What is apple scab?
Apple scab is a major disease of apples and crabapples worldwide. It is caused by the fungus “Venturia inaequalis”. It causes repeated defoliation in spring and summer, resulting in fewer flowers and weakening of the tree, which leads in turn to other diseases. In addition, scab lesions on apple fruit are undesirable to the consumer, making them unsellable and resulting in significant economic loss every year.
What does apple scab look like?
Infected leaf
- First presents as lighter green areas that become olive green with fuzzy margins (spore production) on both sides
- As the disease progresses, the spots develop a more prominent outline and became black and velvety
- Leaves may became distorted and pucker and will turn yellow and drop prematurely
Infected fruit
- First presents as water soaked areas that turn into velvety green to olive brown lesions
- As lesions develop they have a blistered or “scabby” appearance with a distinct margin
- Will lead to cracking and may cause deformities and uneven growth
Infected blossoms
- Dark green lesions at the base of flower, on sepals and on stem pedicels before and during bloom
Are there other hosts of apple scab?
In addition to apples and crabapple (Malus spp.), hawthorn (Crataegus spp.), mountain ash (Sorbus spp.), firethorn (Pyracantha spp.), loquat (Eriobotrya japonica). Different but closely related Venturia species cause scab on European and Japanese pear, cotoneaster species as well as common pear (Pyrus).
Where does apple scab come from?
The fungus Venturia inaequalis overwinters on fallen infected leaves. May overwinter in the buds in milder climates. Ascospores formed in early spring are released and carried by the wind, causing primary infections in the rainy season. Asexual conidial spores cause secondary infections throughout the spring and summer. Ideal temperatures for apple scab infection is 65-75°F with high humidity. Once infected, trees can continually re infect themselves and neighboring trees.
How do I prevent and control apple scab?
- Plant resistant varieties
- Prune for better air movement and sunlight penetration for drying /remove water sprouts
- Remove of fallen leaves and orchard sanitation in the winter
- Spray 5% solution of urea in the autumn to aid in the breakdown of leaf tissue (42 lb urea in 100 gallons of water). Since nitrogen is being applied in the orchard, the fertilizer program needs to be adjusted accordingly.
- Shred leaf litter using a flail mower or remove leaf litter by raking, sweeping, or vacuuming are additional options.
- Use fungicides as appropriate
Short animated disease fact sheet: ‘Apple Scab; A Fungal Disease of Apples ’.
For more information on Apple Scab:
American Phytopathological Society (APS) (Gauthier, Nicole. 2018)