Apple trees are susceptible to several diseases that can reduce fruit yields and tree vigor. As an apple grower, it’s important to be familiar with the most common apple diseases so you can identify symptoms early and take preventative and control measures. In this article, we’ll discuss four of the most prevalent apple tree diseases and provide guidance on how to manage them.
Apple Scab
Apple scab is considered the most destructive disease of apple trees worldwide. It is caused by the fungus Venturia inaequalis, which overwinters in fallen leaves. In the spring, fungal spores are released and spread by wind and rain to new leaf and fruit tissue, causing lesions characteristic of apple scab.
Scab lesions start out as velvety olive-green spots on leaves. As the lesions age, they turn brownish-black. On fruit, scab lesions begin as light green, velvety spots that eventually become cracked, scabby areas. Heavy scab infections can result in tree defoliation and fruit drop.
To prevent scab, choose resistant cultivars when possible. For susceptible trees, options include:
- Sanitation: Rake and destroy leaves and prunings from diseased trees to remove overwintering fungal spores.
- Fungicides: Use protective fungicides starting at green tip and continue applications following label directions through primary scab season when conditions favor disease.
- Cultural practices: Prune trees to open up the canopy and improve air circulation. Water in the morning to allow leaves time to dry out during the day.
Fire Blight
Fire blight is another serious bacterial disease of apple trees that can quickly kill branches, limbs, or entire trees. It is caused by the bacterium Erwinia amylovora. Fire blight infects blossoms, fruit, shoots, and roots.
In the spring, creamy-white bacterial ooze may be apparent on blossoms and shoots. Infected shoots soon wilt, turning brown or black as if scorched by fire. Leaves remain attached to blighted shoots, giving the tree a scorched appearance. Cankers develop on limbs and trunks, exuding bacterial ooze in wet weather. Entire trees can be killed rapidly in warm, humid weather.
To manage fire blight:
- Prune out infections: Remove infected shoots 8-12 inches below visible symptoms during dry weather. Sterilize pruning tools between cuts.
- Antibiotics: Application of antibiotics like streptomycin may help protect blossoms when applied at bloom. However, resistance is a concern.
- Resistant varieties: Choose less susceptible cultivars. Avoid highly susceptible cultivars like Gala, Fuji, Honeycrisp.
- Cultural practices: Promote airflow through proper planting, pruning, and avoiding high nitrogen fertilization which stimulates succulent growth.
Cedar Apple Rust
Cedar apple rust results from a fungal pathogen (Gymnosporangium juniperi-virginianae) that requires two different hosts to complete its life cycle: Eastern red cedar and apple or crabapple. Spores overwinter on cedar trees and infect apple trees in the spring.
On apple tree leaves and fruits, cedar apple rust first appears as yellow spots that develop into bright orange, circular lesions about 1/4 inch in diameter with a darker border. Black pustules eventually erupt through leaves and fruit, releasing spores that will infect cedar trees to complete the disease cycle. Premature defoliation and reduced fruit quality can result.
Cedar apple rust management involves:
- Sanitation: Remove galls on cedar trees which release infectious spores. Rake up and destroy infected apple leaves.
- Fungicides: Protectant fungicides applied starting at pink bud on apples are helpful. Systemic fungicides are most effective.
- Resistant cultivars: Some crabapple and apple cultivars have resistance, including Red Delicious, Winesap and Jonagold. Avoid susceptible varieties like Rome, Jonathan, and Golden Delicious.
- Distance: Locate apple trees as far as possible from Eastern red cedar trees.
Bitter Rot
Bitter rot, caused by the fungus Colletotrichum spp., is a late season fruit rot that can ruin apple crops before harvest. Sunken brown lesions form on the side of fruits, enlarging over time and becoming covered with orange conidial spore masses in concentric rings. Fruit shrivels into a mummified state clinging to the branches.
Bitter rot management focuses on:
- Sanitation: Remove mummified fruit from trees and ground to reduce inoculum. Burn or bury infected fruits.
- Fungicides: Start protective fungicide sprays after petal fall and continue following label directions as fruits develop through harvest if bitter rot has been a problem.
- Resistant varieties: Some newer varieties have resistance, including Enterprise, Williams Pride, and Pristine. Avoid heirloom varieties like Jonathan, Rome, and York.
- Cultural practices: Prune trees for good airflow. Avoid unnecessary irrigation late in the season. Pick fruits regularly as they mature.
FAQs About Apple Tree Diseases
Here are answers to some frequently asked questions about diseases of apple trees:
What causes the lesions on my apple leaves?
If the lesions are velvety olive-green spots that turn brownish-black, the cause is likely apple scab fungus. Other possibilities are cedar apple rust (yellow spots turning orange) or a bacterial disease like fire blight.
How do I know if my tree has fire blight?
Look for dead shoots and branches that appear scorched or burnt, with leaves still attached. Cankers on limbs may ooze a creamy bacterial exudate. The disease spreads quickly in warm, humid weather.
What is the treatment for apple scab?
The most effective controls are resistant varieties, sanitation measures, and fungicide sprays starting at green tip and continuing through primary scab season as needed. Prune for open canopies to improve airflow.
My fruit is covered in orange fuzzy spots. What should I do?
That sounds like cedar apple rust. Management relies on sanitation, fungicide applications, planting resistant varieties, and removing nearby cedar trees. Rake up and destroy fallen leaves to reduce fungal spores.
How do I prevent bitter rot on apples?
Late-season fungicide applications, prompt harvesting, removing mummified fruits, and planting resistant varieties all help protect against bitter rot fungus. Improving airflow helps reduce disease pressure.
What organic options exist for managing apple diseases?
Organic approaches include selecting disease-resistant cultivars, applying preventative sprays like copper fungicides and sulfur at critical times, practicing careful sanitation, and supporting overall tree health through proper care and nutrition.
Conclusion
Diseases like apple scab, fire blight, cedar apple rust, and bitter rot can significantly impact the productivity of apple trees. Careful monitoring for early symptoms along with integrated management practices are key to protecting trees and fruit from damage. Strategies include selecting resistant varieties, fungicide applications at critical timings, pruning for airflow, quick removal of infected plant tissues, and preventative care for overall tree health. With proactive management, the impact of diseases on apple yields can be minimized. A little time and effort goes a long way when it comes to avoiding losses to valuable apple crops.