Weekly Fruit Update - June 21, 2022
UNIVERSITY OF MINNESOTA EXTENSION - www.extension.umn.edu
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| Photo: A strawberry field after harvest and before renovation |
This update discusses: how climate change is impacting berry production; mid-season apple pests; and last call for grape nitrogen applications. Strawberry renovation was discussed in the last update and is still relevant though the content is not repeated in this article.
Apples
Pests that have entered the game this month:
- Woolly apple aphid - the white aphid colonies can be seen on apple tree shoots and watersprouts. Yesterday, we saw a beneficial predator, syrphid fly larvae, approaching a colony of aphids to feed on them. Syrphid fly larvae can consume all or part of a colony. Avoiding broad-spectrum insecticides early in the season, and avoiding unnecessary insecticide applications can help preserve beneficial insects including syrphid flies, green lacewings, and European earwigs.
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| Syrphid fly larvae crawling over to feast on woolly apple aphid - see white "woolly" colonies (Photo: Marissa Schuh) |
- Silver leaf - a disease of woody tissue that manifests as "silvery" leaves - this is a trunk disease, but the symptom we can see is leaves with a silvery-green appearance. The color is more subtle than powdery mildew, and covers the entire leaf evenly rather than in blotches.
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| A leaf displaying "silver leaf" (right) next to another leaf (left) that is just beginning to display the symptom at the leaf margins. Photo: Annie Klodd |
- Cedar apple rust lesions on leaves and fruit - CAR has been active for weeks now, but the lesions are now apparent on leaves and fruit.
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| Cedar apple rust lesions on apple tree leaves. Photo: Annie Klodd |
- Japanese beetles - Adult populations started emerging about 4 weeks ago, but have been most prevalent the last two weeks.
Apple maggot and codling moth are still active. However, the two orchards I scout have minimal to no codling moth or apple maggot because their IPM programs have included effective and well-timed insecticides. In one of the orchards, Assail was sprayed last week for Japanese beetles and also killed off the apple maggots present (source: weekly trap counts).
Grapes
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| This vineyard is demonstrating nitrogen deficiency, indicated by yellowish-green leaves spread evenly across the field. Photo: Annie Klodd |
I visited a couple of vineyards this week that were displaying nitrogen deficiency. If the leaves across the vineyard are consistently light green or yellow in color, the vines may need more soil-applied nitrogen fertilizer. Another sign is that the yellowed vines are low in vigor. This is the "last call" to apply nitrogen fertilizer - applying it during or after August can cause the vines to form new shoots in the fall, which can delay the "hardening off" process for winter. Apply any last nitrogen this week or wait until next spring.
Note: I recommend soil-applying nitrogen fertilizer. Based on objective university research, I do not recommend foliar applications of N. Its benefits are mainly cosmetic and temporary, and can injure the leaves. To quote Texas A&M Extension: "Foliar application - Although having limited benefits, small applications of N in the form of urea may be used to correct visual symptoms of deficiency. With foliar applications, a risk of leaf burn can occur mainly when sprays are done between bloom and harvest." Source: https://aggie-horticulture.tamu.edu/vitwine/2019/06/03/nitrogen-management-in-vineyards/
Heat and drought is impacting Minnesota's berry season:
The current heat wave has reduced yields and season length for strawberries and summer-bearing raspberries. Several raspberry farms have reported on their Facebook pages that the season was short and low-yielding. June-bearing strawberry season was also very brief, with some farms around the metro closing after about 10 days.
Excessive high temperatures impact fruit set for fruiting crops in several different ways:
- Reduced pollinator activity
- Flower and pollen deformation
- Strawberries ripening before sizing up
- Sunburn on raspberries
- Water stress due to high evapotranspiration (water leaving the plants through the leaves)
If climate change increases the frequency and longevity of heat waves and droughts, we will see this occur more often. In other years, we see the impacts of heavy rainfall and storms. Our team is interested in research and outreach projects focused on climate change resilience for specialty crop farmers. Learn about how climate change impacts Minnesota, and what UMN is doing, here. See one of our outreach projects in these two videos:
