Over the last couple of years new research on bitter pit has shed light on this very complex issue. We used to think of bitter pit as a Ca deficiency, but now we have clear evidence that this is an issue of Ca distribution to the fruits, and relates to nutrient imbalances rather than Ca deficiency. There are several factors that affect the incidence of bitter pit: vigor, crop load, rootstock, irrigation, nutrition, and weather conditions. Each site will have their own limitations and identifying these limitations is key to determine the best management strategies to reduce bitter pit incidence.
Nutrition: Peel nutrient levels of cultivars with lower incidence of bitter pit, such as Gala, have higher contents of Ca and lower contents of K and Mg than cultivars with high bitter pit incidence like Honeycrisp, and overall fruit with high incidence of bitter pit have a higher ratio of K/Ca and N/Ca than those with no bitter pit. New research from Cornell University exploring bitter pit prediction shows that nutrient (e.g., K/Ca ratio) content in peel sap of developing fruitlets is a very good indicator of bitter pit susceptibility even on healthy looking fruit. The research compared the sensitivity of different structures of the fruit to detect differences in nutrient content and how they related to bitter pit incidence. They evaluated the nutrient content in fruit peel, the entire fruit, and the peel sap. Nutrient content in peel sap was the best indicator of bitter pit incidence as well as the easiest method. Sap analysis measure the water-soluble portion of the nutrients which makes it more sensitive than measuring the dried tissue that includes the structural nutrient portion. This study concluded that high ratios of K/Ca (>25) in peel sap of developing fruits collected early/mid July are associated with higher risk of bitter pit incidence at harvest and in storage.
Regarding nutrient management to decrease bitter pit incidence, foliar Ca applications have been one of the most utilized tools to increase the concentration of Ca in fruit. Yet, many have questioned the efficiency of these applications. In fact, only 1% of all the foliar Ca applied during the growing season gets in the fruit. However, there are some recommendations that can help increase the efficiency of foliar Ca applications: use the product with the highest concentration of active Ca (CaCl has been the prefer product and works the best); consistent applications throughout the entire season are a much better approach than targeting early or late applications only; there is higher uptake of Ca when the applications finish with cooler weather (<75 ˚F).
The management of nutrients such as N and K are key to reduce bitter pit. In the case of Honeycrisp, this is a cultivar that is very sensitive to N, weak-growing, and prone to bitter pit. These characteristics makes it very challenging to optimize tree N status to be able to achieve good fruit quality while maintaining tree vigor. The optimal N concentration in leaves is 2.0 to 2.2%, and leaf samples should be collected in late June/early July before zonal chlorosis shows up. If you take leaves with zonal chlorosis, the level of N and other nutrients will be much lower than in leaves with no zonal chlorosis. Regarding timing of N applications, the majority of the N for apple trees come from the soil, but when there is not enough N in the soil then we need to apply supplemental. The best timing to apply N in Honeycrisp is between petal fall to end of shoot growth if you are using fertigation. If you are using ground applications, the best timing is between budbreak to petal fall. If you apply N later, you will have higher N in the fruit which will result in a higher N/Ca ratio and consequentially higher bitter pit. In general, rates of 0 to 50 lbs/acre of N split between bud break and petal fall and in the case of sandy soils additional post harvest N applications (3% foliar urea) in early October are recommended. Lastly, N is directly related to vegetative vigor, and the more vegetative growth the higher incidence of bitter pit we will have. Controlling vigor by reducing N fertilization is critical to reduce bitter pit. In Honeycrisp blocks with high content of organic matter (3-4%), high vigor, high foliar N content, and a history of bitter pit incidence, should completely eliminate N applications or reduce them to 50%.
In terms of K fertilization, soils in Wisconsin have high concentration of K, in particular orchards that were previously farms or have received high applications of manure. High levels of K in soil will result in increased bitter pit. This is why many orchards see high incidence of bitter pit even when the trees have low vigor, and they apply foliar Ca during the entire season. Another thing to consider when thinking of K availability in soils is that applications of organic matter such as compost and manure can have really high levels of K, which could exacerbate the bitter pit problem. Lastly, Honeycrisp has lower requirements of K than cultivars like Gala, and the optimal leaf K levels for Honeycrisp is 1.0 to 1.3% while for Gala is 1.3 to 1.8%. In Honeycrisp blocks with high soil and foliar K content and a history of bitter pit incidence, completely eliminate K applications or reduce them to 50%.
Rootstock vigor and nutrient uptake: There is plenty of evidence that more vigorous rootstocks will have higher incidence of bitter pit. Vigorous rootstocks will have more shoot growth resulting in more movement of water and Ca to the developing shoot and leaves in detriment of Ca partitioning to the fruit. Controlling tree vigor is key to reduce bitter pit incidence. In addition to vigor control, there are significant differences among rootstocks when it comes to bitter pit incidence. Rootstocks like B.9, G.65 and G.214 have the lowest bitter pit incidence, while G.210, M.7, G.814 and G.118 have the highest bitter pit incidence. Although the incidence of bitter pit for a given rootstocks is related to their vigor, another aspect when selecting a rootstock for Honeycrisp is the ability of the rootstock to uptake K. The higher the K uptake by the rootstock, the higher the K/Ca ratio in the fruit and thus the bitter pit incidence. Rootstocks like G11 and G41 are more efficient in taking up K than G.214 and B.9. If you are using a rootstock that is more efficient in taking up K then you have to lower the K fertilization.
Although choosing a rootstock that confers low bitter pit incidence is the logical decision, something else to consider is that most of these rootstocks have low vigor and with a weak cultivar as Honeycrisp, it might take too long to fill the allocated space, and this results in economic losses. When we make rootstock decision only based on bitter pit incidence, we might be losing money because we are selecting low vigor rootstocks that take too long to fill their space. Another way of looking at this is to select the rootstock that produces the highest cumulative yield of fruit with no bitter pit (bitter pit free fruit). Research shows that over a 14-year study the best rootstocks in terms of yield and low bitter pit incidence for Honeycrisp with the highest economic returns are: G.214, G.935, G11, G.30 These rootstocks fill their space faster are high yielders and have lower incidence of bitter pit, but maybe not the lowest.
Effect of weather and irrigation: Weather has a larger impact on bitter pit than all other factors, followed by low crop load that will give you the highest risk of bitter pit. Drought around bloom results in less Ca uptake from the soil in the early part of the season when the Ca is still moving towards the fruit (due to the vascular connections that are still active), this results in higher bitter pit incidence. The second critical period in which drought can affect bitter pit incidence is after petal fall in July, when very dry conditions compromises Ca uptake which results in low Ca/K ratio. Dry weather early in the season will increase bitter pit. In 2020 and 2021 we did experience droughts during the above-mentioned critical periods, all of which could have contributed to higher incidence of bitter pit in Honeycrisp. Excessive rain close to harvest also has a negative effect on bitter pit incidence, as fruits will become larger, there will be increased uptake of K from the soil. Unfortunately, controlling high precipitation is less manageable than irrigating early in the season.
In conclusion, bitter pit is a complex nutrient imbalance influenced by many factors which are not necessarily the same for all orchards. Controlling tree vigor, use of adequate rootstocks, reducing K and N fertilization, and providing irrigation during critical periods of fruit development, in addition to supplemental Ca applications, can all help reduce bitter pit incidence.This article was posted in Apples and tagged Amaya Atucha, Apples, Bitter pit, bitter pit control, bitter pit prevention.