Apple scab is a fungal disease that affects apple and crabapple trees. It is caused by the fungus Venturia inaequalis. Apple scab can seriously impact the quantity and quality of the fruit crop if left unchecked. The disease presents itself as olive-green or brownish black velvety lesions on leaves, fruit, twigs, and buds. With the right knowledge and timely action, apple scab can be effectively managed.

Identifying Apple Scab

Knowing how to identify apple scab is the first step in treating and preventing this disease. Here are the key signs to look for:

Lesions on Leaves

The most distinctive symptoms are velvety olive-green to brownish-black spots that appear on the surface of leaves. These lesions are the fruiting bodies of the fungus.
Lesions first emerge on the undersides of leaves in spring as the buds open.
As the disease progresses, lesions will become visible on the upper leaf surfaces as well. They can cover the entire leaf.
Heavily infected leaves often curl, distort, yellow, and drop early. Defoliation can occur by midsummer in severe infections.

Lesions on Fruit

Pinhead-sized black dots appear on developing fruit about a month after petal fall.
As the fruit continues to grow, these lesions develop into fully sunken, dark and velvety patches.
The lesions may be scattered across the surface of the fruit or clustered around the calyx end.
Fruit infections lead to cracking, distortion, early drop, and overall poor quality.

Lesions on Twigs and Buds

Twig lesions start out as dark olive green areas that later turn brownish-black.
Infected buds are misshapen and discolored. They may fail to develop entirely.
Cankers can form on twigs and small branches.

Signs During Dormancy

During winter dormancy, look for small black fruiting bodies protruding from infected buds and twigs. This helps to identify sites of overwintering infections.

Careful monitoring through the growing season is needed to detect apple scab early and distinguish it from other diseases. Compare symptoms to reference images to confirm apple scab infections. Send suspect plant samples to a diagnostic lab for definitive diagnosis if needed.

How Apple Scab Spreads

Understanding how apple scab spreads aids in prevention:

Primary inoculum: The fungus overwinters in fallen leaves and fruit as pseudothecia (fruiting bodies). In spring, these release ascospores which infect emerging buds and leaves.
Secondary spread: Conidia (asexual spores) are then produced in the lesions on young leaves and fruit. Rain and wind disseminate conidia to cause new infections through summer.
The disease cycle repeats, with leaves falling in autumn to harbor overwintering pseudothecia.
Infections require wet leaf surfaces and moderate temperatures. Ascospores discharge after about 0.1 inches of rain. Conidia spread with just a film of moisture.
Optimal temperature range is 60-70°F. Infections can occur from pink bud stage through late summer.
Apple scab spreads rapidly under favorable conditions, expanding lesion populations exponentially.

Treating Apple Scab Infections

If apple scab is detected, prompt action is required to avoid devastating losses. Integrated treatment strategies should be employed:

Fungicide Sprays

Fungicide application is the primary treatment for active apple scab infections. Start spray program at green tip stage before bud break.
Use targeted fungicides like captan, myclobutanil, mancozeb, fenarimol, propiconazole, thiophanate-methyl, and chlorothalonil. Strobilurin fungicides are also effective.
Coverage is key. Thoroughly coat leaves, buds, fruit, twigs, and branches, especially new growth. Repeat applications per label directions to maintain protective coverage through seasons favoring apple scab development.
Tank mixing fungicides with differing modes of action helps minimize resistance risk. Follow label precautions against phytotoxicity and pollinator harm.

Pruning and Sanitation

Prune out twigs with canker lesions to limit spread.
Rake up and remove fallen leaves and fruit debris promptly to reduce overwintering inoculum. Burn or bury this debris.
Disinfect pruning tools after each cut with bleach solution (1 part bleach to 9 parts water).

Protective Coatings

Coating emerging leaves with agricultural glue or antitranspirants prior to bud break may provide some protection from primary apple scab spores.
Frequent applications of agricultural oils may help smother lesions and shield new growth against secondary spread. Take care to avoid plant damage or fruit finish impacts.
Damaged fruit skin can be coated with edible waxes to exclude spores.

Resistant Varieties

Plant scab-resistant cultivars where possible to limit disease susceptibility, e.g. Enterprise, Liberty, Goldrush, Prima.
Remove highly susceptible trees and rootstock to prevent local inoculum buildup.

Rapid action at first symptoms gives the best chance of rescuing an infected apple crop. But prevention is ideal to avoid infestation and damage entirely.

Preventing Apple Scab

Effective apple scab prevention integrates cultural practices, sanitation, protective sprays, and resistant cultivars:

Cultural and Sanitation Practices

Rake and dispose infected leaves and fruit immediately after fall. Don’t compost.
Prune trees for optimal air circulation and quick drying. This prevents the moist conditions fungus needs to sporulate.
Control weeds around trees.
Disinfect tools regularly with 10% bleach solution.
Remove wild crabapples and abandoned apples nearby to limit inoculum sources.

