The Balancing Act of Bitter Pit Management in Apples
What is a bitter pit?
Bitter pit is a physiological disorder, an abnormality that can occur within fruit tissue that is not caused by pests or mechanical damage, rather by several factors such as environmental conditions, genetic background, maturity at harvest, preharvest, and postharvest practices. The disorder is characterized by small, dark brown, sunken spots on the apple’s skin (Fig. 1, 2). The pits typically form towards the calyx end of the fruit. The flesh is brown, and the pit is slightly water-soaked. Bitter pit results from an internal imbalance, rather than a deficiency, of calcium on the calyx end of the fruit. Calcium is an essential element for cell membrane and cell wall development and stability. If the amount of available Ca is restricted, the cell membrane leaks and the cells die. Pockets of dead cells form bitter pits on apples. Ca is taken up by roots along with other nutrients (potassium, magnesium, nitrogen) and transported up the tree via transpirational flow in the xylem. Then it is divided between transpiring organs, the shoots and the fruits. Calcium moves from the stem to the calyx end of the fruit. Finally, it is partitioned to the cell membrane and other parts of the cells. Particularly in Honeycrisp, the xylem tissue has a lower functionality, and thus is less effective in transporting Ca, compared to other cultivars less prone to bitter pit development. The calcium distribution cannot keep pace with the expansion of the fruit, resulting in the expression of bitter pit.
Bitter pit begins to develop after petal fall. Early signs of slight indentations surrounded by darker green may be visible on the tree (Fig. 1). However, the true quantity of fruits with bitter pit cannot be visually detected until the first 60 days of storage (Fig. 2).
What factors cause or contribute to bitter pit?
There are several factors that can cause or contribute to the development of bitter pit, including preharvest management factors, environmental factors, genetic background, and maturity at harvest, as well as postharvest practices.
Regarding preharvest management factors, tree nutrition plays an important role. The nutrients potassium (K) and nitrogen (N) compete with calcium (Ca) for uptake by the tree. When the internal ratio of K/Ca or N/Ca is too high, the amount of available Ca is restricted. To maintain healthy Ca levels in the fruit, the uptake of other nutrients must be limited. In addition to reducing the Honeycrisp tree’s capacity to take in Ca, excessive N can encourage unnecessary vegetative growth and lower yield. If an orchard has a history of bitter pit, nitrogen application should be significantly reduced. Nitrogen should only be applied at bud break and right before petal fall, when the shoots are still growing. After that phase there is a risk of increasing N levels in the fruit, lowering the proportion of calcium. Boron helps the movement of Ca, so if B is low then Ca levels will also be low.
With respect to rootstocks, it has been reported that high rootstock vigor leads to increased bitter pit incidence, because nutrients are diverted to shoots and leaves rather than fruit. However, in Honeycrisp, which is known for its weak growth, the vigor should remain high enough to produce adequate yields. Rootstocks that grow to a larger size may take up more water, K, and N, lowering the K/Ca and N/Ca ratio. The optimal choice of rootstock depends on the specifications of the planting site. In studies under New York environmental conditions rootstocks producing the highest biter pit-free fruit were G.11, G.30, G.214, and G.935, which translated to a higher crop value.
It is also known that large fruits are more prone to bitter pit because low crop loads have an elevated K/Ca ratio.
Environmental factors play a key role in bitter pit development. A drought during bloom or in July after petal fall can stress the xylem and inhibit the movement of calcium into developing fruits. However, high rainfall can lead to increased vegetative growth, limiting Ca partitioning to fruits and causing excessive fruit size.
In terms of genetic background, Honeycrisp has a particularly high susceptibility to bitter pit compared to other cultivars, like Gala. This heightened susceptibility can be attributed to Honeycrisp's limited genetic capacity to utilize calcium at various stages of the uptake process. Other cultivars at high risk of bitter pit development include Cortland, Cox Orange Pippin, Braeburn, and Golden Delicious.
With respect to maturity at harvest, fruit that is harvested before reaching optimal maturity is more susceptible to bitter pit. The plant growth regulators ReTain® (active ingredient: Aminoethoxyvinylglycine (AVG), Valent USA)and HarvistaTM (active ingredient: 1-Methylcyclopropene (1-MCP), AgroFresh) have been found to increase the incidence of bitter pit.
Regarding postharvest practices, it has been shown that a conditioning period at 50℉ before cold storage (38℉), which is known to reduce the risk of chilling injuries soft scald and soggy breakdown in Honeycrisp, may increase the risk of bitter pit.
What are the practices that can be implemented to manage or avoid bitter pit incidence?
Preharvest management practices such as choosing the appropriate rootstock under the specific growing environmental conditions as well as focusing on nutritional management and crop load are key factors to consider. Avoid over-thinning to maintain a moderate fruit load and size. It has been recommended to thin apples to 4 fruits/cm² trunk cross sectional area (TCA) for young trees (year 2 to year 4) and 5–7 fruits/cm² TCA for older trees to prevent growth of large fruits with high internal K/Ca ratios. Furthermore, prune excess shoots and leaves to promote Ca partitioning into fruits. Maintaining soil moisture and the availability of micronutrients B and Zn helps improve Ca uptake from the soil.
Regarding pre harvest foliar calcium sprays, it is important to mention that the effectiveness of calcium sprays intended to improve the internal calcium content of fruits varies significantly by orchard site and the growing season. As little as 1% of the treatment enters the fruits, an insufficient change to prevent bitter pit on its own. Spray treatments can mitigate but not completely prevent bitter pit and should be considered a complement to other management practices.
In terms of stage of maturity at harvest, avoid early harvests as these will enhance bitter pit development. Monitor the maturity of your fruits weekly to determine optimal harvest dates.
With respect to postharvest practices, avoiding preconditioning fruit at 50℉ and instead directly storing fruit at 38℉ has been shown to decrease bitter pit incidence or limit their appearance. Nevertheless, skipping preconditioning may enhance the risk of less destructive disorders such as soft scald and soggy breakdown. Application of the ripening inhibitor 1-Methylcyclopropene (1-MCP) has also been reported to decrease bitter pit risk, together with the use of controlled atmosphere during long term storage. Drenching immediately harvested apples in a calcium solution before storage may work as a complementary practice.
This article is featured in the Vegetable and Fruit News, Vol. 15, Issue 9.