Penn State University has developed a new tool for determining bitter pit in Honeycrisp apples. The predictive model is based on two factors: average shoot length and the N/Ca ratio in fruit peel. Here’s a summary of their two extension articles on the predictive model that can be found:
The average shoot length is estimated by selecting 20 typical trees per block and recording the length of 5 typical terminal shoots around the tree. For best results, select current season shoots with moderate branch angles (avoiding strong vertical shoots or weak shoots hanging below a horizontal orientation). Sum the lengths of the 5 shoots from the 20 trees and divide by 100 to obtain the average shoot length for the block.
To estimate N/Ca ratio of fruit peel select 3 typical fruit from each of the same 20 trees per block three weeks before anticipated harvest. (Shoot length measurements and collection of fruit samples can be done simultaneously if that is more practical for you.) Within a block, select fruit of similar size. Scrub the apples in tap water to remove any residues. Use a potato peeler to remove 1 cm-wide (about 3/8″) strips of peel from around the circumference at the calyx end of the fruit. Be careful to avoid removing flesh with the peel because it is difficult to grind for analysis. If there is flesh attached to the peel, use a dull knife or spoon to scrape the flesh off the peel.
Combine the peel tissue from the 60 apples and place them on a cookie sheet on parchment paper and dry in an oven at 180 degrees overnight. Submit the samples to the Penn State Agricultural Analytical Services Lab with a Standard Plant Analysis Kit, or you can also send them to the UW Soil and Forage Lab.
The table below shows the percentage of fruit that may be expected to develop bitter pit from trees with varying combinations of average shoot lengths and N/Ca ratios in the fruit peel. For example, trees with a N/Ca ratio of 10.0 would be expected to have no bitter pit if average shoot length is less than 5 inches, but 36% of the fruit may develop bitter pit if average shoot length is 15 inches. If a tree has an average shoot length of 25 inches or more, we would expect 59% of the fruit to develop bitter pit even when the N/Ca is only 2.0.
Implications for best management of Honeycrisp in the orchard
The predictive model also has implications for Honeycrisp management in the orchard. Growers have long-understood the importance of a multi-faceted approach for controlling Ca-related disorders in bitter pit-prone cultivars, but in the case of Honeycrisp it has been difficult to ascertain which practices are most important relative to its high susceptibility to the disorder. The two-variable model suggests the focus should be on managing terminal shoot growth and increasing the ratio of Ca to N in the fruit.
This article reprinted and edited with permission from authors, Tara Baugher, Rich Crassweller and Robert Michael of Penn State University Extension .This article was posted in Apples and tagged Bitter pit, Honeycrisp.