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What Causes Apple Mummies and How to Prevent Them

Fruit mummies on a table including apple mummies.
Fruit mummies on a table. Fruit mummies are shriveled up baby fruits. These can happen naturally as part of the fruit thinning process or they can be a sign of disease. Photo credit: OrchardPeople.com.

One of my favorite orchards in Toronto is at Spadina House, a grand estate dating back to the 19th century. This public orchard is never sprayed with fungicides or pesticides, so sometimes the trees get sick. This year, a number of the heirloom apple trees had lots of shriveled up, dead fruitlets - called apple mummies - on the bare branches over the winter. 

So, in episode 104 of the Orchard People radio show and podcast, I interviewed Kerik Cox, Ph.D., a plant pathologist, mycologist, and bacteriologist, from the Cox Program at Cornell University College of Agricultural and Life Sciences to get the low down on apple mummies. I wanted to learn why these fruitlets form, when they are a sign of disease, and how to prevent them.

So let’s dig in! 

What Causes Apple Mummies?

Apple trees naturally produce more fruitlets than they are able to support. So, trees will naturally “thin out” the harvest. Early in the growing season you may find lots of small apples that have fallen from the tree, a phenomenon sometimes called “June drop” even if it sometimes happens in July! These are substandard fruits that your apple tree has to let go of so that it can conserve energy and ensure that the remaining fruit can grow to full size. 

But not all of the apples left on the tree’s branches will be good! So, if a tree doesn’t have enough nutrition to support all of its growing fruit, it will channel its energy into the best of the bunch and the remaining ones may stay small. Slowly, those neglected fruitlets will die. Some will fall to the ground. Others will remain on the tree.

While “thinning” is a natural process that happens to healthy trees, apple mummies can also be a sign of disease. Examples include bitter rot (Colletotrichum), a fungal disease that can damage the apples,  or the deadly bacterial disease fire blight (Erwinia amylovora), which can take out an entire orchard in a matter of weeks. 

Apple mummies can also form as a result of environmental damage. For instance, a late frost can wreak havoc with fruit trees, killing the smaller and more vulnerable fruitlets while the stronger and larger ones may survive. The problem is that the dead apple mummies that remain on the tree can be an attractive destination for disease pathogens. And that is not a good thing.  

Why are there dead FRUIT MUMMIES on my fruit trees? Preventing fungal diseases.

Can Apple Mummies Spread Disease? 

According to Kerik, fruit mummies on stone fruit trees are actually more dangerous than apple mummies. Stone fruit mummies often get fuzzy due to a disease called brown rot (Monilinia fructicola). That fuzz is what Kerik calls “Fungal babies,” which are baby spores that will spread around your garden or orchard and affect other fruit trees. 

But apple mummies don’t usually spread disease in the same way. Kerik explains: 

“If you have a stone fruit and it looks fuzzy, maybe worry. If it's an apple tree, I would not worry so much. The mummies that are typically on apples will contribute a little bit to what we like to refer to as disease inoculum, which are the propagules of disease that are present in the orchard.”

While the apple mummies that result from diseases may not always be vectors for disease, they will probably result in a poor-quality harvest with much of the fruit being rotten inside.

How do you know if an apple mummy is a symptom of disease or if it is part of the tree’s natural thinning process? Let's explore a number of the apple diseases that result in apple mummies next.

Close up of smooth apple mummy on a branch indicating fire blight. What causes apple mummies.
Apple mummies with fire blight are smoother, oozing, and less shrivelled than regular apple mummies. Photo credit: Kerik Cox.

Fire Blight Apple Mummies

One of the deadliest infections that apple trees can get is fire blight (Erwinia amylovora). An apple mummy on a tree with fire blight looks smoother and less shriveled than typical mummies. 

“If it feels sticky to the touch, that’s just loaded with fire blight cells,” Kerik explains. “Or, if you get to the middle of the summer and droplets are forming on your fruit, that means fire blight has ridden the water slide straight to the fruit and is bursting out. It will get into the vascular tissue of the tree and move to the active growing tissues.”

If you find this type of apple mummy, inspect the rest of your tree carefully. Look for new growth that has turned brown or black and curled up into a “shepherd’s crook”—a symptom of fire blight. This bacterial disease can spread quickly in warm, wet weather.

Large apple trees on full-sized rootstocks have extensive root systems, and they might be able to tolerate fire blight. In that case, carefully cut off and destroy the affected branches or bag them up and send them to municipal waste. 

