers will choose to apply with one of the Group 3 fungicides for their first application. Group 3 fungicides move rapidly into the plant after application and most have a rain fastness of one hour. They are also very locally systemic though, and windy conditions during application can effect coverage and decrease effectiveness.  

If you are unable to get an application on before the rain, Group 3s have decent post infection activity and maybe applied up to 72 hours after an infection period and still provide some protection.  However, there maybe some risk to relying on post infection activity, so growers should plan on applying ahead of wet infection periods.