Bitter Pit, Crop Nutrition, and Calcium Transport
By Thomas Skernivitz, Senior Editor, American Fruit Grower
Life is good for Emma Grant. Just 25 years old, she is already in her third season as the Orchard Manager at 2,500-acre Cherry Bay Orchards in Suttons Bay, MI.
While killer frosts, spotted wing drosophila, and leaf spot can wreak havoc on her cherries and apples any given year, she typically takes solace knowing she has either fought the good fight or rolled with the circumstances.
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But mention bitter pit, and Grant’s disposition takes a turn all too familiar to fellow apple growers across the country. For starters, she concedes there is probably no such thing as a completely bitter-pit-free year. Worse, that may be the only predictable trait of the physiological disorder, which is characterized by brown, sunken lesions on the apple’s skin and underlying spongy, brown tissue.
“Nobody really understands bitter pit. I can’t find a pattern yet,” Grant says. “One day somebody will figure this out, and their name will be in lights.”
What is common knowledge about bitter pit is that it involves, among many factors, nutritional imbalances, largely calcium deficiency. But other than that, for Grant and many of her peers, “it’s just a black hole of throwing calcium,” she says.
“Nobody really understands bitter pit. I can’t find a pattern yet. One day somebody will figure this out, and their name will be in lights.” - Emma Grant, Cherry Bay Orchards
Photo by Emily Schneider
Like a Box of Chocolates
To help address Grant’s concerns and demystify bitter pit and calcium delivery, we reached out to experts from five crop input companies: AgroLiquid, Rovensa Next North America, Timac Agro USA, Tessenderlo Kerley Inc. (TKI), and Valent USA.
Danica Kluth, TKI’s PNW Regional Agronomist, Crop Nutrition, finds Grant’s vexation both common and valid. “Bitter pit is unpredictable,” she says. “And even when growers feel like they have done everything correctly, it can still happen. Agronomists and researchers have helped narrow down contributing factors, but it is still somewhat of a mystery.”
Abe Isaak, Field Agronomy Manager with AgroLiquid, agrees. “Many times, out of frustration, we wonder if anyone understands our issue or problem,” he says. “Other tree fruit issues, such as disease and pests, are easy to diagnose and treat. When treating them, you spray a fungicide for a disease or a pesticide for a pest, and the problem remains under control. Figuring out the root cause of bitter pit, on the other hand, can be tricky.”
Fortunately, the industry’s knowledge of bitter pit has increased in recent years, according to Rick de Jong, Rovensa Next’s Canadian Regional Manager. “We are now aware that there are a multitude of related factors that can impact bitter pit severity in a given season,” he says.
Building a proactive strategy becomes the vital component, Kluth says. However, as Grant laments at her northwest Michigan operation, even the best-laid strategies are often clouded by uneven results. “It is so inconsistent!” Kluth says. “Neighboring blocks with similar programs can have drastically different levels of bitter pit. It is a system of a physiological imbalance, not just a single deficiency. This makes it harder to treat with a one-size fits all approach.”
Isaac concurs, saying, “We know that calcium deficiency, primarily in surface cells, is the most common cause of bitter pit, and we know what should work to prevent or cure the symptoms. What makes it more frustrating is that we often get mixed results from applications, and it can be difficult to repeat the same results in another field.”
“Confounding,” Greg Clarke, Senior Field Market Development Specialist with Valent USA, concludes. “Producers are adding more calcium and doing everything correctly in their nutrition and irrigation programs,” he says, “yet they are still ending up with high bitter pit some years.”
Never as Simple as 2 + 2 = 4
Complicating matters, according to Randy Ross, Regional Agronomy Manager with Timac Agro, is that in-season calcium applications are only one part of the bitter pit equation. Several other factors can have a big influence on the level of severity of bitter pit any given year, including, he says, the following:
Tree vigor: Over-vigorous trees can exacerbate bitter pit. Vegetative growth competes for calcium availability in trees. Any calcium being used by the trees to build cell walls for leaves and wood is not available to the fruit. That is why bitter pit varieties are often put on the most dwarfing rootstocks available. Growers should be very conscious of factors that promote tree vigor, like aggressive pruning and nitrogen applications.
