Apple Scab Management - Solution Guide
Apple scab is one of the most common diseases of apple and crabapple trees. It is a fungal disease caused by Ventura inaequalis. In edible apple trees, apple scab can lead to repeated foliage loss, fruit drops, and deformed, unusable fruits.
NOTE: Pears can also be susceptible to a similar fungal scab disease caused by Ventura pirina. Although pear scab cannot cause infections on apples or vice versa, the symptoms and prevention methods for these fungal diseases are similar.
Trees Affected
This fungal pathogen can infect crabapple, Hawthorn varieties, and Sorbus spp. (Mtn. Ash), but apple scab is a disease of great concern to apple fruit growers since it can affect fruit quality and potential yields.
Symptoms
Heavily infected fruits become misshaped, lumpy and may also crack or drop. Initial infection symptoms appear on the newest leaves around flower buds as small lesions or spots are olive-green. The underside of leaves shows gray, velvety spots or scars, with severe infections causing puckered, stunted, or twisted leaves. Eventually, the leaves turn yellow and drop.
Early infections can occur on the blossom end of fruits or the stems of the fruit. More severe infections typically lead to fruit drops. Scab infections are cumulative, with the first season's symptoms being less noticeable or severe, but subsequent seasonal symptoms become more severe.
Infection Occurrence
Fungal spores are discharged into the air during spring rains or via overhead irrigation, causing water to splash with spores that are carried to develop flower buds. Infections occur most commonly in temperatures between 60°-75°F while the leaves are wet. However, infections can occur in temperatures as low as 40°F. Peak infections occur when buds show their first color through to petal fall. However, gardeners need to know that secondary infections can happen later in the season from new spores that germinate on wet leaf surfaces and then cause new lesions that can infect harvested fruits without detection, leading to misshapen fruits while they are in storage.
Managing Apple Scab - 4 Important Steps to Prevention
- Sanitation: Spores survive and spread from dropped leaves, stems, and fruit litter. Spores can travel but not too far. If your neighbors have apples or other host trees, talk to them about the importance of sanitation. Remove all leaf debris by raking old leaves to put into the trash or burn them. Keep potentially disease-infected leaf debris out of the compost pile since infecting spores do not die in compost piles.
- Apply fungicides early: Fungicides are preventative, not curative. Fungicides are most effective for areas that experience wet or rainy weather. Multiple applications and timing of each is essential. Start from bud emergence, spray again 3-4 weeks after petal drop. Again, one more application during the growing season may also benefit severe infections in wet areas.
- Rotate fungicide products: Utilize non-resistance forming bio-fungicides like Revitalize® by Bonide® with Bacillus strain D747 or mineral-based fungicides like Sulfur. Follow up later in the season with a synthetic, broad-spectrum product containing active ingredient chlorothalonil such as Fung-onil® by Bonide®.
- Plant resistant cultivars: 'Freedom', 'Liberty', 'Enterprise', 'Duchess of Oldenburg', 'Golden Sentinel™ Columnar', 'Northpole™ Columnar', 'Scarlet Sentinel™ Columnar', 'Honeygold', 'Wealthy', 'White Icicle™ Columnar', and 'Wolf River'. Crabapples - 'Whitney', 'Prairiefire' and 'Royal Raindrops®'.
Cumulative fungal diseases can take over a year to mitigate effects or symptoms successfully. Be patient but diligent to achieve desired results and protect your apple trees.