But most apple trees today are grafted onto dwarfing or semi-dwarfing apple rootstocks, and in those cases fire blight can be deadly, killing small trees in a matter of weeks.  Prune out diseased tissue here too....but if you are planting new apple trees, it’s wise to invest in cultivars from your local fruit tree nursery that are resistant to fire blight to avoid this devastating disease that affects both apples and pears. 

Puckered looking apple mummy on tree indicating bitter rot. What causes apple mummies.
This wrinkled and leathery apple mummy is a sign of bitter rot. Photo credit: Kerik Cox.

Bitter Rot Apple Mummies

Apple mummies resulting from bitter rot, caused by the fungus Colletotrichum, have distinct features. A tell-tale sign is dark, ringed spots on the skin. If you cut the fruitlet open, the flesh under the spots will be black or brown and rotten. 

Eventually, these fruitlets become more wrinkled and leathery and they might have tiny salmon colored droplets on them. In this situation, affected fruitlets can easily spread this disease to other apple trees, infecting healthy fruit, reducing yield, and causing significant crop loss. In severe cases, the disease can weaken the tree, making it more susceptible to other infections and pests.

Bitter rot, like fire blight, thrives in warm, humid conditions.

An apple mummy with black rot is greyish and less shrivelled than ordinary apple mummies.
Black rot (Botryosphaeria obtusa) is another disease that can result in apple mummies. This diseased fruitlet looks like it is covered in a layer of soot. Often these mummies will be firm and slightly oily feeling. Photo credit: Kerik Cox.

How to Prevent Apple Mummies and Fruit Tree Fungal Diseases

Fruit mummies aren’t good for fruit trees, and apple mummies are no exception. Here are five ways to prevent apple mummies:

1. Remove Apple Mummies from Your Tree

If you have one or two trees, check them weekly and remove any mummy fruits you see. Often, it’s easy to pluck off the mummies, but according to Kerik, some cultivars, like Fuji and Cortland apples, hold on tight! In those cases, you may need to cut off the mummies with heavy-duty scissors or pruners.

2. Prune Your Fruit Trees Correctly

Proper apple tree pruning creates an open canopy that allows air flow and sunlight in, keeping leaves dry. Most disease pathogens prefer dark, damp conditions. As Kerik says, “Air is the best fungicide and bactericide.” Don’t just trim around the edges; remove entire branches strategically to reduce crowding. Learn more in Orchard People’s online course Certificate in Fruit Tree Care.

3. Irrigate Your Trees Correctly

Sprinklers waste water and can damage fruit trees by wetting the entire canopy instead of focusing on the roots. Keeping the canopy dry helps protect your fruit tree from diseases that cause apple mummies.

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4. Practice Good Fruit Tree Hygiene

Whether you’re growing apple, pear, or stone fruit trees, sanitation is key. Kerik advises, “If it’s not making you fruit and it looks dead, remove it—whether it’s a branch, a piece of wood, a canker, a shoot, or a mummy. All of that is a reservoir for bacteria and fungi.” Remove leaf litter around the tree to prevent pests and pathogens from overwintering and returning in greater numbers in spring.

Fruit mummy with brown rot in soil overwintering.
Fruit mummies can overwinter in the leaf litter. If that happens and the fruit mummy is diseased, the pathogen will return the following year. Photo credit: Kerik Cox.

5. Use Microbe-Rich Mulches

For small-scale growers, homemade compost, composted manure, or arborist wood chips can serve as microbe-rich mulch. For larger orchards, Kerik suggests using urea pellets, a nitrogen-rich fertilizer. “In commercial orchards, we put feed-grade urea prills on the ground. The urea enhances the microbial communities in the soil, breaking down leaf litter and apple mummies like a compost pile.” This creates a “free buffet” for natural soil microbes, boosting their ability to break down organic matter.

Should you put apple mummies in your compost? 

So, if active compost can kill fruit mummies, should you put infected fruit mummies in your compost? Well, that’s something I personally would avoid. As a home or community grower, our compost pile is relatively small and never gets very hot. My concern would be that the pathogen would survive and come back to haunt me another day. 

Instead, we rake up the leaf litter and fallen fruit (including apple mummies) from around our trees. We bag it up and send it off to our city’s leaf litter collection program. They then dump these bags into huge, steaming hot compost piles that will kill any pathogen unlucky enough to find itself hiding inside. 