Fruit load: A light fruit load can lead to more bitter pit. The fewer fruit on the tree, generally the bigger they get and the more bitter pit they are going to develop. Biennial varieties like ‘Honeycrisp’ are very prone to this. Over thinning can be an issue as well. Using growth regulators during the growing season has become more widespread to even out the fruit load of biennial trees from one year to the next.
Ground moisture: On unirrigated land, bitter pit is always worse during a dry year. In order for calcium to move from the soil to the root hair and then be translocated through the vascular system of the tree to the fruit there has to be adequate moisture available. Even under ideal moisture conditions calcium has limited mobility in plants and is only somewhat mobile in the soil.
Nutrient imbalances: Certain positively charged nutrients (cations) like magnesium and potassium can be antagonistic to calcium uptake and assimilation. Base saturations should be examined to make sure your soils have the proper calcium, magnesium, potassium ratios. An imbalance of magnesium and potassium relative to calcium can inhibit the uptake of calcium and lead to more bitter bit. You want to make sure you have enough magnesium and potassium to make the crop, but potassium fertilization should be particularly scrutinized on bitter pit varieties.
“So, in-season calcium foliar applications are necessary as a supplemental source of calcium,” Ross says, “but should only be one part of a calcium management plan.”
“So, in-season calcium foliar applications are necessary as a supplemental source of calcium,” Ross says, “but should only be one part of a calcium management plan.” - Randy Ross, Timac Agro
Calcium: the Crux of the Matter
Calcium deficiency, Rovensa’s de Jong reminds, is the effect rather than the cause of bitter pit. “The cause can be any physical, cultural, or environmental action that disrupts or antagonizes calcium uptake, distribution, or supply,” he says.
"Calcium deficiency is the effect rather than the cause of bitter pit. The cause can be any physical, cultural, or environmental action that disrupts or antagonizes calcium uptake, distribution, or supply.” - Rick de Jong, Rovensa Next
In turn, bitter pit becomes a calcium transport issue, according to Valent’s Clarke. If a grower cannot deliver calcium to the outer areas of an apple, that tissue breaks down and bitter pit occurs, he says.
“That’s where the ‘black hole’ Grant mentions comes into play — soils, trees, leaves, and even fruit can be tested to have enough overall calcium, but it’s not ultimately getting to where bitter pit is formed,” he says.
Clarke lauds the work of Michigan State University’s Todd Einhorn and his PhD candidate, Chayce Griffith. Their research has shown how bitter pit is related to xylem function in developing fruit. ‘Honeycrisp’, for example, loses xylem function much earlier than other varieties, resulting in a reduction in calcium distribution and a greater risk of bitter pit.
“We can apply adequate calcium and other nutrients, but if the internal transport system in the fruit isn’t working, it’s for naught,” Clarke says. “That’s the exciting new work being done by researchers that might help with this issue in the near future. Einhorn and Griffith’s work at Michigan State has also shown that some PGRs can help to extend xylem function and thereby reduce bitter pit. So, we may soon have bitter pit management programs that involve PGRs combined with well-designed nutritional programs.”
"[Research] has shown that some PGRs can help to extend xylem function and thereby reduce bitter pit. So, we may soon have bitter pit management programs that involve PGRs combined with well-designed nutritional programs.” - Greg Clarke, Valent USA
Responding to a calcium deficiency with a “more the merrier” calcium application strategy is oversimplifying the issue, according to TKI’s Kluth. “Dumping more calcium into the program will not necessarily guarantee that the calcium will make it to the fruit,” she says. “Translocation depends on tree health, root function, ET, and nutrient balance. It’s a case of ‘just because it is in the soil doesn’t mean it is getting to the fruit.’”
“Dumping more calcium into the program will not necessarily guarantee that the calcium will make it to the fruit. Translocation depends on tree health, root function, ET, and nutrient balance. It’s a case of ‘just because it is in the soil doesn’t mean it is getting to the fruit.’” - Danica Kluth, TKI
Per AgroLiquid’s Isaak, there are two things growers should keep in mind when addressing bitter pit with calcium applications: timing and solubility.
“If you apply calcium at the right time but can’t get it into the plant (solubility), the tree will not be able to absorb it either by the roots or leaves if foliar applied,” he says. “If you apply soluble calcium at the wrong time, the plant won’t need or take it up.”
