Author: Madeline Wimmer
This fruit update contains information about…
Images: Black rot (Diplodia seriata, syn Botryosphaeria obtusa) can show up as different symptoms on apple leaves, blossoms, fruits, branches, and trunks. Sanitation is important to managing black rot, and removing mummified fruits, diseased branches, and dying trees can help mitigate issues for next year.
Black rot is caused by the fungal pathogen Diplodia seriata, syn Botryosphaeria obtusa. It can be an issue for infected trees throughout the growing season and can show up on various apple anatomy. Our June 5th Fruit Update briefly covered black rot foliar symptoms, which look like concentric rings and are referred to as frogeye leaf spot. In addition to leaves, black rot can affect flowers, branch wounds, and fruit, and around this time rot symptoms can be seen on any infected apples that have continued to ripen on an apple tree.
Life cycle and symptoms
Similar to apple scab and some other fungal diseases, black rot spores develop during warm (above 48° F.) rainy periods in the spring and are spread by rain splash, wind, or insects. Infections can successfully occur when spores contact the point of infection (e.g., a flower, fruit, or branch wound) at a temperature (between 78–80° F. for a leaf infection, or 68–75° F. for a fruit infection) and wetness duration adequate for fungal growth (1). Early blossom and fruit infections can lead to fruit drop or complete mummification, while other fruit infections develop into a rot, usually starting at the blossom/calyx end. Occasionally, black rot fruit infections can also give way to early fruit ripening with rot at the fruit core/ovary.
When apple branches are wounded by winter damage, hail, or other injuries, they become vulnerable to infections, and this is how black rot can make its way into trunks and branches. Sometimes infected branches can display symptoms similar to fire blight canker, however symptoms on branches and trunks can vary in appearance depending on how long the infection has been taking place. Cankers can be sunken, and reddish brown, eventually leading to cracking bark and detectable black pycnidia, or fruiting bodies that house spores for future infections. Serious or chronic black rot infections can lead to defoliation in a single season, which decreases winter survivability for trees, and can lead to overall tree decline.
Management
If black rot is problematic within an orchard section or on a specific apple variety, prioritizing good sanitation practices can help to remove the inoculum that will carry the disease over to the following year. Because black rot overwinters in mummified fruits, as well as branches, and the trunk, removing mummified fruits, dead limbs or branches, and whole trees when necessary, can slow down and decrease its ability to spread. Flagging infected limbs now can effectively help branch removal when dormant pruning next year. Additionally, remember to sanitize pruning shears, loppers, and saws with a light dilution of bleach or other non-corrosive disinfectant when transitioning between pruning infected and non-infected areas.
Some apple varieties, like McIntosh and Cortland are more prone to black rot infections than other varieties, which may influence varietal selection for some growers. For chemical management recommendations, refer to the Midwest Fruit Pest Management Guide starting on page 53 where black rot management on apples can be found under the tab, “summer rots (black and white).”
Black rot vs opportunistic fungi and sooty mold
There are a lot of fungi that produce black spores, black fruiting bodies that house spores, or have a darker mycelium (i.e., the body of the fungus). This can occur on pruning wound sites where the woody tissues lose integrity partway through the growing season and begin to lose sap, or in areas where aphids or other sap sucking insects leave honeydew behind. There are a number of different fungi that lead to these conditions that are not the black rot pathogen D. seriata, syn B. obtusa (e.g. sooty mold and other opportunistic fungi). Knowing this is important because the consequences of having one or the other are very different. Additionally, black rot on apples is not the same pathogen as black rot on grapes (Guignardia bidwellii) or black rot in brassica crops, which is actually caused by the bacteria Xanthamonas campestris pv campestris.
Additional reading:
Black rot of apple (University of Minnesota Extension)
Resources cited:
Images: Two examples of apple scab (Venturia inaequalis) lesions on fruits, one showing minor blemishes (left image) while the other has a more severe infection which led to fruit cracking (right image).
As mentioned in last week’s update, the high rain frequency we experienced during the spring and early summer this year made it challenging to find dry windows of time to apply fungicides for apple scab (Venturia inaequalis) management. For growers who were not able to make it out and spray or routinely manage with a low or no spray program, more apple scab infections might be present on leaves and fruits for this year’s harvest.
Late season fruit infections truly look scab-like as they become brown and corky over time and can eventually lead to fruit deformation and cracking as the fruit develops. While there isn’t much that can be done at this point for the current year’s fruit and leaves, now is a time to consider sanitation measures for managing next year’s disease.
Here are some things to think about if you faced a high frequency of infected apples and leaves this year:
Additional reading:
Apple scab of apples and crabapples (University of Minnesota Extension)
Apple diseases- Apple scab (PennState Extension)
Resources cited:
Image: Netting can be lifted up and clipped to an upper trellis wire to expose clusters for hand-harvesting, as seen here on this row of Frontenac Gris.
