Madeline Wimmer- Fruit Production Extension Educator, wimm0035@umn.edu
It’s almost May, which means it’s that time of the year again to share some updates related to Minnesota fruit production. The Fruit updates series share updates related to fruit crop growth stages, report on pests and diseases found throughout the season, and deliver general guidance and tips related to Minnesota fruit production. Stay tuned for more updates throughout this year’s growing season.
Fruit phenology:
Apples:
Images: Apple blossoms in an first pink stage taken at ApplesRus in Rochester Minnesota (04/25; Zone 4b) and at a slightly earlier growth stage at the University of Minnesota Horticulture Research Center, near Victoria, Minnesota (04/26; Zone 4b).
Which growth stage apples are in depends on a few factors. This includes the USDA hardiness zone they’re grown within, influences from local topography, the specific apple variety grown, as well as general growth habit and canopy composition. Right now, many apple varieties grown in the Southern two-thirds of Minnesota are somewhere between a tight cluster and first pink growth stage.
Apple scab:
From the point apples enter the green tip phase of leaf emergence, they can be vulnerable to apple scab (Venturia inaequalis) infection if the weather conditions are right and spores are present. The chance for infection increases with a rainy spring like many parts of Minnesota have experienced this growing season. Rain presents an additional challenge by decreasing opportunities to apply fungicide; however, cooler weather during wetting events can also slow down the pathogen and decrease the likelihood for infection.
And with all of these variables involved, this is why it can be extremely helpful to follow models like the Network for Environment and Weather Applications (NEWA) that track and predict infection event dates, which are based on the average temperature, leaf wetness, and total rain during a given time period.
To learn more about apple scab, visit the University of Minnesota Extension webpage, “Apple scab of apples and crabapples.” Access to information related to apple scab models from the Horticulture Research Center’s weather station can be found on the NEWA website.
Grapes:
Images: A grape spur with two buds roughly in the wooly growth stage at the University of Minnesota Horticulture Research Center near Victoria, Minnesota (04/26; Zone 4b) and a similar growth stage for a vineyard near Rochester, Minnesota (04/25; Zone 4b).
Grape growth stages in this series will be based on the E-L system, which puts buds in both of these pictures at early “wooly” bud phase, after which point the buds will continue to grow, develop, and eventually leaf out, which they will be quickly progressing toward, this week.
From this stage until leaf out, grape buds will be tender and growers should take caution when tying fresh canes to the fruiting wire and when doing late season pruning. Buds at this phase will also begin to be vulnerable to grape flea beetle, especially for vineyard rows planted close to wooded areas. To learn more about grape flea beetles, check out this University of Minnesota webpage.
Blueberries:
Images: Two pictures of highbush blueberries taken at the “early pink bud” (left) and “late pink bud” (right) growth stages at Little Hill Berry Farm near Northfield, Minnesota (04/27; Zone 4b; photo taken by Aaron Wills). Both of these varieties are early producers and are farther along than some other varieties might be.
These pictures show a snapshot of some recent development stages of highbush blueberries grown in Southeastern Minnesota. The early pink bud growth stage shown here can still tolerate cold temperatures as low as 23-25F. After this point, these blueberries will continue to progress into bloom.
Cedar-apple rust telial horns sited in Zone 5b.
Image: A taylor eastern redcedar (Gymnosporum juniperi-virginianae) showing cedar-apple rust telial horns, one of multiple sporing stages at the University of Wisconsin Arboretum within Madison, Wisconsin (Zone 5b).
Cedar-apple rust can affect apple foliage and fruits, leading to unmarketable fruit as well as defoliation in more serious cases. It’s a fungal disease that requires multiple hosts, spending parts of its life cycle on cedars and other plants in the Cupressaceae family (see lifecycle image below) and the rest of its life cycle on apple trees and other plants in the Rosaceae family. The above image of telial horns was spotted on a cedar tree in a Zone 5b region.
Image: The cedar apple life cycle shares multiple hosts to complete its cycle, including cedar and apple trees. Image sourced from Cornell Cooperative Extension.
Cedar-apple rust can be managed by excluding susceptible plants of the Cupressaceae family from an orchard region. Additionally, some apple varieties exhibit a range of cedar-apple rust resistance. Visit the University of Minnesota Extension webpage, “Cedar-apple rust and related rust diseases” to learn more about cedar-apple rust management and view lists of resistance varieties.
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