Because we have such a wide spectrum of damage – from 5 to 100% – in our apple crop this year, I thought it would be a good idea to provide some guidelines on thinning strategies and considerations for this unusual season.

First of all, it is imperative for growers to keep track of what is happening in terms of fruit set and drop, especially if Promalin was applied. To help predict fruitlet abscission in a “normal” season, Michigan State University’s Precision Crop load Management tool (http://msue.anr.msu.edu/uploads/files/PredictingFruitset1-21-14.pdf) recommends tagging 15 blossom clusters at the pink stage on 5 trees per block at pink (75 total clusters) and use them to track growth starting at 6 mm. After a frost you can use this same tracking methodology to predict which fruits will drop due to cold damage. Pay special attention to blocks located in high and low ground, to assess the variability of the damage within your orchard.

Growers with substantial damage should wait longer before thinning (12-18 mm), as fruit set will be more easily determined at that growth stage. Waiting until fruit is 12-18 mm is also recommended if damage to spur leaves is present. Fruit under 12 mm rely mostly on carbohydrates provided by spur leaves, however, after 12 mm the bourse shoot will become the main source of carbohydrates to the growing fruits. When spur leaves are damaged, thinning before 12 mm could result in over thinning.

On blocks with light frost damage where Promalin was applied it is possible to have a higher percentage of fruit set than desired. If waiting to thin after 12 mm, an increase in the NAA rate of 10-20% is recommended. However, take in consideration that trees with frost damage and those sprayed with Promalin will thin easier. In sites with damage to spurs a lighter thinner spray would be recommended (e.g. MaxCel at 32 oz/100 gal). Carbaryl alone or MaxCel alone are both weak thinners.

Do not apply nitrogen fertilizer in blocks with substantial damage, as this will encourage vegetative growth and reduced fruit set. Applications of Apogee to control shoot growth can help in trees with low fruit set.

If you have a high percentage of damage (90-100%), most likely any fruit that will survive will have some damage or blemishes (i.e., frost rings) and poor internal fruit quality. In this scenario growers might consider removing all the fruit that is left, so that they can reduce some of their sprays for controlling insects and diseases that affect exclusively fruit, however growers should be cautious regarding reducing all spraying as this might result on higher pest pressure for next year. To drop all the fruit off the trees you can use a combination of NAA + Carbaryl at petal fall and another application at 8-10 mm, or you can also use 6BA (MaxCel, RiteWay, or Exilis Plus) + Carbaryl. If you are past 18 mm, then there are only two chemistries that will have thinning activity, Carbaryl and Ethephon. Avoid nitrogen fertilizer, as these trees will be very vigorous due to the lack of fruit to control shoot growth. Application of Apogee, at a higher rate than normal years, will help control vigor. Apogee can be applied starting at full bloom and repeated 2, 4 and 6 weeks after full bloom.

Always make sure you read the label before using any products.

This article was posted in Apples, WFN, Vol. 1-4 and tagged Amaya Atucha, apple thinning, Apples, frost, frost damage.