Author: Madeline Wimmer
This fruit update contains information about…
Bitter pit is not a disease, even though it may look similar to some disease symptoms like apple scab (Venturia inaequalis); the lesions could also be confused with other physiological issues like cork spot or lenticel breakdown, or even stink bug injury. Bitter pit results from cell wall breakdown and the appearance of sunken lesions on the outside and throughout the apple flesh due to issues with calcium metabolism within the apple. Some apple varieties are more prone to it, and Honeycrisp is an especially susceptible variety. To make things more complicated, bitter pit has a long window from which it can start showing symptoms, from as early as 4-6 weeks after petal fall and as late as postharvest, while the fruit is in storage.
When thinking about how to manage bitter pit, it’s helpful to know adding more calcium to the soil is not likely to solve the issue. Most soils in Minnesota have adequate amounts of calcium. Thus, this is often more of an issue with calcium getting into the fruit, rather than calcium getting into the roots and the plant in general*; although, competition with other nutrients within the soil can be an issue in some cases. Additionally, foliar sprays may be more effective than ground calcium applications, but how effective calcium sprays are for bitter pit has been challenged more recently. See the additional reading section at the end of this section for more detailed readings related to the topics discussed below.
Timing is extremely important for good results with these products, and researchers at Cornell University recommend applying prohexadione calcium products at the pink growth/development stage. For more specific considerations with prohexadione calcium, refer to the additional reading below.
One final consideration more specifically about Honeycrisp, is that it is not only is prone to bitter pit, but it is also susceptible to mottled, or zonal chlorosis. This type of chlorosis is a physiological disorder caused by a build up of carbohydrates in the leaves, rather than an actual nutrient deficiency. Don’t make the mistake of applying nitrogen to "fix" the mottled chlorosis as this is not the root cause of the chlorosis and can increase the chances for bitter pit issues.
The effects of hail damage on grapes can be destructive and is an event most grape growers are aware of in Minnesota. The grapes in the photo above were subject to two storms recently: one with high winds (wind speeds up to 70 miles per hour) and heavy rain on August 26th, followed by a second storm on August 29th when small hail fell for about one minute. The combination of all of these weather occurrences physically damaged berries and led to bruising. Although the symptoms looked fairly superficial, for some berries there may be some amount of cracking. When longer hailstorm durations, or hailstorms with larger hail pieces occur, more severe damage is possible, not only to the fruit, but also to the grapevine shoots, trunks, cordons, and so on.
For growers who have experienced similar weather and are seeing these conditions, check your clusters to see if you have any berry cracking or splitting. When berry skin is damaged this late in the season, bunch rot diseases like sour rot and botrytis may become problematic. For more information on managing late season bunch rot diseases, refer to the additional reading section below.
Additional reading:
Don’t let grape bunch rots spoil your 2020 harvest (University of Minnesota)
Midwest Fruit Pest Management Guide (starting on page 179)
Can grapes rebound from a hailstorm? (Good Fruit Grower)
Thank you to Erin Treiber and Colin Zumwalde for their contributing knowledge about the HRC weather for this week's grape article.
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This fruit update contains information about…
- Postharvest fruit handling
- Apples
- Bitter pit management: Thinking ahead for the next growing season.
- Grapes
- Berry bruising from late-season heavy wind, rain, and small-hail.
- 2024 Fruit and Veg Updates survey
This article will conclude the University of Minnesota (UMN) Extension fruit update series for the 2024 growing season. While our regular updates are wrapping up for this year, the growing season is not done for everyone. If you find yourself needing production support during these final weeks, reach out through the contact link above. Additionally, the newsletter will continue to publish more fruit-related content throughout the year, and into next year until the next fruit update series begins. I hope you have found value in this series for your production needs and look forward to joining the growing season with you, next year!
-Madeline Wimmer, UMN Extension educator, fruit production
Postharvest fruit handling
There are a number of practices that can help maintain fruit quality from harvest into storage. For example, if some apples happen to cool down too quickly from a very warm outdoor temperature to a cold storage temperature, it can lead to chilling injury, which tends to be more problematic from some apple varieties like Honeycrisp. Postharvest handling can also relate to food safety considerations (e.g., maintaining hand-washing station for fruit handlers, cleaning equipment and bins, preventing bird-feces contaminated fruit from being harvested) to exclude harmful bacteria from contaminating stored fruit. Additional considerations for postharvest care can be found on our UMN Extension web page, “Postharvest handling of fruit and vegetable crops in Minnesota.” This resource provides information related to sorting, grading, and storing late-season fruit crops like apples and grapes, as well as strawberries, which is still relevant to day-neutral strawberries at this point in the season.Apples
Bitter pit management: Thinking ahead for the next growing season.
Image: Are you seeing late-season bitter pit symptoms? Bitter pit symptoms show up as sunken depressions on the apple skin, and can extend deeper throughout the flesh. Photo taken of a Honecrisp apple at the UMN Horticulture Research Center (HRC; Zone 5a).
Bitter pit is not a disease, even though it may look similar to some disease symptoms like apple scab (Venturia inaequalis); the lesions could also be confused with other physiological issues like cork spot or lenticel breakdown, or even stink bug injury. Bitter pit results from cell wall breakdown and the appearance of sunken lesions on the outside and throughout the apple flesh due to issues with calcium metabolism within the apple. Some apple varieties are more prone to it, and Honeycrisp is an especially susceptible variety. To make things more complicated, bitter pit has a long window from which it can start showing symptoms, from as early as 4-6 weeks after petal fall and as late as postharvest, while the fruit is in storage.
