In a three-year study in six commercial Honeycrisp orchards, the percentage of fruit on a tree that developed bitter pit ranged from 0 to 74%. When the percentage of fruit with bitter pit was correlated with concentrations of various mineral nutrients measured in the fruit peel three weeks before harvest, bitter pit was negatively correlated with Ca and consistently positively correlated with K, P and ratios of Mg/Ca, K/Ca and N/Ca. Bitter pit also increased as average shoot length increased and decreased as crop load (number of fruit per cm2 trunk cross-sectional area) increased. The best combination of variables for predicting bitter pit was average shoot length plus the N/Ca ratio in the fruit peel.
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.
Table: Estimated percent of apples on Honeycrisp trees developing bitter pit after cold storage as affected by shoot length and N/Ca.
|Average shoot length (inches)|
Estimating average shoot length
Select 20 typical trees per block and record 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.
Estimating N/Ca ratio of fruit peel
Three weeks before anticipated harvest select 3 typical fruit from each of the same 20 trees per block. (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 (Fig. 1). 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. When filling out the information form, include the average shoot length. Results sent to the person submitting the sample will be limited to values for mineral elements and there will be no interpretation. Results will also be sent to a fruit specialist who will send the grower an interpretation.
Sometimes a block of trees will contain trees with varying crop loads or tree vigor may vary across a block. In those cases it may be preferable to submit samples from different sections of the block or from trees with light or heavy crops.
Fig. 1. Illustration of how to sample peel tissue: The peel already on the plate shows the size of the sample needed from one apple. Peel from the second apple is being removed from the calyx end of the fruit, exercising care to not cut into the flesh.
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.