Fungicide for Crabapple Trees: When To Use & Best Options

Fungi commonly infect a range of tree species, including crabapples, which can cause various symptoms like leaf discoloration, decay, and potentially, the formation of cankers and rot.

If left untreated, fungal diseases can severely damage or even kill trees.

The best fungicide to use is often dependent on the targeted fungi. Fungicides are either contact, (eliminate fungus upon direct contact) or systemic (absorbed into the plant). Common fungicides include captan, mancozeb, thiophanate-methyl, myclobutanil, propiconazole, and chlorothalonil.

Below, you’ll find a comprehensive guide to understanding and managing fungal diseases in crabapple trees, including information on common types of fungal diseases, how to identify them, prevention strategies, and treatment options.

Looking for an in-depth resource on caring for crabapple trees? Find comprehensive information in my article, Crabapple Care and Maintenance.

Common Fungal Diseases in Crabapple Trees

Here is a brief rundown of the most common fungal problems seen in crabapple trees.

Apple Scab

Apple scab, caused by the fungus Venturia inaequalis, is one of the most common diseases in crabapple trees.

It is characterized by dark, scaly lesions on the leaves, fruit, and twigs of the tree. The disease is most prevalent in wet, cool weather and can cause significant defoliation if not treated.

Cedar-Apple Rust

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

The disease, caused by the fungus Gymnosporangium juniperi-virginianae, produces bright-orange, gelatinous galls or “rust” on the leaves and fruit of the crabapple tree.

Fire Blight

Fire blight, caused by the bacterium Erwinia amylovora, is not a fungal disease but is worth mentioning due to its severity.

It causes wilting, blackening, and a “burned” appearance of branches. Fire blight can spread rapidly and kill a tree if not managed.

Sooty Blotch and Flyspeck

Sooty blotch and flyspeck are two different fungal diseases that often appear together on the fruit of crabapple trees.

They cause dark, smudgy spots (sooty blotch) or clusters of tiny black dots (flyspeck) on the fruit surface but do not affect the fruit’s credibility.

Red crabapples on a tree with only a few, sick-looking leaves.

Identifying Fungal Diseases

Identifying fungal diseases in crabapple trees involves careful observation of the symptoms.

Leaf spots, blights, cankers, rots, wilting, and unusual growths or discolorations can all be signs of a fungal disease.

However, symptoms can vary depending on the specific type of fungus, the tree’s overall health, and environmental conditions.

Preventing Fungal Diseases

Prevention is the first line of defense against fungal diseases in crabapple trees. Here are some strategies to help prevent these diseases:

Choose Disease-Resistant Varieties

Some varieties of crabapple trees have better resistance to certain fungal diseases than others. When planting new trees, consider choosing these disease-resistant varieties.

Practice Good Sanitation

Regularly cleaning up fallen leaves, pruned branches, and other plant debris can help prevent the spread of fungal spores. It’s also a good idea to sanitize pruning tools between uses and after every cut.

Provide Proper Care

Trees that are healthy and well cared for are more resistant to diseases. This includes watering properly (avoiding wetting the leaves when possible), fertilizing appropriately, and pruning to promote good air circulation.

Use Fungicides

Preventive fungicide sprays can help control fungal diseases. It’s important to apply these sprays properly and at the right time for them to be effective.

Treating Fungal Diseases

If a crabapple tree is infected with a fungal disease, there are several treatment options:


Pruning and destroying diseased branches can help stop the spread of the disease. It’s important to prune during dry weather and to sanitize pruning tools between cuts (not just once before you begin).


Fungicides are chemicals that kill or inhibit the growth of fungi. They can be used to protect crabapple trees from a variety of fungal diseases.

Here are some types of fungicides that are commonly used on crabapple trees:

  • Captan is a general-use fungicide that is effective against a wide range of fungal diseases. It is often used as a preventative treatment, applied before disease symptoms appear.
  • Mancozeb is a broad-spectrum fungicide that is effective against many types of fungi. It is often used in rotation with other fungicides to prevent the development of resistance.
  • Thiophanate-methyl is a systemic fungicide, meaning it is absorbed by the plant and can protect against fungi from the inside out. It is effective against a variety of fungal diseases.
  • Myclobutanil is a systemic fungicide that is particularly effective against rusts, powdery mildew, and certain other fungal diseases.
  • Propiconazole is another systemic fungicide that is effective against a variety of diseases. It is often used for diseases that have already infected the tree as it can stop the spread of the disease within the tree.
  • Chlorothalonil is a broad-spectrum fungicide that is used to control a variety of fungal diseases. It is often used as a preventative treatment.

When using fungicides, it’s important to follow the manufacturer’s instructions carefully.

This includes wearing protective clothing, applying the fungicide at the right time, and using the correct dosage.

Overuse of fungicides can lead to the development of resistance in fungi, making the fungicides less effective over time.

Biological Control

Recent research has shown promising results using biological control agents to manage fungal diseases.

For example, a study published in Horticulturae found that the bacterium Bacillus licheniformis PR2 was effective in controlling several major fruit fungal pathogens in jujube orchards, a tree similar to the crabapple.

This bacterium secretes chitinase, an enzyme that breaks down chitin, a major component of fungal cell walls. This leads to the inhibition of fungal growth and a reduction in disease symptoms.

Integrated Pest Management

Integrated Pest Management (IPM) is a comprehensive approach to pest and disease control that combines different methods and strategies.

It includes regular monitoring, the use of disease-resistant varieties, good sanitation practices, appropriate use of fungicides, and biological control methods.


Fungal diseases can pose a significant threat to crabapple trees, but with proper identification, prevention, and treatment, it’s possible to manage these diseases and maintain healthy, productive trees.

Remember that every tree and every situation is unique. If you’re unsure about the best course of action for a particular tree, don’t hesitate to consult with a local extension service or a professional arborist.

They can provide guidance based on the specific conditions and challenges you’re facing.

When you’re familiar with the methods the pros use, ensuring the wellbeing of a Crabapple tree can be straightforward. Don’t forget to explore our other insightful articles on this topic.