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

Tree Fruit Recorded Message for Tuesday, May 24, 2011

Thinning Suggestions
Here are my notes from yesterday’s field meetings. Steve Hoying and Dr. Terence Robinson indicated that the next 7 days will be the necessary apple chemical thinning window in much of our region. During this period, growers should reduce application of thinning agents below your “normal” rates for the following reasons:

1. Many blocks, including some perennial heavy-bearing varieties like ‘Gala’, had moderate to less bloom due to 2 consecutive past years of heavy crops and some winter injury.

2. Foliage on trees is currently greater than normal for this time of year and very succulent. More leaf area and soft tissue will mean thinning chemicals will be drawn into the plant at greater-than-normal rates.

3. Predicted warm temperatures over the next week will mean trees will respond more to the thinners.

4. Trees have been running a carbohydrate deficit since green tip this spring and the deficit is expected to increase this week (see updated carbohydrate models for Marlboro and Hudson). This means that reserves of carbohydrates (i.e. energy source) in the trees are likely to be reduced from “normal” and natural thinning or “June drop” may be greater than normal. (I am already seeing drop in some blocks, but we expect to see significantly greater amounts once the trees undergo some stress with the upcoming heat.

Based on the above observations, apple thinning recommendations for our area include:

1. Wait as long as possible to thin in the hope you can determine what level of drop will occur naturally and/or the result of your petal fall thinner applications. You can get effective thinning out to about 15mm king fruit size. At least wait until Wednesday or Thursday, as the coming heat will cause fruit to start to differentiate in size and you can get some handle on how much is likely to stay and what will come off in the “June drop”.

2. Reduce “normal” thinner rates by 25% where blocks appear to have had good to snowball bloom and good pollination. Example: if you normally use 10 ppm NAA, drop back to 7.5 ppm. Where blocks appeared to have poor-moderate bloom, reduce rates further and use 50% of the “normal” rate or less. There are some blocks where natural June drop will be sufficient and no chemical thinning may be needed this year.

3. This might be a year where NAA alone or carbaryl alone are options in blocks of moderately hard-to-thin cultivars (Jersey Mac, Paula Red, Aceymac, Spartan, Stamen) where you would normally combine the two. Given the forecast for repeated wetting this coming week, you might want to choose NAA alone because repeated wetting of carbaryl may lead to excessive thinning.

4. Consider still using the combination carbaryl plus NAA or Maxcel on hard-to-thin cultivars (examples include Cameo, Gala, Honeycrisp, Fuji, Jonamac, Golden Delicious, Red Delicious, Empire, Lady Apple).

5. Many growers prefer to use Maxcel instead of NAA on small-fruited cultivars like Gala and Empire. Do not use Maxcel without tank-mixing it with carbaryl as it is too weak a thinner on its own. Just drop the rate of Maxcel by 25-50% depending on the bloom.

6. Do not concentrate the carbaryl this year to reduce residues and thinning from rewetting. Example: if you have a 500 gallon tank, only put in 1 pint per 100 gal or a total of 5 pints, regardless of the number of “X’s” you spray.

7. This would be a year to use dry powder carbaryl to reduce the greater possibility of phyto associated with the liquid formulations, particularly the XLR+ formula. However, we learned yesterday that the powder is in short supply or not available. Again, this is even more reason not to concentrate the carbaryl.

8. It may also be wise to leave captan out of your fungicide treatments in your next spray if you are thinning this week with carbaryl, and certainly don’t tank mix captan with carbaryl this week.

9. You may not be able to see the results in time this coming week before you must re-thin if you already applied carbaryl, NAA or the combination at “petal fall”. In that case, reduce the rate of NAA or Maxcel to no more than 50% of your normal rate in your next application. Some blocks thinned earlier may not need another thinning application, but we won’t know that until we have a few days of heat to stress the trees. Warm night temperatures are the key factor.

The following comments by Dr. Robinson are related to pear thinning:

1. Recent research at Geneva showed that Maxcel at 4 quarts per acre (in 100 gal of water per acre) provided excellent thinning of Bartlet pears in a “normal year”. Half that rate or 2 quarts per acre was the recommended rate for Bosc. Not that 2011 is not a normal season and those rate should be reduce by 25% or more this year. This is an expensive treatment but produces great fruit size.

2. Carbaryl is not labeled on pears, can cause damage, and should not be used with Maxcel when thinning pears.