Mummy Berry 2020 - Post #3

Over the past weekend we expected some major development in bud but given the cool temperatures of late, we really have not seen things progress all that much.  To date, development is late and the cool weather has resulted in slow bud development.  As you can see in Figure 1, temperatures from 1 March to 4 May, 2020 are tracking significantly behind the 5- and 10-year averages.

Figure 1. Heating degree day accumulations for plant (above 5°C) and insect (above 10°C) development from March 1st to April 20 th for the past 17 seasons. Provided by Jeff Franklin (AAFC).

That said, with wet days and some mature apothecia cups confirmed, if you haven't applied a treatment in the last 7 days, it may be prudent to do so.  Please confirm bud stage in individual fields/blocks on your farm.  In advanced fields (40-50% or more at F2), there is risk of infection if all other conditions are present.  For the primary infection of monilinia to occur you need all three of these conditions to occur:

1. Inoculum present: history of monilinia blight and mature apothecia cups (Figure 2).
2. A susceptible host: vegetative buds are more than 6mm exposed green tissue and bud scales have separated (F2 stage) in the flower buds (Figure 3).
3. Ideal environmental conditions: wet periods with relatively warm temperatures (more explanation below).

 Figure 2. Mature apothecia cups.

Floral bud counts on Monday May 4, 2020 from the Annapolis Valley Nova Scotia: 

• Duke block: 50-65% of floral buds are susceptible to mummy berry infection (Figure 3)
• Second block of later varieties: Buds are swollen but not susceptible yet. 

Figure 3. Floral buds at F2 stage of development.

Based on the wetness period and different air temperatures, Paul Hildebrand and Rick Delbridge developed the Mummy Berry Forecast System in Nova Scotia (Table 1). By visually assessing when the floral bud scales have separated and leaf buds are more than 0.6 cm green, growers can determine when 40-50% bud break has occurred. If temperature and leaf wetness fall within the moderate to high infection range on the table, it is recommended that the grower apply a fungicide within 72 h from the start of the wet period.

     Table 1. Temperature and wetness conditions required for Mummy Berry infection.

Table 2 presents the susceptibility of common highbush blueberry cultivars to fruit infection by Mummy Berry (M. vaccinii-corymbosi). Susceptible (fruit infection incidence 30-50.9%), moderately resistant (fruit infection incidence 11-29.9%), and resistant (fruit infection incidence 0-10.9%) varieties. (Data from Stretch and Ehlenfeldt, U.S. Department of Agriculture).

Table 2. Highbush blueberry cultivar susceptibility to Mummy Berry

As a reminder, these recommendations are based on conditions observed in the Annapolis Valley, Nova Scotia and may not be applicable to your location. The decision to treat should be based on appropriate conditions observed on your farm and in particular fields/blocks. For detailed information please check out Perennia's Management of Mummy Berry Disease in Highbush Blueberry. 

A complete list of registered products can be found in Perennia's Highbush Blueberry Disease and Insect Management Guide.