VT Apple IPM: Heading into bloom

Trees at the UVM orchard in South Burlington are at early to late pink bud stage and we expect bloom to start this coming week. This is go-time for setting the crop for 2024, and there are a lot of considerations to take into account as we make management decisions.

First- pest management. This looks like a doozy of a week for apple scab, so stay covered and plan to include both a protectant (mancozeb or captan) and a single site (FRAC groups 3,7,9,11, always rotating between them) fungicide (opens to spray table in New England Tree Fruit guide) in any sprays. This will help with postinfection action for missed apple scab infections as well as help out with cedar apple rust and powdery mildew. Organic growers, I would be applying sulfur with every ½ to 1 inches of rain- you have both washoff and rapid leaf expansion working against you. Today and tomorrow look great for reapplying any sketchy coverage before the next rains come Wednesday and Thursday. The good news on the disease front is that, for now, fire blight is a non-issue as the cool weather hasn’t allowed sufficient bacteria to multiply to make for an infective dose once blooms do open. Keep an eye on NEWA, though, as conditions can rapidly change with just a few warm / hot days.

Insect activity is pretty quiet. We have caught a few tarnished plant bugs at the UVM orchard, and have just hung European apple sawfly, obliquebanded leafroller, and codling moth traps. This is a good time to get those up. As we approach bloom, it is best to keep the insecticides on the shelf to protect both managed and wild pollinators.

Most orchards that were frost affected last year have abundant fruit buds. But look carefully and methodically in your orchards, especially across varied topography, as the cold snap on April 25-26 appears to have done a bit of damage. I am seeing everything from complete bud kill to undamaged buds in the same block. Here is a quick video I made in the UVM orchard of the damage we’re seeing: https://www.youtube.com/watch?v=TPn0s_klGPI

Normally I would be suggesting that folks start thinning at bloom with a mild thinner, but I do suggest a good scout of your buds to see if you have significant damage first. I am inclusion longtime UMASS thinning expert Duane Greene’s recommendations for bloom thinning this year from the Healthy Fruit newsletter (always worth the cost of the subscription, which can be found here):

Chemical thinning suggestions for bloom-time (Duane Greene)

• Weather during the chemical thinning season has been widely unpredictable the fast few years. Given this uncertainty it seems prudent to take advantage of each opportunity to thin with the hope that an application or two will be followed by favorable weather to thin. Even if the weather may not be ideal, chemical thinner applications may predispose fruit to drop following subsequent applications. Given this fact we suggest that you take advantage of applying a thinner at each stage of development, especially early. Apples are less susceptible to hormone type thinners at bloom, so the risk of over-thinning is extremely small.

• We experienced an extremely challenging growing season last year and there may be repercussions from that, which may require special attention. Some blocks of trees were damaged last year immediately following bloom which resulted in trees having few or no seeds. Seeds inhibit flower bud formation for the following year. On these trees and on trees that carried a light crop last year we have noted exceptionally good return bloom which will require special attention to thinning. If nearly all spurs have flowers, it will be almost impossible to thin these trees down to an appropriate crop load using only our usual post-bloom hormone thinners. In these special cases it may be appropriate to apply the blossom thinner ATS at bloom at 2.5 to 3%. Cornell recommends 2 applications with the first being made at 60% bloom. (You can also use the Pollen Tube Growth Model on NEWA, however, that is considerably more involved.) There may be some phytotoxicity but this injury does not last long. Normal hormone thinner applications should probably follow based upon observation of initial set. (Note that use of ATS should be recorded as a fertilizer application in your spray records.)

• Generally, an orchardist chooses to use a hormone type thinner at bloom, especially NAA (Fruitone-L, Pomaxa, Refine) or Amid-Thin. I would like to emphasize again that thinners applied at bloom are at least 50% less effective than when applied post-petal fall. I have never over-thinned any apple tree with a bloom application of a hormone thinner, but I think that this application is necessary to start the thinning process. I have noted that thinner application at bloom may not thin by itself but if followed by a thinner application at petal fall or even at the 10 mm stage greater thinning has been noted on tress that also received a bloom spray.

-NAA is often the default thinner selected for the bloom application. Check the production guide for recommendations for a specific varieties that you are thinning. A 10 ppm rate is often standard when applied at bloom.

-An equally viable choice for a bloom thinner is Amid-Thin, although it is less frequently used. Amid-Thin is a mild thinner and over-thinning is very unlikely. The Amid-Thin label has been changed and updated for the 2024 thinning season. In general, higher rates of application are allowed. We suggest that you read the new label. The application rates suggested for specific varieties have been revised and modern varieties have been included. The highest suggested rate of application remains at 8 oz/100 gallons, although higher rates are now allowed. The maximum amount that can be applied in a single application is 20.9 oz with a maximum amount that can be applied in one year cannot exceed 62.7 oz per acre. In previous years I have added a surfactant (Regulaid) to Amid-Thin to improve thinning capability, but this addition did not have a measurable effect on thinning. Since my experience with Amid-Thin is that it is a mild thinner, I would not hesitate to increase the rate applied a little above 8 oz/100 gallons on harder to thin varieties. Proceed cautiously.

Pest and Pollinator Showcases this week:

Orchard Pests and Pollinators On-Farm Meetings – two locations.

Thursday May 9, 4:30-7:00, Sweetland Farm, 97 Kerwin Hill Road, Norwich, VT

Friday May 10, 4:30-7:00, UVM Catamount Farm, 65 Green Mountain Drive, S. Burlington, VT

These are free, hands-on field meetings to increase your identification skills and understanding of common orchard pests, wild pollinators, beneficial insect habitat in tree fruit production. Pesticide applicator (2) and Certified Crop Adviser (2) credits available.

Questions? Laura.o.johnson or 802-656-4827

Where trade names or commercial products are used for identification, no discrimination is intended and no endorsement is implied. Always read the label before using any pesticide. The label is the legal document for the product use. Disregard any information in this message if it is in conflict with the label.

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