Weekly Fruit Update - May 26, 2021

 

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

This week's fruit update includes:

• Apples - chemical thinning, codling moth, plum curculio
• Grapes - shoot thinning, training one-year old vines, fungicides
  
• Strawberries
• Raspberries
• Stone fruit
  

Apples

Fruit set is generally very strong this year in orchards I have visited across Minnesota in the last week. Honeycrisp yield is expected to be slightly below average in several states this year; I do not know if this is true in Minnesota. Fruitlets are growing about 1 mm per day right now. Yesterday in White Bear Lake, SweeTango fruitlets were between 11-12 mm. Codling moths have been found in traps in southern MN. 

Apply chemical thinners and monitor for plum curculio and codling moth. Apply effective insecticides for PC and CM once economic thresholds are reached.

SweeTango fruitlets following petal-fall NAA thinning spray, with king fruit at 9 mm on 5/21/2021, Preston, MN.

 

Chemical thinning: In order to avoid hand thinning, it is important to apply chemical thinners during a specific window while fruitlets are small. It is best to thin before 15 mm. Between 11-15 mm, the following products may be used: 

• NAA + Sevin
• Maxcel + Sevin
• Maxcel + NAA
• NAA will assist with return bloom, making it a good choice for use with Honeycrisp
• For Honeycrisp, one example program recommended by UW-Madison Extension is 3 oz NAA in 100 gallons water + 1 pt. of Sevin in 100 gallons water.

Once fruitlets are over 15 mm, NAA is no longer effective, and oils are commonly added to Maxcel, Sevin, or Etherl to increase the thinning response at that point. The use of oils can complicate other applications if using sulfur or copper, due to the phytotoxicity risk associated with mixing these products.

Read about how calculating sprayer tank mixes based on Tree Row Volume helps improve precision thinning.

Pests: Growers have begun to find codling moths in traps in Minnesota. This is an important time to spray for both codling moth and plum curculio, which are targeting the developing fruitlets. 

Plum curculio (PC): Scout varieties that size faster, first. Look for PC feeding and crescent-shaped oviposition scars. PC migrate into the orchard gradually once temperatures are consistently above 60F, especially if nighttime temperatures remain warm. They then mate for about a week before oviposition. They lay eggs in fruitlets that are over 5 mm. Therefore, PC risk is high at this point in Minnesota. Cover sprays are recommended if PC is found inside the perimeter (past the first 4-5 rows) of the orchard. If it is limited to the perimeter, then perimeter sprays may be used to limit insecticide. Thinning sprays of carbaryl are unlikely to control PC because the rates for thinning are too low for this insect. Pyganic is the most effective organic option. Consult the Midwest Fruit Pest Management Guide for insecticide options.

Codling moth (CM): I heard from orchards in Afton and Farmington that caught CM in pheromone traps over the weekend (May 22-23). Time sprays based on trapping and visual fruitlet injury, with one trap used per 2.5 acres. Spray once the traps catch 2 CM for 2 weeks in a row, or when more than 5 CM are caught in a single week. If you are not using traps, WSU Extension recommends spraying once 0.5% of fruit display injury. CM fly in the evenings (6-11pm) when wind speeds are between 3-5 mph, temperatures are above 60F, and there is no rain. Females lay most of their eggs the first evening they emerge. If conditions are not ideal when they emerge, egg-lay is decreased. Consult the Midwest Fruit Pest Management Guide for insecticide options.

Grapes

As shoots approach 12 inches long, the small time window for shoot thinning is coming to a close. We have released several warnings in the last few weeks to remind growers to mark out time for this time-sensitive task. Watch a short video we made earlier this week demonstrating shoot thinning. 

A healthy one-year old Frontenac grapevine with shoots between 6-14 inches long. Scandia, MN, 5/25/2021.

 

For grapevines that were planted last year: 

• If the existing trunk is healthy and producing viable shoots: De-sucker each vine, or leave one sucker if you desire double-trunk vines. Then, remove the majority of the shoots growing along the length of the trunk, leaving about 4 healthy shoots toward the top of the trunk. Two of those will become the cordons, and the others will be removed during winter pruning. Leaving 4, instead of 2, gives you more options for the best cordons come pruning season and helps reduce the risk of bull canes forming by distributing the vine's energy among multiple shoots. If the vines are growing on heavy soil and/or are exhibiting strong vigor, consider leaving 1-2 "kicker shoots" - a shoot somewhere along the trunk that helps distribute the vine's energy and reduces the risk of bull canes. 
  
• If the existing trunk is weak and has few vigorous shoots, it may have vascular damage that makes it a poor trunk in the long-term. Train up at least 1-2 suckers as potential replacement trunks. Thin minimally.

Grape disease management: We all know that this early part of the season is the most critical time for disease control in grapes. You probably applied your bud break fungicides last week. Because of the several days of rainfall we have experienced, the products you applied then are most likely no longer active. Make a similar application, including an effective protectant fungicide (i.e. mancozeb) and a powdery mildew fungicide, ideally before the next rainfall. If 1-2 inches of rainfall occurs, you may need to reapply it. Systemic fungicides are more effective during periods of heavy rainfall than protectant fungicides, because they are less susceptible to runoff during rain. Therefore, you may consider using a systemic for powdery mildew rather than a protectant, if many rainfall events are predicted. Apply systemic products post-infection (post-rainfall event) and protectants pre-infection (pre-rainfall) but allow several hours for the product to dry on the leaves before the next rain. This 2012 article from MSU gives some great tips on fungicide application and weather. Use the Midwest Fruit Pest Management Guide to view a grape spray program and an extensive list of fungicide options.

Strawberries:

Flower density appears to be very high this year at the farms I have visited or heard reports from. Most farms seemed to escape any significant frost damage. I have seen a small bit of frost damage in instances where the king blooms had started to open during the frost 2 weeks ago, but this will not significantly impact overall yields due to very high flower set. 

Bloom-timed fungicide applications are critical for botrytis and anthracnose control. Remember that even though we cannot see symptoms of these diseases at this time, they are active and reproducing, especially with so much recent rainfall. Refer to the Midwest Fruit Pest Management Guide for fungicide recommendations.

Continue scouting for tarnished plant bug and implementing control measures if necessary. Do not apply insecticides if not necessary, to protect pollinators and other beneficial insects active during bloom.

Tarnished plant bug on a leaf. Photo: Jeff Hahn.

Raspberries:

There is nothing new to report at this time, as summer-bearing raspberries are not quite at bloom yet; please refer to last week's update and the Midwest Fruit Pest Management Guide. for more information.

Please contact me at kloddann@umn.edu with any raspberry questions.

Stone fruit:

In Excelsior, MN, tart cherries are between 3-6 mm. Plums and apricots are between 5-15 mm, depending on the variety. Monitor for plum curculio feeding and oviposition injury and implement control measures if necessary; find information above, under Apples.