Southern Apple Growers Faced Reduced Crop In 2010

According to Dr. Steve McArtney, apple specialist, North Carolina Sate University, Mountain Crops Horticultural Research Station in Mills River, NC, this year has produced an interesting thinning season like none he’s ever seen before, with “severe over-thinning of some cultivars, and even a total loss of the crop where no thinners were used in some instances.”

Here, McArtney describes what he believes to be responsible for the over-thinning:

Spring came late to the Southeast with budbreak delayed by around ten days compared to historical averages, but high temperatures during the following weeks sped things along so that bloom was delayed by only 2-3 days. With a snowball bloom on most varieties, good weather during bloom and good bee activity there were no apparent problems with pollination or initial fruit set. All the indications suggested that we were setting up for a huge potential apple crop and that an aggressive approach to chemical thinning would be needed. Several growers started their thinning program with a petal fall Sevin application but the majority chose a more traditional approach, waiting until the fruit reached the 10 mm stage to make their initial thinner application. Although the petal fall Sevin sprays gave some good early thinning it was still obvious that additional post-bloom thinner application(s) would be needed to further reduce the crop to a manageable level.

Beginning in mid May reports of over-thinning started coming in from across the Southeast. In many cases this was severe over-thinning; in some areas all varieties dropped all their fruit while in others the heavy drop was specific to only one or two varieties. In Henderson County, NC, many Red Delicious and Gala trees were significantly over-thinned. In research plots at the MHCREC we observed that spur Red Delicious trees that were sprayed with a thinner when they were at the 10 mm stage on April 30 were defruited regardless of what chemicals they were thinned with. Yet unthinned control trees in the same trial set heavy crops of fruit, and trees that only received a petal fall Sevin application on April 21 thinned down to an acceptable commercial crop load.

How could conventional thinning sprays that were applied at the normal time end up defruiting something like spur Red Delicious strains which are typically very difficult to thin, especially after initial fruit set had been so high and the fruit appeared to be growing so strongly? The most severe cases of over-thinning occurred on trees that were sprayed around April 30. To try and understand what happened I requested temperature and light records for Henderson County from the State Climate Office of North Carolina.

While light levels were generally good in the weeks following bloom there were occasional instances of cloudy days, the longest of these was for two consecutive days (May 2 and 3). The records showed that while daytime maximum temperatures appeared to be relatively normal throughout this period the nighttime temperatures on May 2 and May 3 were unusually high (66F). This combination of reduced light levels and high night temperatures was recorded 2-3 days after what appeared to be the critical time when thinners were applied to Red Delicious and Gala trees that were over-thinned. Chemical thinners that were applied either before or after this time were certainly effective, but did not result in over-thinning.

Both low light levels and high night temperatures can create a stress in the tree by reducing the amount of carbohydrates available for fruit growth. Low light will reduce photosynthesis while high night temperatures will increase the rate of dark respiration, consuming any carbohydrates that might have been produced by the tree during the day. The light and temperature conditions 2-3 days after application of chemical thinners on April 30 were likely to have created a severe carbohydrate deficit in the trees that greatly increased the efficacy of chemical thinning sprays.

Knowing that the level of carbohydrates in the tree can have a pronounced effect on fruit growth and the efficacy of chemical thinning sprays I sent the weather data to Dr. Terence Robinson at Cornell University in Geneva, NY and asked him to run the carbohydrate balance model that his colleague Dr. Alan Lakso has developed for New York conditions. This model uses light and temperature data to calculate the amount of carbohydrates available for fruit growth of a standard ‘Empire’/M.9 tree each day. These researchers have observed over many years that chemical thinners tend to be more effective when the application coincides with a period of carbohydrate deficit, and less effective when applied at a time of carbohydrate surplus. Output from the model showed two periods of severe carbohydrate deficits following bloom. The first was for a period of six days beginning the day before petal fall. Robinson notes that periods of deficit around bloom do not result in over-thinning because the fruit are relatively insensitive to chemical thinners at this time. The second deficit which was more severe occurred between April 30 and May 4, corresponding to the period immediately after application of chemical thinners to trees that were over-thinned, and to the period when fruit are most sensitive to chemical thinners.

Daily carbohydrate balance of apple trees in NC according to the Lakso Carbohydrate Balance Model. The model was kindly run by Dr. Terence Robinson using light and temperature data from Henderson County.

The model appears to explain why over-thinning occurred on many North Carolina orchards in 2010. While it’s academically interesting to look back and understand why this happened, the explanation provides little comfort to the many growers who may suffer financially this year as a consequence. It would be much more useful to see if the model might have predicted this response. Cornell University research and extension personnel report that the Lakso Carbohdyrate Balance Model has provided useful predictions of chemical thinner efficacy under New York conditions over the past few seasons. Of course, the accuracy of any prediction is dependent on the accuracy of short term (3-5 day) forecasts of light and daily maximum/minimum temperatures. While such forecasts may be more accurate in the apple production regions of New York than in the mountains of North Carolina this will not stop us from evaluating the model as a tool to help growers make chemical thinning decisions here in the future.