Bitter pit showing up on apples at harvest.
Every growing season seems to be different. Hot, dry weather in the 2016 season resulted in considerable bitter pit problems in apples. The 2017 season saw little problems with bitter pit because the season was much cooler and wet. The 2018 season seems to be a heavier than normal moisture season depending on where you live in the state. Although, the western regions of the state do have lower moisture levels. The wet spring gave way to a short dry period in late June to early July in some regions. Here in central Pennsylvania, it does not seem like it will ever stop raining!
Normally we would expect bitter pit to be lower in wet years, but we can never be certain. Excessive vegetative growth can serve as a “magnet” to draw more calcium to the leaf tissue at the expense of the fruit. We know that apple xylem, the primary conduit for calcium becomes dysfunctional as the fruit develops (Amarante et al., 2013). Recent studies by Lee Kalcsits in Washington State have also shown that Honeycrisp fruit have less functional xylem tissue in the lower portions of Honeycrisp fruit (Prengaman, 2018) and this may help explain why this cultivar tends to show more bitter pit.
Bitter pit severity varies with cultivar and season, but Honeycrisp is particularly susceptible. Results from a Penn State study in commercial Honeycrisp orchards led to the development of a model for predicting bitter pit potential (Baugher et al., 2017). In the three-year study in six orchards, the percentage of fruit on a tree that developed bitter pit ranged from 0 to 74 percent.
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 calcium (Ca) and consistently positively correlated with potassium (K), phosphorus (P), and ratios of magnesium to calcium (Mg/Ca), potassium to calcium (K/Ca), and nitrogen to calcium (N/Ca). Bitter pit also increased as average shoot length increased, and decreased as crop load (the number of fruit per square centimeter of 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 would be expected to have no bitter pit if average shoot length is less than 5 inches, but 36 percent 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 percent of the fruit to develop bitter pit even when the N/Ca is only 2.
Total rainfall (inches) at locations in Pennsylvania for the months of May and June and through July 24 from the NEWA weather stations.
|Rock Springs||Biglerville||Cabot||Reading||North East||Scott Township||Breinigsville||Pittsburgh|
Estimating average shoot length
Select 20 typical trees per block and record the length of five 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 five 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 three 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-centimeter-wide (about 3/8-inch) 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.
How to sample peel tissue. The peel in the plate shows the size of a sample needed from one apple. The apple being peeled shows the peel being removed from the calyx end of the fruit. Exercise care to not cut into the flesh. To remove excessive flesh use a spoon to remove the flesh from the peel.
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.
Implications for best management of Honeycrisp in orchards
The predictive model also has implications for Honeycrisp management in the orchard. Growers have long understood the importance of a multifaceted approach for controlling calcium-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 calcium to nitrogen in the fruit.
Amarante, C., A. Miqueloto, C. Steffens, A. dos Santos & L. Argenta. 2013. Changes in xyleme functionality during apple fruit development: Implications on calcium concentration and incidence of bitter pit. Acta Hort. 1012:135-140.
Baugher, T., R. Marini, J. Schupp & C. Watkins. 2017. Prediction of bitter pit in Honeycrisp apples and best management implications. HortScience 52:1368-1374.
High-res Honeycrisp: Cell structure scans offer new insight into why Honeycrisp is so prone to bitter pit. Good Fruit Grower, Kate Prengaman, Apr 10, 2018