Apple Scab: How to Identify, Treat and Prevent Effectively
Apple scab is a serious disease that affects apples and crabapples, as well as other plants in the same family.
It occurs everywhere where apples are grown around the world and causes more damage than any other apple disease. However, the disease is more common in areas that have cool, wet spring weather.
This disease produces lesions that can be observed on leaves, blossoms, and the fruit itself. If you suspect that the apple scab fungus is infecting your trees, this article will lay out everything you need to do to effectively manage and treat it using tried and tested methods.
What is Apple Scab?
A serious disease of apples and ornamental crabapples, apple scab (Venturia inaequalis) attacks both leaves and fruit.
It produces lesions that can be observed on leaves, as well as blossoms, sepals, pedicels, petioles, and even the fruit itself.
The fungal disease first appears as pale yellow or olive-green spots on the upper surface of leaves. Dark, velvety spots may appear on the lower surface of young leaves as they are exposed to the infection.
These lesions have poorly formed borders and, for this reason, can sometimes be mistaken for sooty mold or even leaf fuzz. Severely infected leaves and tissues become twisted and puckered, turn yellow, and may drop early in the summer.
Unfortunately, two to three years of consecutive defoliation in this manner can weaken the tree significantly, making it vulnerable to other diseases, insects, and stresses such as freeze damage.
Host Plants for Apple Scab
The fungus called Venturia inaequalis is what causes apple scab. It can infect apples and crab apples (Malus spp.), mountain ash (Sorbus spp.), hawthorn (Crataegus spp.), mountain ash (Sorbus spp.), loquat (Eriobotrya japonica), and firethorn (Pyracantha spp.).
A related fungus, Venturia pirina, infects pears (Pyrus spp.) and causes similar symptoms. The apple scab pathogen, on the other hand, will not infect pears, and the pear scab pathogen will not infect apples.
How to Identify Apple Scab?
Symptoms on fruit are similar to those found on leaves. Scabby spots are sunken and tan and may have velvety spores in the center.
The first signs of apple scab depend on the type of tree being infected, but generally initial symptoms include leaves turning dull, with olive-green round spots, usually on the leaves closest to the buds.
As these spots mature, they become larger and turn brown and corky. Infected fruit becomes distorted and may crack allowing entry of secondary organisms. Severely affected fruit may drop, especially when young.
Lifecycle of Apple Scab
Apple scab overwinters primarily in fallen leaves and in the soil and is caused by the fungus Venturia inaequalis.
Disease development is favored by wet, cool weather that generally occurs in spring and early summer.
Fungal spores are carried by the wind, rain, or splashing water from the ground to flowers, leaves, or fruit.
During damp or rainy periods, newly opening apple leaves are extremely susceptible to infection. The longer the leaves remain wet, the more severe the infection will be.
The spores require several hours of moisture to start new infections. Apple scab spreads rapidly between 55-75 degrees Fahrenheit.
Unfortunately, the infection cycle might occur multiple times throughout the growing season if the leaves remain wet for an extended period of time.
Leaves that have many spots eventually turn yellow as the disease progresses and fall off early. This weakens the fruit trees and consecutive years of defoliation can decrease growth, reduce blooms, and even increase susceptibility to other diseases as well as winter injury.
How to Manage and Treat Apple Scab
Managing and treating the apple scab fungus is an integrated process that combines sanitation, resistant cultivars, and fungicides. Let’s look at the steps you can take to treat and manage this disease:
Choose Scab-Resistant Cultivars
The best line of defense is to choose resistant varieties when possible. Some great scab-resistant apple cultivars include:
- Crimson Crisp
- Crimson Gold
- Enterprise
- Freedom
- Goldrush
- Jonafree
- Liberty
- Nova Easygro
- Novamac
- Priscilla
- Pristine
- Redfree
- Scarlet Prima
- Sir Prize
Similarly, some crabapple varieties that are resistant to apple scab include:
- Anne E
- Bob White
- Molten Lava
- Ormiston Roy
- Prairiefire
- Red Jewel
- Strawberry Parfait
- Sugar Tyme
Plant Trees Correctly
It’s important to make sure to plant your apple and crabapple trees properly in full sun and to leave sufficient space around the trees for them to mature. This will allow air to circulate and allow water to evaporate properly after watering.
This will make sure that trees aren’t too crowded next to each other, as this can increase susceptibility to the disease.
The apple scab fungus needs moisture on leaves to start infecting, so this will help dry the leaves and reduce the severity of the disease if already infected, and reduce susceptibility if you’re trying to prevent the infection in the first place.
Prune Infected Leaves
Another great way to manage apple scab is to rake under trees and prune and destroy infected leaves to reduce the number of fungal spores available to start the disease cycle over again next spring.
Apply urea to trees after harvest, but before fall leaf drop as this will help speed up leaf decomposition.
Change Watering Schedule
Water in the evening or early morning hours (avoid overhead irrigation) to give the leaves time to dry out before infection can occur, thereby reducing risks of apple scab fungus taking over.
Cover with Compost
Spread a 3- to 6-inch layer of compost under trees, keeping it away from the trunk, to cover the soil and prevent splash dispersal of the fungal spores. Not only will this give your tree the nutrients it needs to fight any disease, but it’ll also prevent it from spreading if currently infected.
Spray Liquid Copper Soap
For best control, spray liquid copper soap early, two weeks before symptoms normally appear. Alternatively, begin applications when the disease first appears, and repeat at 7 to 10 day intervals up to blossom drop.
Use Wettable Sulfur
Bonide® Sulfur Plant Fungicide, a finely ground wettable powder, is used in pre-blossom applications and must go on before rainy or spore discharge periods. Apply from pre-pink through cover (2 Tbsp/ gallon of water), or use in cover sprays up to the day of harvest.
Wettable sulfur of this kind is one of the materials available to home growers to control apple scab in edible crabapples and apples.
Apply Fungicides
Organocide® Plant Doctor is an earth-friendly systemic fungicide that works its way through the entire plant to combat a large number of diseases on ornamentals, turf, fruit, and more.
Apply as a soil drench or foliar spray (3-4 tsp/ gallon of water) to prevent and attack fungal problems in your apple and crabapple trees.
Use Sulfur Based Sprays
Containing sulfur and pyrethrins, Bonide® Orchard Spray is a safe, one-hit concentrate for insect attacks and fungal problems. For best results, apply as a protective spray (2.5 oz/ gallon) early in the season.
If disease, insects, or wet weather are present, mix 5 oz in one gallon of water. Thoroughly spray all parts of the plant, especially new shoots.
Eric is now retired, but is still a renowned gardener known for his expertise in composting, organic gardening and pest control, utilizing pesticide-free options, such as beneficial insects.
Eric believes when you do something good for the environment, the effects will benefit generations to come.
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