When thinking about how to manage bitter pit, it’s helpful to know adding more calcium to the soil is not likely to solve the issue. Most soils in Minnesota have adequate amounts of calcium. Thus, this is often more of an issue with calcium getting into the fruit, rather than calcium getting into the roots and the plant in general*; although, competition with other nutrients within the soil can be an issue in some cases. Additionally, foliar sprays may be more effective than ground calcium applications, but how effective calcium sprays are for bitter pit has been challenged more recently. See the additional reading section at the end of this section for more detailed readings related to the topics discussed below.
Prehexadione calcium applied at the pink growth stage.
If you’re seeing bitter pit this year, it’s a good time to start thinking about practices that can help suppress it next year. One product that can be used is prohexadione calcium (i.e., Apogee, or Kudos), which is different from a calcium foliar spray in that it acts as a growth regulator, rather than a direct calcium supplement. It works by slowing down vegetative growth, which then lowers the amount of water and calcium being pulled toward vegetation, making more available for developing fruits.Timing is extremely important for good results with these products, and researchers at Cornell University recommend applying prohexadione calcium products at the pink growth/development stage. For more specific considerations with prohexadione calcium, refer to the additional reading below.
What else can you plan to do to suppress bitter pit next season?
- Prune this winter and thin fruit in the spring for a balanced crop. Over pruning and over thinning can both lead to an under cropped tree. Too few fruits left on an apple tree can then lead to larger fruit sizes, and larger fruits tend to be more susceptible to bitter pit.
- Maintain consistent soil moisture when possible. It was not possible to avoid the fluctuations in rain for the earlier part of the growing season this year, but maintaining adequate and consistent soil moisture levels in drier years. During years like 2023
- Avoid applying nitrogen after shoot growth has ended. For trees at risk of bitter pit, applying nitrogen later in the season and before harvest can lead to a nutrient competition between calcium and nitrogen in the fruit and increase the likelihood of bitter pit. If you’re planning to apply nitrogen to correct a deficiency, a good time to apply it is between petal fall and the end of shoot growth (1).
Image: An example of the variation of mottled, or zonal chlorosis characteristic of Honeycrisp. This type of chlorosis, or yellowing, is not caused by a nutrient deficiency, but is actually a physiological conditional related to a build up of carbohydrates in the leaves.
One final consideration more specifically about Honeycrisp, is that it is not only is prone to bitter pit, but it is also susceptible to mottled, or zonal chlorosis. This type of chlorosis is a physiological disorder caused by a build up of carbohydrates in the leaves, rather than an actual nutrient deficiency. Don’t make the mistake of applying nitrogen to "fix" the mottled chlorosis as this is not the root cause of the chlorosis and can increase the chances for bitter pit issues.
*Physiology explained: Because plant calcium goes where the water goes within a plant, it is more heavily pulled toward leaves that are transpiring (i.e., “sweating”) water and thus pulling in more water and calcium. Fruits, underground structures like potatoes, and other plant parts that are not transpiring do not pull as much water and overall calcium with it and are less likely to have adequate calcium overall.
Additional reading:
Calcium for bitter pit: Save it, don’t spray it (Good Fruit Grower)
Bitter pit suppression is all about timing - Video (Good Fruit Grower)
Prohexadione calcium (Apogee, Kudos) for bitter pit control (University of Wisconsin Fruit Program)
Additional reading:
Calcium for bitter pit: Save it, don’t spray it (Good Fruit Grower)
Bitter pit suppression is all about timing - Video (Good Fruit Grower)
Prohexadione calcium (Apogee, Kudos) for bitter pit control (University of Wisconsin Fruit Program)
Controlling bitter pit in apple: Best practices for growers (contains tips for apple storage; University of Maryland Extension)
Resources cited:
Atucha, A., June 30, 2022, Apple Tree Nutrition: Bitter Pit Control, Wisconsin Fruit, University of Wisconsin Fruit Program.
Resources cited:
Atucha, A., June 30, 2022, Apple Tree Nutrition: Bitter Pit Control, Wisconsin Fruit, University of Wisconsin Fruit Program.
Grapes
Berry bruising from late-season heavy wind, rain, and small-hail.
Image: A table grape cluster with berries that developed bruises after high-winds, rain, and small hail recently passed through at the UMN Horticulture Research Center (Chaska, MN; Zone 5a).
The effects of hail damage on grapes can be destructive and is an event most grape growers are aware of in Minnesota. The grapes in the photo above were subject to two storms recently: one with high winds (wind speeds up to 70 miles per hour) and heavy rain on August 26th, followed by a second storm on August 29th when small hail fell for about one minute. The combination of all of these weather occurrences physically damaged berries and led to bruising. Although the symptoms looked fairly superficial, for some berries there may be some amount of cracking. When longer hailstorm durations, or hailstorms with larger hail pieces occur, more severe damage is possible, not only to the fruit, but also to the grapevine shoots, trunks, cordons, and so on.
For growers who have experienced similar weather and are seeing these conditions, check your clusters to see if you have any berry cracking or splitting. When berry skin is damaged this late in the season, bunch rot diseases like sour rot and botrytis may become problematic. For more information on managing late season bunch rot diseases, refer to the additional reading section below.
Additional reading:
Don’t let grape bunch rots spoil your 2020 harvest (University of Minnesota)
Midwest Fruit Pest Management Guide (starting on page 179)
Can grapes rebound from a hailstorm? (Good Fruit Grower)
Thank you to Erin Treiber and Colin Zumwalde for their contributing knowledge about the HRC weather for this week's grape article.
2024 Fruit and Veg News reader survey
Our 2024 Fruit and Veg News reader survey is available and accessible by the link below or through the embedded survey below.2024 Fruit and Veg News Reader Survey
We thank you greatly for your feedback!
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