Cox lab team from Cornell Agritech in the orchard. What causes apple mummies.
The Cox Lab team at Cornell University tests new tools and pesticides that can prevent fungal and bacterial apple tree diseases including those that result in apple mummies. Photo credit: Kerik Cox.

Are there Pesticides or Fungicides that are Effective Against Apple mummies? 

Growers can use different organically accepted fruit tree sprays to protect their apple trees throughout the growing season. And while there are many sprays on the market, new ones are also being developed. Kerik and his team at Cornell’s Cox lab spend their time testing sprays and other tools. The new trend is to use natural products like cinnamon, thyme, and garlic to create pesticides that are both effective and mostly environmentally safe.

You might think these botanically based products will be harmless since we can eat them. But they can be hot! In the right proportions, these pungent herbs and spices can burn the thin cell walls of fungal and bacterial pathogens. However, if you mess up the proportions, they can also burn plant tissue. The trick is to develop a botanical pesticide that is both effective and safe.

One of the products that Kerik’s team is testing is Cinnerate, and so far, the results are promising. Other options his team is testing use thyme, garlic, or even UV light to protect fruit trees.

Online, you can find recipes to make your own botanical sprays with these ingredients. An old favorite is garlic fruit tree spray, and I’ll include a recipe for it below.

How Do You Protect Your Tree From Apple Mummies?

In the end, think about apple tree mummies as a way for your tree to communicate with you. If your apple tree gets lots of fruit mummies, it’s time to start thinking about how you care for your tree. Are you pruning it properly each year? Are you giving it the nutrients it needs?

You can learn how to do that and much more in my online course Certificate in Fruit Tree Care.

Make Your Own Fruit Tree Garlic Spray

Want to make your own homemade fruit tree spray? There are lots of options to try.

Below a recipe to try. While this spray is all-natural, it can harm beneficial insects like bees, so be sure to spray at a time when bees aren’t active (early morning and late evening). Keep an eye out for bird nests and avoid spraying near them. And remember, you wouldn’t want to get this in your eyes or on your skin, so handle it with care!

Garlic Fruit Tree Spray

Garlic is known for its strong scent, which can deter many types of pests, from aphids to caterpillars and even some types of fungi.

Ingredients:

  • 1 pound of garlic
  • 1 gallon of water (rainwater is best)

Instructions:

  1. Peel and Mince: Start by peeling the garlic cloves. Then, mince or finely chop them to release the essential oils. The finer, the better.
  2. Simmer: Combine the minced garlic with water in a large pot. Bring the mixture to a simmer (not a full boil) and let it cook for about 20 minutes. This process infuses the water with garlic's active compounds.
  3. Cool and Strain: After simmering, take the pot off the heat and let it cool. Once it's cool, strain the mixture to remove the garlic pieces. A cheesecloth or a fine mesh strainer works great for this. You want the liquid as clear as possible to prevent any sprayer clogs.
  4. Dilute (Optional): Depending on what you're dealing with, you might want to dilute the mixture. A common approach is to dilute the concentrated garlic water with more water. This step is adjustable based on your needs and the sensitivity of your plants.
  5. Bottle Up: Pour the strained, cooled liquid into a spray bottle or a larger sprayer for application. It’s ready to use!
  6. Apply: Spray it on the leaves of your fruit trees, covering both the tops and undersides where pests like to hide. Early morning or late afternoon is the best time to apply, avoiding the hot midday sun to reduce the risk of burning the leaves.

Tips:

  • Storage: You can store the garlic spray in the refrigerator for up to a week. For longer storage, freeze it in an ice cube tray and thaw cubes as needed.
  • Test: Always test the spray on a small area of the plant first to ensure it doesn't react badly.
  • Reapply: After rain or watering, you might need to reapply, as it can wash off.

Susan Poizner

Director, OrchardPeople.com Fruit Tree Care Education Online

Susan Poizner is an urban orchardist in Toronto, Canada and the award-winning author of three fruit tree care books.  Susan trains new growers worldwide through her award-winning fruit tree care training program at Orchardpeople.com and is a former instructor of Fruit Production at Niagara College in Ontario. Susan is also the host of Orchard People, a monthly radio show and podcast, and Susan is an ISA Certified Arborist.

Susan Poizner and the cover of her eBook Grow Fruit Trees That Thrive

FRUIT TREE CARE NEWSLETTER

Sign up for our monthly newsletter and we will send you our eBook "Growing Fruit Trees That Thrive." You can unsubscribe at any time.
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