Fungus on Crabapple Tree: 12 Types, Prevention & Treatment

Crabapple trees are a lovely addition to any yard with their bright blooms and colorful fruits. However, as with all plants, they can encounter occasional issues.

One such issue is fungal diseases, which can cause a variety of symptoms and can potentially harm the tree’s health.

This article will explore 12 types of fungal diseases that can affect crabapple trees and provide prevention and treatment methods for each.

If you’re wanting to learn more about common issues that plague crabapple trees, my comprehensive article, Crabapple Tree Problems, is for you.

1. Powdery Mildew (Multiple Pathogens)

Powdery mildew is a common fungal disease that can affect a wide range of plants, including crabapple trees.

It is characterized by a white, powdery coating on the leaves, stems, and sometimes fruit.

The affected leaves may become distorted or stunted, and in severe cases, the disease can cause leaf drop. Powdery mildew is caused by several different species of fungi and thrives in warm, dry climates.

Prevention & Treatment

Preventing powdery mildew involves a combination of cultural practices and, if necessary, fungicide applications.

Ensure your crabapple tree is planted in a location with good air circulation as this can help reduce the humidity that the fungi thrive on.

Avoid overhead watering, which can create a moist environment on the leaves. If powdery mildew appears, fungicides can be used.

Choose a product labeled for use against powdery mildew, like this one, and follow the manufacturer’s instructions for application rates and timing.

2. Sooty Mold (Capnodium citri)

Sooty mold is a type of fungus that grows on the honeydew excreted by certain pests, such as aphids, scale insects, and whiteflies.

It appears as a black, sooty coating on the leaves, stems, and fruit of the tree.

While sooty mold itself does not directly harm the tree, it can block sunlight, which can interfere with photosynthesis.

Additionally, the presence of sooty mold indicates that there is a pest problem that needs to be addressed.

Prevention & Treatment

Preventing and treating sooty mold involves managing the pests that produce the honeydew the mold grows on.

Regularly inspect your crabapple tree for signs of these pests, and use appropriate pest control methods if necessary.

This could include introducing beneficial insects, using insecticidal soaps or oils, or applying a systemic insecticide.

If sooty mold has formed, it can often be washed off with a strong stream of water.

3. Apple Scab (Venturia inaequalis)

Apple scab is a fungal disease that is particularly common in regions with cool, wet springs. It causes dark, scaly lesions on the leaves, fruit, and sometimes the twigs of the tree.

The affected leaves may eventually turn yellow and fall off.

Prevention & Treatment

Preventing apple scab involves a combination of cultural practices and fungicide applications.

Rake and dispose of fallen leaves, which can harbor the fungus. Choose resistant varieties of crabapple trees if possible.

If apple scab has been a problem in the past, consider applying a fungicide in the spring when the leaves are emerging.

Choose a product labeled for use against apple scab, and follow the manufacturer’s instructions for application rates and timing.

4. Cedar-Apple Rust (Gymnosporangium juniperi-virginianae)

Cedar-apple rust is a unique fungal disease that requires two different host plants to complete its life cycle: a juniper and an apple or crabapple tree.

On crabapple trees, it causes bright orange-yellow spots on the leaves, and in severe cases, it can cause leaf drop.

The disease is most noticeable in the spring when the fungal galls on the juniper host produce orange, gelatinous spore horns during wet weather.

A leaf from an apple tree infected with cedar-apple rust.

Prevention & Treatment

Preventing cedar-apple rust can be challenging due to its unique life cycle. If possible, remove any nearby juniper trees that could serve as a host for the fungus.

Fungicides can also be used to protect susceptible trees. These should be applied in the spring when the orange, gelatinous spore horns are visible on the juniper host.

5. Cedar-Hawthorn Rust (Gymnosporangium globosum)

Cedar-hawthorn rust is a similar disease to cedar-apple rust, but it can also affect hawthorn trees in addition to crabapples.

The symptoms are similar to those of cedar-apple rust: orange-yellow spots on the leaves.

Prevention & Treatment

The prevention and treatment methods for cedar-hawthorn rust are similar to those for cedar-apple rust.

This includes removing any nearby juniper or hawthorn trees that could serve as a host for the fungus and using fungicides to protect susceptible trees during the spring when the fungus is most active.

