*
But, you can generally consider values between +20.00 and -20.00 to result in 'baseline' thinning. Values more negative than -20.00 suggests that the carbohydrate deficit is significant and you can expect fruit to come off easier. (Either with help from thinner -- use lower rate -- or by itself.) Values more positive than +20.00 suggest the tree is producing enough carbohydrates to support most of the developing (growing) fruit. Hence, thinning will be more difficult -- use higher rates of thinner or wait (if possible) to apply chemical thinner.
In addition, the Thinning Index is best applied when fruitlets are in the 5-15 mm size range. (Vs. petal fall or much greater than 15 mm which is past the normal thinning window.) Fruitlets approaching 10 mm in size is are more susceptible to chemicial thinner application(s), hence that is typically the optimum timing to be applying chemical thinner(s).
For another take on the thinning situation, look at Glen Koehler's Orchard Radar. (Select a location and then 'Horticulture').
May 31 update -- all Advanced IPM models have been updated. Note there is a significant uptick in carbohydrate deficit (to the positive side) beginning Thursday. Realistically, this means the successful chemical thinning window is closing as early as tomorrow. In addition, fruit size is approaching 15 mm where the activity of chemical thinners (NAA and BA in particualr) declines substantially. Any grower anticipating the need for chemical thinning should do it ASAP. Otherwise, if you feel you have already done your thinning, wait and see what you get!
May 25 update -- all orchards are still showing a carbohydrate deficit, thus fruit will be susceptible to 'easy' thinning. Advanced apple IPM growers are advised to use no more than moderate rates (2-3 oz. per 100 gallons water dilute spray) of Fruitone-L on easy-to-thin McIntosh. Another option would be to use Maxcel, again at moderate rate, 48-64 oz. per 100 gallons water dilute spray. Research at UMass suggests the addition of Sysstem-Cal (Agro-K) to Maxcel increase uptake and efficacy. (This could be a substitute for carbaryl.) Maxcel by itself is a pretty weak thinner. (Which might be good this year.) As we move into the weekend and early next week, thinning will become more difficult as fruit may be getting too large (>12 mm) and/or we are seeing more sun which will decrease the carbohydrate deficit. The next 5 days will be good days to apply chemical thinner if it is appropriate in your orchard. Be sure to read Mike Fargione's 'Thinning Suggestions' for added insight into the current situation.
May 20 update -- all the TI charts have been updated effective either yesterday (19-May) or today (20-May). In general, there is a significant carbohydrate deficit going into the next few days, which means chemical thinner sprays could be quite efffective. If using Fruitone-L, one might want to reduce the rate a bit to 2 to 3 ounces per 100 gallons (5-7.5 ppm) particularly where bloom or and/or pollination conditions may have not been the best. If you are confident of good initial fruit set the 4 to 5 ounces (10 to 12.5 ppm) is advised. (Non-Advanced IPM growers can also use carbaryl.) If fruit size is moving past 5 mm, then you might want to consider using Maxcel too, particularly if daily temperatures are in the mid-70's or above. Maxcel can be used alone (at higher rate, 2 to 2 quarts per 100 gallons) or in combination with Fruitone-L (or carbaryl if allowed) at 1 to 2 quarts. Combinations with Maxcel -- particularly carbaryl -- can be very effective thinners. Be careful.
One other thing to keep in mind, until we get some sunny, dry weather, foliage will be rather sensitive and uptake will be high. Lots of things to think about when applying these most important sprays. But don't be shy, we rarely over-thin and far more often under-thin.
Comments (16-May) from Mike Fargione, courtesy Cornell Cooperative Extension Hudson Valley Regional Fruit Program:
"If a dry window does present itself today, you might think about getting some carbaryl on blocks of hard-to thin cultivars to provide short-term insect control and to start the thinning process. You can review thinning strategies and rates for specific cultivars at http://ipmguidelines.org/TreeFruits/content/CH11/default-6.asp . On Friday, Steve Hoying suggested he would leave out the NAA in petal fall thinning of hard-to-thin cultivars sprayed early this week, since temperatures will be too low to promote the action of this thinner. The multiple wetting events we expect this week should help increase the thinning effect of carbaryl and make up for some of the NAA’s absence. I will present results from the carbohydrate model in one of the next 3 morning messages."
May 16 update -- not much has changed, although there is an indication the TI (Thinning Index) may rise towards the end of the week with (presumably) more sunny weather. Temperatures forecast this week seem to be lower than first thought. Fruitone (NAA) and most growth regulators are less active when applied during cooler temperatures (< 65 F.) Keep that in mind when timing your petal fall or later application(s).
May 13 assessment -- all orchards are showing a significant present and predicted carbohydrate deficit. Some ignore or discount the pre-bloom deficit, however, all indications point to a potentially effective application of thinner at petal fall. (This will have to be assessed with time.) A rate of 3 to 4 ounces Fruitone-L in 100 gallons dilue at petal fall is indicated in McIntosh blocks where carbaryl is not being used.*
*Note: these recommendations are largely for your consideration among many other factors when applying chemical thinners. The intent is to give you more information in helping you decide on your thinning strategy. Only you know the history of your orchard, the bloom conditions, and what your cropping objective is. Also consider the fact weather forecasts are notoriously fickle (and regionally quite variable) this time of the year.
**This assumes a normal bloom with no frost and reasonably good pollinating weather and bee activity.