Bitter pit isn’t only a problem of Honeycrisp. While Honeycrisp may be a newer cultivar, bitter pit is nothing new. Although Honeycrisp has become notorious for it’s susceptibility to developing this physiological cell breakdown, many other cultivars also have a tendency to develop the disorder.
Bitter pit is primarily found around the blossom half of the apple. It appears under the skin, as soft brown spots, indicative of damaged cell walls. The outside of the fruit may appear to have slightly depressed spots over this necrotic area. Often, the damage is not visible until post-harvest. If visible at harvest, it can be easily mistaken for insect damage, diseases or even hail injury.
Apples with bitter pit are edible. If you direct market your apples for immediate sale, losses may be mitigated if consumers are willing to eat an apple with some soft brown spots which have a bitter taste. But for most producers, bitter pit makes apples unmarketable.
But bitter pit doesn’t have to happen. It can be impacted by management practices. Its development depends on some factors which are under the control of growers, and some – like the weather – that are not. But knowing when conditions favor bitter pit development, and managing to mitigate the risks, can decrease its incidence.
While calcium is often cited as the cause of bitter pit, other nutrients play a role. Nitrogen and potassium levels, which fluctuate throughout the growing season, or are excessive comparative to the calcium level, are important factors in bitter pit development. The ratio of nutrients may be more important than the amount of each individual nutrient present in the fruit.
According to a recently published research report, “Soil and ambient temperatures, soil pH level, soil physico-chemical structure and microbial activities, rootstocks, cultivar, irrigation, relative humidity, scion fruit size and crop load are among these major factors that influence Ca balances and partitioning in the fruit tissue, and thus severity of fruit bitter pit in apples.”
Calcium is needed to maintain the integrity of cell walls, and anything that impedes calcium uptake in cell walls and cell storage sites can mean a greater chance of bitter pit. During early summer, fruits obtain all of the calcium they are going to obtain from the tree. But the fruit is still maturing, and the amount of calcium may not be adequate for the needs of the fruit as it grows.
Low calcium levels can occur if there are high fruit loads. Each fruit is competing for the calcium, meaning there is less for everyone. Calcium within the tree isn’t shared between fruit and leaves or roots – it isn’t very mobile. Even if the leaves or soils have adequate levels of calcium, the fruits may not. Conversely, larger fruits can require more calcium than is available to them, causing bitter pit.
Nitrogen plays a role also. Too much nitrogen can increase fruit size and decrease calcium levels per fruit. Nitrogen is associated with excessive tree vigor, and nitrogen applied in late spring or beyond has been shown in studies to increase bitter pit occurrences.
Low boron levels may predispose to bitter pit, as boron moves calcium into the fruit.
Higher potassium levels are thought to be more detrimental when calcium is relatively low, while higher levels of calcium combined with higher potassium levels may be beneficial for some cultivars. Apples going into controlled atmosphere storage should have higher levels of calcium in relation to potassium.
Analysis of the calcium levels in the fruits can be taken beginning in mid-summer to determine if there are adequate levels in relation to other nutrients. Each cultivar will have different acceptable nutrient levels.
Environmental conditions can cause changes in how minerals are taken into the tree and utilized. Soil nutrient levels, water availability, tree age, vigor and rootstock all are believed to play a role in determining the ratio of calcium to other nutrients in the fruit.
Soil moisture fluctuations, including both low and high moisture levels, can predispose fruit to bitter pit development. Low soil moisture decreases calcium levels in fruit, and also causes fruit to be small. Excessive soil moisture levels can cause large fruit and lower the calcium levels in relation to potassium and nitrogen.
Low soil pH can lead to excess of certain nutrients, which in turn limit calcium availability to the tree, no matter whether soil calcium levels are adequate or not.
Tree vigor increases bitter pit as the ratio of fruit to leaves decreases. Leaves will compete with the fruit for available calcium. (Rootstocks which favor bitter pit development tend to be the more vigorous ones.) Irrigation management can lead to excessive vigor, including large fruits, if too much water is applied. The type of irrigation system is also thought to impact tree mineral uptake.
Heading new shoots on overly vigorous trees can decrease the likelihood of bitter pit. Over-pruning in the dormant season should be avoided; summer pruning on trees with a history of biter pit may be beneficial.
Maturity at harvest is another factor that can play a role in bitter pit development. According to the University of Minnesota Extension, “A lesser known cause of bitter pit is premature harvest and storage.” Researchers found that late season cultivars were regularly being harvested too soon. If harvested several weeks prior to the proper maturity as indicated by starch index readings, Mutsu and Northern Spy apples in controlled atmosphere storage were highly likely to develop bitter pit.
Weather and factors not under a grower’s control can influence whether or not bitter pit develops. Fruit position on the tree, as well as air temperature, also play roles.
Heat stress early in the growing season has been shown to inhibit calcium uptake into fruit. About 90% of the fruits’ calcium uptake happens in the six week after full bloom. Excessive heat, which increases vegetative growth, also decreases crop load. Both of these consequences will decrease fruit calcium levels, and increase the incidence of bitter pit.
Calcium sprays are the common response to prevent bitter pit damage. Multiple sprays of calcium, beginning in summer and continuing up to harvest if needed, are applied to fruits. Calcium chloride, calcium nitrate, calcium sulfate and amino acid-calcium chelates are all used. A calcium drench post-harvest can increase calcium levels, but applications during the growing season are recommended.
With many factors impacting bitter pit development, growers do have opportunities – even with those pesky Honeycrisp – to reduce or prevent incidences of this disorder. The bite of bitter pit can be reduced through management practices.