Bird netting does not need to be taken down before harvest for those who practice hand harvesting. Most vineyards are busy with many tasks during harvest season and choose to clip up their netting to expose grape clusters when harvesting by hand. At some point,however, taking down netting and storing it for next year becomes necessary. This isn’t usually the most exciting task of the season, and it can take much more time without the proper organization and planning. Now is the time to decrease the amount of effort it takes to sort through and apply netting for next year.
Note: If you missed our article earlier this year, “Bird netting and other bird deterrents,” from our August 8th Fruit Update provided a good foundation about the concepts behind different bird deterrents and netting types.
Below are a few considerations for taking down and storing bird netting this year:
Timing:
This fruit update contains information about…
- 2024 Fruit and Veg News reader survey: We want to hear from you!
- Apples
- Black rot (Diplodia seriata, syn Botryosphaeria obtusa).
- Late–season apple scab (Venturia inaequalis) fruit infections and mitigating next year’s outbreak.
- Grapes
- Taking down and storing bird netting.
- Post–harvest nutrient management.
- (Avoiding) post-harvest nitrogen applications.
- Applying potassium post-harvest.
2024 Fruit and Veg News reader survey: We want to hear from you!
The end of the 2024 growing season is approaching, which means our weekly fruit updates will also be concluding. If you have been following our updates, we want to hear from you about what was useful and enjoyable to read, as well as what you would like to read more about next growing season. Plus, if some information wasn't so helpful we want to hear that too!You can take our 3 minute anonymous survey through the link below or complete the embedded survey at the bottom of this newsletter.
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Apples
Black rot
Images: Black rot (Diplodia seriata, syn Botryosphaeria obtusa) can show up as different symptoms on apple leaves, blossoms, fruits, branches, and trunks. Sanitation is important to managing black rot, and removing mummified fruits, diseased branches, and dying trees can help mitigate issues for next year.
Black rot is caused by the fungal pathogen Diplodia seriata, syn Botryosphaeria obtusa. It can be an issue for infected trees throughout the growing season and can show up on various apple anatomy. Our June 5th Fruit Update briefly covered black rot foliar symptoms, which look like concentric rings and are referred to as frogeye leaf spot. In addition to leaves, black rot can affect flowers, branch wounds, and fruit, and around this time rot symptoms can be seen on any infected apples that have continued to ripen on an apple tree.
Life cycle and symptoms
Similar to apple scab and some other fungal diseases, black rot spores develop during warm (above 48° F.) rainy periods in the spring and are spread by rain splash, wind, or insects. Infections can successfully occur when spores contact the point of infection (e.g., a flower, fruit, or branch wound) at a temperature (between 78–80° F. for a leaf infection, or 68–75° F. for a fruit infection) and wetness duration adequate for fungal growth (1). Early blossom and fruit infections can lead to fruit drop or complete mummification, while other fruit infections develop into a rot, usually starting at the blossom/calyx end. Occasionally, black rot fruit infections can also give way to early fruit ripening with rot at the fruit core/ovary.
When apple branches are wounded by winter damage, hail, or other injuries, they become vulnerable to infections, and this is how black rot can make its way into trunks and branches. Sometimes infected branches can display symptoms similar to fire blight canker, however symptoms on branches and trunks can vary in appearance depending on how long the infection has been taking place. Cankers can be sunken, and reddish brown, eventually leading to cracking bark and detectable black pycnidia, or fruiting bodies that house spores for future infections. Serious or chronic black rot infections can lead to defoliation in a single season, which decreases winter survivability for trees, and can lead to overall tree decline.
Management
If black rot is problematic within an orchard section or on a specific apple variety, prioritizing good sanitation practices can help to remove the inoculum that will carry the disease over to the following year. Because black rot overwinters in mummified fruits, as well as branches, and the trunk, removing mummified fruits, dead limbs or branches, and whole trees when necessary, can slow down and decrease its ability to spread. Flagging infected limbs now can effectively help branch removal when dormant pruning next year. Additionally, remember to sanitize pruning shears, loppers, and saws with a light dilution of bleach or other non-corrosive disinfectant when transitioning between pruning infected and non-infected areas.
Some apple varieties, like McIntosh and Cortland are more prone to black rot infections than other varieties, which may influence varietal selection for some growers. For chemical management recommendations, refer to the Midwest Fruit Pest Management Guide starting on page 53 where black rot management on apples can be found under the tab, “summer rots (black and white).”
Black rot vs opportunistic fungi and sooty mold
There are a lot of fungi that produce black spores, black fruiting bodies that house spores, or have a darker mycelium (i.e., the body of the fungus). This can occur on pruning wound sites where the woody tissues lose integrity partway through the growing season and begin to lose sap, or in areas where aphids or other sap sucking insects leave honeydew behind. There are a number of different fungi that lead to these conditions that are not the black rot pathogen D. seriata, syn B. obtusa (e.g. sooty mold and other opportunistic fungi). Knowing this is important because the consequences of having one or the other are very different. Additionally, black rot on apples is not the same pathogen as black rot on grapes (Guignardia bidwellii) or black rot in brassica crops, which is actually caused by the bacteria Xanthamonas campestris pv campestris.
Additional reading:
Black rot of apple (University of Minnesota Extension)
Resources cited:
- West Virginia University Extension Service, 2018, Black Rot Diseases in Apples - Botryosphaeria obstua (Schwein), ANR-IPM-18-002
Late–season apple scab fruit infections and mitigating outbreaks for the following growing season.
Images: Two examples of apple scab (Venturia inaequalis) lesions on fruits, one showing minor blemishes (left image) while the other has a more severe infection which led to fruit cracking (right image).
As mentioned in last week’s update, the high rain frequency we experienced during the spring and early summer this year made it challenging to find dry windows of time to apply fungicides for apple scab (Venturia inaequalis) management. For growers who were not able to make it out and spray or routinely manage with a low or no spray program, more apple scab infections might be present on leaves and fruits for this year’s harvest.
Late season fruit infections truly look scab-like as they become brown and corky over time and can eventually lead to fruit deformation and cracking as the fruit develops. While there isn’t much that can be done at this point for the current year’s fruit and leaves, now is a time to consider sanitation measures for managing next year’s disease.
Here are some things to think about if you faced a high frequency of infected apples and leaves this year:
- Superficial scab infections like the one in the left picture above are safe for consumption and some may be suitable to sell in some capacity when consumers are well informed about the blemishes that are present.
- Mowing infected leaves can encourage their decomposition, which also encourages the breakdown of apple scab fruiting bodies. Additionally, a 5 percent urea solution (46-0-0; this equates to 42 pounds of urea to 100 gallons water) can be applied to the orchard floor to encourage leaf decomposition when leaf fall occurs.
- Consider creating a management plan for the next growing season if your goal is to grow infection-free fruit. Organic and conventional products exist for its management and the Midwest Fruit Pest Management Guide provides guidance for conventional management schedules starting on page 15. Information about organic approaches to managing apple scab can be found in A Grower’s Guide to Organic Apple Management, published by Cornell Extension, starting on page 52.
- Pruning can alter the canopy microclimate and a good pruning program aims to increase canopy airflow and decrease humidity and overall leaf wetness durations throughout the season. These factors all impact apple scab development and can contribute to any management strategy as a tactic to decrease the likelihood of apple scab infections.
Additional reading:
Apple scab of apples and crabapples (University of Minnesota Extension)
Apple diseases- Apple scab (PennState Extension)
Resources cited:
- Peter, K., 2023, Apple Diseases - Apple Scab, PennState Extension.
Grapes
Taking down and storing bird netting.
Image: Netting can be lifted up and clipped to an upper trellis wire to expose clusters for hand-harvesting, as seen here on this row of Frontenac Gris.
Bird netting does not need to be taken down before harvest for those who practice hand harvesting. Most vineyards are busy with many tasks during harvest season and choose to clip up their netting to expose grape clusters when harvesting by hand. At some point,however, taking down netting and storing it for next year becomes necessary. This isn’t usually the most exciting task of the season, and it can take much more time without the proper organization and planning. Now is the time to decrease the amount of effort it takes to sort through and apply netting for next year.
Note: If you missed our article earlier this year, “Bird netting and other bird deterrents,” from our August 8th Fruit Update provided a good foundation about the concepts behind different bird deterrents and netting types.
Below are a few considerations for taking down and storing bird netting this year:
Timing:
- Remove bird netting before complete leaf fall or hardening off has occurred. As leaves begin to fall off the vines, the chance for netting to catch on canes and woody tendrils can make removal more difficult.
- Depending on whether you are using plastic mesh netting or draped knit netting, you will want to either roll up the netting or store it bunched up in stuff sacks. Know which type of netting you have and gather the supplies needed to store it for next year.
- Think ahead and have a method for gathering the netting that suits your needs ready to go.
- Questions: Is a crank, multiple spools, and/or other supplies needed to make spools that will hold rolls of netting? What will keep the netting in place to a spool or other item used for rolling up the bird netting? Are the sacks or bags that are used for bunching up knit netting intact or do they need to be replaced?
- If your vineyard has varying row lengths, be sure to label nets based on row length, or by region.
- When rolling up multiple lengths onto one spool, write the order of the netting from the outermost net (the first net placed on the spool) to the innermost net (the last net placed on the spool) with the intention of following that order during the following year when nets are reapplied.
- Similarly, if you have a method to how you apply netting each year, save yourself some time by creating a system for storing nets. The nets that are used first should be the easiest and first nets that are accessed.