Dormant Sprays

Apply lime sulfur or horticultural oil sprays while trees are dormant to kill overwintering pseudothecia on twigs and buds.

Protective Fungicides

Start applying targeted fungicide sprays at green tip and maintain coverage through multiple applications over the season, especially during wet weather. Rotate modes of action.
Strobilurin, DMI, chloronitrile, anilinopyrimidine, and multisite contact fungicides are options. Follow label directions.
Mist-blowers can supplement coverage on dense foliage.

Resistant Varieties

Plant apple cultivars bred for scab resistance like Liberty, Enterprise, Goldrush. They have varying immunity depending on the Vf gene sources.
Use resistant rootstocks like Geneva series. These limit fungal growth systemically.
Remove highly susceptible trees and rootstocks which serve as local inoculum reservoirs.

Integrated management combining sanitation, fungicides, resistant varieties and vigilant monitoring offers the best protection against apple scab. Be prepared to respond promptly at first signs of infection. Consistent prevention over successive seasons helps keep the fungus in check and avoid widespread crop impacts.

FAQs About Apple Scab Identification, Treatment and Prevention

Here are answers to some frequently asked questions about identifying, treating, and preventing apple scab disease:

How can I tell if my tree has apple scab?

Look for olive-green to black velvety spots on leaves, distorted fruit with sunken lesions, and cankers on twigs. Lesions first appear on undersides of leaves at budbreak. Fruit lesions emerge around a month after petal fall.

What is the best time to treat apple scab?

Treat preventatively starting at green tip, before infections start. Treat curatively immediately at first sign of lesions to limit spread. Maintain fungicide protection until late summer.

How often should I spray for apple scab control?

Spray every 7-10 days from green tip through 2-3 weeks after petal fall, when fruit is most susceptible. Spray again every 10-14 days until late summer if conditions favor fungal growth.

Can apple scab spread to other trees?

Yes, apple scab readily spreads locally to uninfected apple and crabapple trees via wind and rain-splashed spores. Eliminate nearby sources of inoculum.

Should infected apples be removed from the tree?

Yes, remove and destroy infected fruit immediately to prevent secondary spore production. Don’t compost diseased fruit.

How long do apple scab spores live in fallen leaves?

The overwintering fruiting bodies survive in leaf litter through winter dormancy until spring. Rake and dispose infected leaves.

What chemicals kill apple scab fungus?

Captan, myclobutanil, chlorothalonil, mancozeb, and strobilurin fungicides are effective. Rotate between modes of action.

Is apple scab worse some years?

Yes, wet springs favor apple scab epidemics. Dry weather limits fungal growth and spread. Monitor annually and adjust treatment accordingly.

Can you eat apples infected with apple scab?

Lightly infected mature fruit is edible but poor quality. Severely scarred apples should be discarded. Fungicide residues may be a concern.

How do I know if my apple tree has resistant rootstock?

Check rootstock tags, invoices, or graft union scars for identifiers like G.30, G.935, G.969, indicating Geneva series. Contact your nursery to verify.

Conclusion

Apple scab can significantly reduce yields and fruit quality if left unmanaged. Identifying signs like leaf and fruit spots early allows for prompt treatment. Preventing inoculum buildup through sanitation and dormant sprays limits primary apple scab sources. Fungicide applications before and through wet periods curtails spread of secondary infections. Planting resistant varieties provides systemic protection against the fungus. Apple scab is most effectively controlled through vigilant monitoring, early detection, and integrated treatment and prevention practices over successive seasons. Consistent management keeps apple scab damage to a minimum and allows for bountiful, high quality harvests.

How to Identify, Treat, and Prevent Apple Scab

This is a detailed and informative article on how to identify, treat, and prevent apple scab disease in apple trees. It provides extensive information to help identify apple scab based on symptoms, understand how it spreads, treat active infections, and prevent future outbreaks. The article covers when and how to use fungicide sprays, sanitation methods, resistant varieties, and integrated management programs. It includes a symptom overview, disease life cycle and spread, treatment recommendations, prevention strategies, and FAQs. With thorough coverage of the topic and useful subheadings, this comprehensive guide serves as a valuable resource for identifying, managing, and controlling destructive apple scab infections. The article aims to help orchardists and backyard growers recognize signs of apple scab early, take prompt curative action at first symptoms, implement preventative measures before infection starts, and adopt consistent integrated disease management practices. With multiple integrated solutions, apple scab can be effectively tackled to avoid significant apple crop losses and produce the best quality fruit.