Macronutrients Beyond Calcium (Plus a Micro)
Bitter pit is not solely about calcium, Kluth says. Excessive potassium, magnesium, and even nitrogen can antagonize uptake of calcium. “Therefore, flooding the system isn’t always effective,” she says. “Trying to balance the nutrients can be the more effective approach to support efficient plant uptake and allocation to the fruit to help get calcium into the cell walls to prevent bitter pit.”
Potassium/calcium ratio is one of the biggest factors, de Jong says. Other ratios, such as potassium/magnesium and nitrogen/calcium, along with the availability of zinc throughout the season can also be impactful, he adds. Much research regarding this balance of calcium, potassium, and magnesium has been done at Cornell, Penn State, and Michigan State, while trying to identify the optimum ratios of the macronutrients, Clarke adds.
Isaak notes the importance of boron, a micronutrient. When boron is low, he says, the plant cannot take up calcium from the soil, no matter what the form of calcium is or if it is in excess. Boron helps move calcium to where it is needed in the plant.
For example, in California, Isaak has seen similar problems in stone fruit to what Michigan has experienced in apples. After a few hours, some varieties of peaches would start to show staining, and some would even have a corking under the skin.
“This started to show up in varieties of fruit that never showed this symptom in the past,” Isaak says. “We applied calcium, and that helped, with mixed results, but it never really solved the issue.”
During that time, Isaak also had issues with fruit set on plums. This was directly related to boron. “I had low boron levels in the soil in the range of 0% to 0.25%. I started to apply boron in the soil and foliar in the spring with bloom sprays. This worked very well, so I did the same for my peach blocks. My fruit set and quality improved, and more importantly, the consistency of fruit set on the trees improved. I also noticed that the staining and corking started to get less,” Isaak says.
The takeaway, he notes, is that growers must always start with the soil.
“Boron is a double-edged sword. If there isn’t enough, deficiencies develop and create other issues; the plant also won’t take it up from the soil,” Isaak says. “On the other hand, excessive boron creates burn and plant death. An excess of any nutrient will always have a negative impact on other nutrients by tying them up and making them less available to the plant.”
“Boron is a double-edged sword. If there isn’t enough, deficiencies develop and create other issues. On the other hand, excessive boron creates burn and plant death.” - Abe Isaak, AgroLiquid
Words of Advice
Turning back to Grant and her quest to someday conquer bitter pit, our supplier sources offer these parting tips:
Isaak: “Start early and be consistent with the applications. Start applying right after petal fall when the small fruitlets are starting to form and continue this weekly until two or three weeks before harvest. Remember that calcium does not translocate in plants. When leaves and fruit are small, they move much more easily when sprayed with an air blast sprayer, and coverage is better. If calcium is not applied to the surface of a fruit, it will migrate there. One to 2 pounds of calcium per application per acre is a good rule of thumb. Potassium is vital at this time of year as well. It works alongside calcium, filling the cell walls that calcium builds during cell division.”
Ross: “The amount of calcium applied foliar isn’t nearly as important as to when it is applied and the source of the calcium applied. The most critical time is early in the fruit’s development during the cell division stage that begins immediately following pollination of the flower and continues for approximately five to six weeks after. This is when cell walls are being built. So application timing can start at bloom but should be no later than petal fall and through the cover sprays until about 30 days before harvest. Calcium applications should be from a product that uses calcium chloride (CaCl) as its source. CaCl has been found to be the most readily available and usable source of calcium to the apples.”
Kluth: “A combination approach is most effective. Applications of calcium to the soil should be targeted with periods of high root activity, which is typically early bloom and also mid-season. For foliar applications, petal fall is a good start for foliar calcium and at least another application two weeks later, maybe more depending the performance of those applications.”
de Jong: “A grower should use all the tools available to them in the management of bitter pit. This holistic approach should include soil management, foliar crop nutrition, and fertigation where possible. Soil management, including fertigation, can address nutritional concerns and optimize soil pH setting the stage for calcium uptake. The tree’s root system is the primary way to move calcium into the crop. Foliar nutrient applications can then be used to enhance calcium levels during windows of opportunity, improving fruit calcium levels.”