Post–harvest nutrient management
(Avoiding?) post–harvest nitrogen.Managing grapevine nutrition is essential to maintaining healthy vines and harvesting a good crop. Each time a grape crop is harvested, some amount of nitrogen is removed from the soil, which means that grapevines are more nutrient depleted postharvest. Deciding to fertilize to replenish the vines postharvest may seem appealing, but specifically applying nitrogen late-season can be controversial because application timing and rates impact how the vines grow and acclimate. With too little nitrogen, vines become unhealthy, but excess nitrogen at a non-optimal time can also put a vine out of balance.
Concerns related to applying nitrogen post-harvest is that vines may experience unnecessary leaf growth and a reduced ability to harden off, which can potentially decrease a grapevine’s ability to overwinter. This is especially true for situations where the time period between harvest and leaf fall is short. Because nitrogen is highly mobile in soils, there is also the concern that most of what is applied and not taken up during the fall may leach into the environment.
It should be noted that applying compost, which can contain nitrogen, post-harvest is ok to do. Compost is best applied after harvest, but before the ground freezes. If you do end up applying chemical nitrogen post-harvest, be sure to apply it shortly after harvest and use a reduced rate.
When can grapes be fertilized with nitrogen?
It is generally recommended to apply nitrogen fertilizer on grapes as a split application of a rate between 30-60 pounds total per acre, depending on a site's soil organic matter content (i.e., vineyards with more organic matter require less nitrogen). One time of year when grapevines are rapidly depleting their nitrogen reserves is from the time after bud burst until around bloom; thus, applying nitrogen a few weeks before bloom can be helpful in keeping up with its supply and demand (1). The second application can be done sometime after fruit set and before veraison. Bear in mind that splitting a nitrogen application does not mean equal amounts need to be applied for each application, and applying a larger portion of nitrogen before bloom and a smaller portion for the second application is recommended.
Applying potassium and other nutrients post-harvest.
Grapes use potassium to help expand and grow berries, thus a good amount is depleted by the time harvest has finished. Unlike nitrogen, potassium will not stimulate more vegetative growth and is less controversial to apply in cold climates post-harvest. Additionally, potassium and other nutrients are less likely to leach from the soil. Note that the decision to apply nutrients and the application rates should be based on prior soil testing to determine the need for any nutrient applied.
Additional reading:
Applying fertilizer to vineyards after harvest (UMN Extension)
Growing Grapes in Minnesota: Fertilization and Nutrition (Minnesota Grape Growers Association)
Resources cited:
Concerns related to applying nitrogen post-harvest is that vines may experience unnecessary leaf growth and a reduced ability to harden off, which can potentially decrease a grapevine’s ability to overwinter. This is especially true for situations where the time period between harvest and leaf fall is short. Because nitrogen is highly mobile in soils, there is also the concern that most of what is applied and not taken up during the fall may leach into the environment.
It should be noted that applying compost, which can contain nitrogen, post-harvest is ok to do. Compost is best applied after harvest, but before the ground freezes. If you do end up applying chemical nitrogen post-harvest, be sure to apply it shortly after harvest and use a reduced rate.
When can grapes be fertilized with nitrogen?
It is generally recommended to apply nitrogen fertilizer on grapes as a split application of a rate between 30-60 pounds total per acre, depending on a site's soil organic matter content (i.e., vineyards with more organic matter require less nitrogen). One time of year when grapevines are rapidly depleting their nitrogen reserves is from the time after bud burst until around bloom; thus, applying nitrogen a few weeks before bloom can be helpful in keeping up with its supply and demand (1). The second application can be done sometime after fruit set and before veraison. Bear in mind that splitting a nitrogen application does not mean equal amounts need to be applied for each application, and applying a larger portion of nitrogen before bloom and a smaller portion for the second application is recommended.
Applying potassium and other nutrients post-harvest.
Grapes use potassium to help expand and grow berries, thus a good amount is depleted by the time harvest has finished. Unlike nitrogen, potassium will not stimulate more vegetative growth and is less controversial to apply in cold climates post-harvest. Additionally, potassium and other nutrients are less likely to leach from the soil. Note that the decision to apply nutrients and the application rates should be based on prior soil testing to determine the need for any nutrient applied.
Additional reading:
Applying fertilizer to vineyards after harvest (UMN Extension)
Growing Grapes in Minnesota: Fertilization and Nutrition (Minnesota Grape Growers Association)
Resources cited:
- Hannam, K.D. et al., 2013, Late-season foliar urea applications can increase berry yeast-assimilable nitrogen in winegrapes (Vitis vinifera L.)
- University of Missouri Grape and Wine Institute, Vinews, No.3 - May 11, 2020.
End of year UMN Fruit and Vegetable News reader survey:
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