6. Fire Blight (Erwinia amylovora)

Fire blight is a bacterial disease that can cause a variety of symptoms, including wilting, blackening, and a “burned” appearance of branches, blossoms, and fruit.

It can also cause cankers on the branches and trunk. Fire blight can spread rapidly in warm, wet weather.

Note: Although fire blight is not technically a fungus, it can be so devasting that we thought it should be included on this list.

Prevention & Treatment

Preventing fire blight involves keeping your tree at its healthiest and, if necessary, using antibiotic sprays.

Prune and dispose of infected branches during the dormant season, and avoid excessive nitrogen fertilization, which can promote susceptible new growth.

If fire blight has been a problem in the past, consider applying an antibiotic spray in the spring when the blossoms are opening.

7. Black Spot (Diplocarpon rosae)

Black spot is a fungal disease that is most commonly associated with roses, but it can also affect crabapple trees.

It causes black spots on the leaves, which may eventually turn yellow and fall off.

Prevention & Treatment

Preventing black spot involves good cultural practices and, if necessary, fungicide applications.

Ensure your crabapple tree is planted in a location with good air circulation, and avoid overhead watering, which can create a moist environment on the leaves.

If black spot appears, fungicides can be used. Choose a product labeled for use against black spot, and follow the manufacturer’s instructions for application rates and timing.

8. Frogeye Leaf Spot (Botryosphaeria obtusa)

Frogeye leaf spot is a fungal disease that causes circular, brown spots on the leaves, which are often surrounded by a red halo.

The spots may eventually coalesce, causing large areas of the leaves to turn brown.

Prevention & Treatment

Rake and dispose of fallen leaves, which can harbor the fungus. If frogeye leaf spot has been a problem in the past, consider applying a fungicide in the spring when the leaves are emerging.

9. Entomosporium Leaf Spot (Entomosporium mespili or E. maculatum)

Entomosporium leaf spot is a fungal disease that causes red or brown spots on the leaves, which may eventually turn black.

The spots are often surrounded by a yellow halo, and in severe cases, the disease can cause leaf drop.

Prevention & Treatment

To manage Entomosporium leaf spot, remove and dispose of fallen leaves, which can harbor the fungus. Fungicides can also be used and should be applied in the spring when the leaves are emerging.

10. Botryosphaeria Canker (Botryosphaeria spp.)

Botryosphaeria canker is a fungal disease that causes cankers, or sunken areas, on the branches and trunk of the tree.

The cankers are often dark and may ooze sap. In severe cases, the disease can cause dieback of branches.

Prevention & Treatment

Prune and dispose of infected branches during the dormant season, and avoid wounding the tree as the fungi can enter through wounds.

If Botryosphaeria canker has been a problem in the past, consider applying a fungicide in the early spring.

11. Flyspeck (Schizothyrium pomi)

Flyspeck is a fungal disease that causes small, dark spots on the leaves and fruit of the tree. The spots are often clustered together, giving the appearance of flyspeck.

Prevention & Treatment

Ensure your crabapple tree is planted in a location with good air circulation, and avoid overhead watering.

If flyspeck appears, fungicides can be used. Choose a product labeled for use against flyspeck, and follow the manufacturer’s instructions for application rates and timing.

12. Sooty Blotch (Gloeodes pomigena)

Sooty blotch is a fungal disease that causes dark, sooty blotches on the leaves and fruit of the tree.

The blotches are often irregular in shape and can merge together to cover large areas of the leaf or fruit surface.

Prevention & Treatment

Be sure your crabapple tree receives good air circulation, and avoid overhead watering, which can invite disease development.

If sooty blotch appears, treat with fungicides. Choose a product labeled for use against sooty blotch, and follow the manufacturer’s instructions for application rates and timing.

Understanding Fungal Disease Management

Managing fungal diseases in crabapple trees involves a combination of good cultural practices, regular monitoring, and, if necessary, the use of fungicides.

By providing proper care and promptly addressing any signs of disease, you can help keep your crabapple tree healthy and vibrant.

Conclusion

Fungal diseases can pose a significant threat to crabapple trees, but with the right knowledge and tools, you can protect your tree and keep it healthy.

By understanding the potential diseases, practicing good tree care, and taking action at the first sign of trouble, you can enjoy the beauty of your crabapple tree for many years to come.

Be sure to check out other issues Crabapple trees often face: