Apple Scab

Hosts

Apple, crabapple, hawthorn, pyracantha, mountain-ash

Biology

Apple scab is caused by the fungus Venturia inaequalis.  It is an infrequent disease of apple in the Intermountain West, because it requires humid, cool weather during the spring months.  The fungus overwinters as fruiting bodies within infected leaves and fruit on the orchard floor. 

The table at right shows the amount of moisture needed at various temperatures for infection to occur.  After the first infection, later infections occur within 9 to 17 days, and these are the main means of disease spread during the summer months.

Symptoms

The most obvious symptoms occur on the leaves and fruit.  Leaf infections consist of velvety brown to olive green spots that look like dark mold on the leaf surface.  Infected leaves become deformed over time. 

Fruit infections also begin as velvety brown to olive green spots that become brown, corky, and cracked.  Late season infections may produce very small spots (known as pin-point scab) that may not become visible until the fruit is in storage.

Monitoring

Infections require different lengths of time of continuous leaf wetness for success.  The effective wetness periods depend on temperature.  At 61-75°F, only 6 hours are needed, while at 48°F, 12 hours are required. 

Post-infection (curative) fungicide sprays need to be applied within 24 - 96 hours. following infection in order to be effective. 

Management

Some newer cultivars are more resistant to apple scab; the best of these at present include Liberty and Prima.  Sanitation helps minimize future infections.  This includes raking and burning leaves, disking (in clean cultivated orchards), or applying nitrogen (e.g., as urea) to accelerate the rotting process of the fallen leaves in areas where winter temperatures are mild and moisture is enough to at least partially rot the leaves.  Often, however, these approaches are impractical in commercial orchards, and fungicide sprays are needed.

Correct timing of sprays is essential for good control.  The period between the start of bud growth and when the young apples are 12 mm in diameter is the most critical.  Protective materials are applied as soon as susceptible tissue is exposed in the spring, and every 7-10 days throughout the season if conditions warrant  The post-infection approach requires accurate monitoring of orchard temperatures and the length of time the leaves remain wet.