Fruit size for Gala apples in the Central Virginia area is ranging between 10-12 mm now, which is a perfect size window for post-bloom thinning applications (e.g. by 6-BA and NAA products). Therefore, I ran the carbohydrate thinning model on NEWA website today, April 28, to predict: a) the tree carbohydrate level and b) thinning rates. Since 2017, we have been using the Batesville weather station for the central Virginia area and we will keep using the same location to help us compare among the year. We did not detect any considerable differences in the model outputs when we used the Afton weather station. However, if you have access to the model from your own computer, I would recommend that you run the model yourself based on the nearest weather station to your location. As you may be aware, any differences in the elevation levels would change the average daily temperature which is a critical input for calculating the tree carbohydrate level, and this will in turn change model outputs and recommendations.
As you will see on the NEWA website, there are two carbohydrate models, the old version which we have been using in the past three years and the 2019 new model (Apple CHO Thinning 2019) which I will use this year. One of the differences in the new model is the percent flowering spurs as one of the inputs you have to add to the model. Despite the fact that Gala is considered as an annual bearing variety and has no major issues with return bloom, I would recommend setting the model at 50-75% flowering spurs instead of 75-100%. The reason for that is to account for the extended period of cold weather we had during bloom and the frost damage the has been reported in some locations. These two factors might have affected flower pollination and total fruit set to some degree. The other two modifications in the 2019 model are: a) the tree carbohydrate balance column in the new model is using 7-day average values instead of 4-day average which makes it more accurate; b) the accumulated 4 oC degree days (DD) column which predicts the number of DD since bloom at which thinning would be highly effective. As a rule of thumb, 200-250 DD from bloom is considered the sweetest spot for thinning applications. Interestingly, this period often matches with the 6-12 mm fruit size window that we have long been recommending for thinning.
With all of that in mind, I ran the new model today for Central Virginia based on Gala’s green tip and full bloom dates (March 10 and April 4, respectively) that were kindly provided by our respected growers in the Central Virginia area. As shown in the image below, the model predicts that we have between 214-251 DD between April 28 and May 1st, which is our target window. The model also suggests carbohydrate surplus during that period which on one hand is good for supporting cell division and consequently good fruit size; but on the other hand, it will make thinning a bit harder. Therefore, the recommendation is to increase thinning rates by 30% during that period. For instance, if you would normally use 6-BA at 48 oz/100 gal, you should increase it to 62 oz/100 gal. This is still among the rate range of 48-64 oz/100 gal we were recommending. To convert from dilute to concentrate applications, please refer to my blog post at (https://blogs.ext.vt.edu/tree-fruit-horticulture/2020/04/02/how-to-calculate-the-tree-raw-volume-trv-and-chemical-rates-for-concentrate-spraying/). Also, for thinning materials to be used for other varieties, e.g. Fuji, Honeycrisp, Pink Lady, Golden Delicious, Red Delicious, and Ginger Gold, please refer to my other blog post at (https://blogs.ext.vt.edu/tree-fruit-horticulture/2020/04/02/fruit-thinning-scenarios-2020/).
One final point to highlight here is the effect of temperature on the activity of thinning materials. It has always been suggested that the best thinning activity by 6-BA and NAA is achieved when temperatures at and after the day of application are greater than 65 oF. By taking this into consideration and based on the 10-day weather forecasting for Batesville, this coming Saturday and Sunday, May 2 and 3, would be good days for thinning applications.
Should you have any questions/comments, please feel free to use the comments box below, call me at 5402326035, or shoot me an email at: firstname.lastname@example.org. Wish you all good luck with your thinning applications.