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Weekly Fruit Update - 7/24/2020

In this week's update:

  • Apple grower survey - help us help you!
  • Apple maggots
  • Apple bitter rot prevention
  • Blueberry leaf issues
  • Sunburn on berries and apples
  • Grape bunch rots

Apple grower survey

UMN Extension and the IPM Institute have put out a short survey about the technical assistance needs of apple orchards in Minnesota. Whether you grow organically or use conventional practices, we want to hear from you so that we can better serve the needs of the growers in this state. The survey focuses on pest management challenges & strategies.

If you grow apples, we would greatly appreciate if you could take a few minutes to take the survey. 

Apple maggots 

According to the IPM Institute's weekly "AppleTalk" call, apple maggot is more widespread this year than in recent years. Populations typically peak in early August, and control efforts should be focused around that time. Eggs that are laid close to harvest time can hatch in the apple bins in the coolers. 
Using traps and checking them regularly aids with timely management. 

If trap counts are consistently high, a powerful insecticide like Exirel or Assail at a high rate has good activity on apple maggot. However, we are not yet at that point. Small populations at this point can be managed with Delegate, imidaculprid (Wrangler) or another less expensive product, saving the other products for later use once populations expand. 

Apple bitter rot prevention

The recent hot, humid weather of mid-July is ideal for the formation of bitter rot on apples. However, the fruit are susceptible to bitter rot pathogens from petal fall to harvest. Infections present during the season can also survive and expand in storage, so in-season control and correct identification of symptoms on the sorting line are important. 

Look for tiny brown, circular, slightly sunken lesions on the fruits. As these spots enlarge, they become large brown or cream colored circles on the fruit. (See this article for photos). If the fruit is cut open, a brown "V" will be present at the sight of the large circular lesion.

Good sanitation (removal of dead plant matter) throughout the year is important for reducing bitter rot pressure. Additionally, bitter rot is most common on stressed, unhealthy, or winter damaged trees. Maintaining good fertility, weed management, and pest management are essential for preventing bitter rot. 

Fungicides can be used for control of bitter rot from the first cover spray until harvest, but they are more effective on trees that are already healthy. See the Midwest Fruit Pest Management Guide for fungicide recommendations. Captan is an effective protectant, and must be reapplied after each rain.

Blueberry leaf issues

These disease-friendly weather conditions also mean blueberry growers may be seeing a variety of symptoms on leaves. Additionally, nutrient stress, high pH, or water stress can cause leaf discoloration and browning. I have seen a number of different leaf symptoms while scouting in the last week. Good identification of the problem is important for treatment, and can prevent unnecessary fungicide or fertilizer application. 

Problems identified in the last week include powdery mildew, phomopsis, water stress, iron deficiency, nitrogen deficiency, and phosphorus deficiency. Nutrient deficiencies often have more to do with excess soil pH rather than lack of nutrients. 

University of Minnesota Extension has a good diagnostic tool for identifying problems on blueberry plants: What's Wrong With My Plant? Another disease collection is here, on eXtension. Cornell University also has a photo guide to blueberry leaf symptoms here. Growers are also welcome to send photos to me for help with identification ( 

Sunburn on berries and apples

I have also received a few photos of sunburn on berry crops the last couple of weeks. Please see this article for information on what sunburn looks like on raspberries, grapes, and apples, and how to prevent it. 

Grape bunch rots

Botrytis "grey mold" on a Marquette grape cluster, Sept. 2019. Photo: Drew Horton.

It is almost time for grape bunch rots to show up in vineyards. Bunch rot infections become apparent on ripening clusters after veraison (early August). Sometimes, the clusters can be clean for most of the post-veraison period, and then bunch rots appear right before harvest. Please see this article on botrytis and sour rot, and watch last week's UMN/UW Extension webinar on sour rot, featuring Dr. Megan Hall from University of Missouri. 

Author: Annie Klodd, UMN Extension Educator - Fruit and Vegetable Production

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