Cornell Cooperative Extension Hudson Valley Regional Fruit Program
Michael J. Fargione, Extension Educator

Tree Fruit Recorded Message for Wednesday, May 11, 2011

To Thin or Not to Thin (at Petal Fall) – That is the Question! (“slings and arrows” should be directed back at Steve Hoying and Mike Fargione)
We are getting close to petal fall thinning time, and undoubtedly some growers are wondering what to do. Let’s review the situation as we see it. Remember to always assess your own situation since conditions can vary widely across the region and do differ from block to block.

Winter – There have been some colder spots with temps going lower than we would like to see. It has shown up in this year’s bloom with some missing flowers, stunted stems, and generally ratty looking clusters. If you have peaches nearby, it is quite evident with the winter die back of shoots.

Pre-bloom – What a difference a year makes with this season’s slow developing cool season. We have already been making preparations for this thinning season by running the Lakso/Robinson Carbohydrate Accumulation Model. This model predicts carbohydrate accumulation by using temperature and sunlight. According to the model, trees have been running at a deficit all of this spring. There just hasn’t been enough sunlight and warm temperatures for trees to manufacture sufficient amounts of their own food. This means that trees has had to draw on stored food from within the buds, shoots, and wood. In years past, we have ignored carbohydrate accumulation/deficit during the pre-bloom period when considering thinning regimes, thinking the tree has sufficient stored carbohydrate for this period. We have decided to discount the deficit during pre-bloom again this year.
Bloom – In general, bloom is abundant on both spur and 1- year wood on most varieties and the crop potential looks excellent in most locations. Yes this is a generalization and there are exceptions. You need to look carefully at each of your blocks and we are happy to look with you if you want a second opinion. There have been reports of areas with less than sufficient bloom, missing clusters, etc., but as we look around, these are scattered and usually associated with too heavy a crop in 2010 (like some Fuji and Golden Delicious blocks). It would appear this is the “on” year for Honeycrisp, as many blocks bloomed densely. We have not experienced any frosts that would have affected viability of bloom. There has been some funky bloom timings – like Red Delicious coming out after Fuji, but in general bloom has been synchronized between varieties with full bloom occurring between the 4th and 8th of May (yes we are sure there are blocks that fell outside of those dates!). Although it has been very windy, both wild and imported bees have been working steadily assuring adequate pollination. And this week’s mild temperatures and sunlight will strengthen flowers and assure good set.

We think that this is a year to use petal fall thinning to start off your program, especially on those difficult to thin varieties such as Honeycrisp, Empire, Jonamac, Macoun, and Rome. Remember that the petal fall timing very rarely if ever over-thins, and has been shown to increase ultimate fruit size and make subsequent thinning trips easier and more successful. Another additional reason to consider petal fall thinning this year is the long-term forecast. After Thursday, there does not appear to be a cloudless day in the forecast for the next 2 weeks. By the time we get to the traditional thinning window, temperatures look like they will warm up. We could be going into an even greater carbohydrate deficit by the 10-12 mm thinning window, making over-thinning a real concern at that time. Petal fall may turn out to be the safest thinning window this season!

The minimum program for varieties that need petal fall thinning is to use 1 pt of carbaryl per 100 gallons. For harder to thin varieties, add some Fruitone to the mix. Generally 5ppm NAA for hard to thin varieties and 7.5ppm for impossible to thin varieties. For specific suggestions and recommendations be sure and see pp146-152 in the 2011 Cornell Pest Management Guidelines for Commercial Tree Fruit Production or on the web.