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Apple Chemical Thinning Advisory for Southeastern PA, May 3, 2012

Posted: May 3, 2012

The diameter of the largest fruits in many orchards is in the 12-14 mm range, meaning that we are in the advanced stages of the primary thinning window.

So what does the Carbon Balance Model say? The Model reports that there has been a mild carbon deficit for the last two days, even though temperatures were only in the 60s yesterday, and much cooler than was forecasted. This mild deficit is due to low sunlight levels for the last three days. A much stronger deficit is predicted for the next several days, due to the much warmer temperatures plus cloud cover that is forecasted.

Background

Many locations in the Midwest and Eastern United States have suffered significant apple crop losses due to freeze and frost this Spring. In south central and southeastern Pennsylvania we are still expecting to harvest a full crop of apples and peaches. It is my understanding that northeastern Pennsylvania may have some damage, perhaps similar to the 30 to 40% crop loss predicted in the nearby Hudson Valley. Western Pennsylvania seems to have suffered damage as well, but I haven't heard a numerical estimate. Thus the Southeastern Pennsylvania region appears to be least affected by cold injury.

In this region, our main cold event was the March 27 freeze, which occurred at Pink on apples. Apple blossom mortality was 30 to 40%, and this was usually the king and 1st lateral flower. That left approximately 3 lateral blossoms per cluster, but the subsequent cool weather spread the opening of these surviving blossoms out, such that we still have a range of fruit sizes to address with chemical thinners. There is some frost damage to the oldest spur leaves, but most spurs appear to have adequate healthy foliage to feed the fruit. In some orchards, there are just a few of the larger 12 to 14 mm fruits, and a majority of the fruits are smaller diameter. Other blocks, especially those of of later-flowering varieties seem to have a large proportion of fruits in the large diameter category. The “early thinning” caused by the March 27 event also seems to have stimulated a large amount of lateral bloom on one-year-old shoots. Initial set of most varieties looks good, except for certain strains of Delicious.

The Current Situation

The diameter of the largest fruits in many orchards is in the 12 to14 mm range, meaning that we are in the advanced stages of the primary thinning window. The current forecast is for temperatures to rise to near 80 degrees F this afternoon, reaching the lower to mid-80s on Friday and Saturday, before dropping back to the 70s early next week. Skies this afternoon, Friday and Saturday are forecasted to be partly cloudy, followed by mostly cloudy-to-overcast skies for Sunday through next Thursday. So what does the Carbon Balance Model say?

The Model reports that there has been a mild carbon deficit for the last two days, even though temperatures were only in the 60s yesterday, and much cooler than was forecasted. This mild deficit is due to low sunlight levels for the last three days. A much stronger deficit is predicted for the next several days, due to the much warmer temperatures plus cloud cover that is forecasted.

Some Advice

So, here we are, at the moment of decision, with what appears to be a valuable crop on our hands if we can thin it correctly. What to do? I offer these thoughts.

I think the model is right (if the forecast is right). Thinners that have just gone on and those about to go on are going to produce a strong thinning response. This doesn’t mean you shouldn’t chemical thin if your initial set calls for it – you should thin soon, especially in blocks that have a large proportion of fruits in the large diameter category!

The first consideration: the “2 x 4” weather. Mild stress the last two days, higher stress predicted for the next 4. Reduce the strength of your tank mixes. This is probably not the year to throw the kitchen sink at your trees. Stick to the low-to-middle range of what you believe to be the best effective chemical thinners for your orchards. Serve your thinner “straight up”: Leave the oils and surfactants out. The notable exception to this advice is that a acidifier/ buffer should be used when mixing NAA with alkaline (high pH) well water.

Nibble at it. Apply thinners where you have no doubts as to the strength of set. Delay thinning in orchard blocks in which there are only a few fruit in the largest size category, and in which you need a portion of the smaller fruits to set to make a crop. If you are targeting removing only a portion of those fruits in the 8 to 9 mm class, you still have a few days to go. Consider holding off on these blocks until Sunday or Monday, which may allow some of the high temperatures to pass.

Glue yourself to the weather report of your choice and adjust to changes in the forecast accordingly. If the heat doesn’t materialize, (like yesterday, May 2nd: Forecast 78 degrees F- actual 63 degrees F), you may want to get going. Also watch for deteriorating spraying conditions and don’t get stuck in a gale with no thinner out.

Remember you are thinning for two crops--2012 and 2013. Just because other regions don’t have a full crop doesn’t mean we can serve consumers small, poorly-colored, insipid tasting fruits and command top dollar for them. And remember that there is no magic spray that will promote return bloom on over-set trees.

Good Luck!

Prepared by Dr. Jim Schupp, Penn State Fruit Research and Extension Center Pomologist.