Apple scab, caused by the fungus Venturia inaequalis, is not considered a serious threat to crabapple or apple trees; however, repeated annual infections can weaken the tree making it more susceptible to other insect or disease problems. Apple scab is more of an aesthetic issue because of the early defoliation and blemishing to the leaves and fruit that it causes. Heavy infections can reduce growth and apple yields. Apple scab is dependent on cool, wet weather. If spring weather is dry, scab may not be a problem.
The need for fungicides depends on the susceptibility of the cultivar and weather conditions. Since apple scab is dependent on wet weather, fungicide applications may be needed for control during wet seasons. Proper sanitation, pruning, and watering may suffice during dry seasons. Fungicide applications are preventive only and need to be timed properly for effective control.
Other Treatment Practices
- Avoid wetting the foliage with irrigation during the growing season.
- Rake and remove fallen leaves from yard in autumn to reduce spores.
- Each winter, prune suckers and branches throughout the crown to improve air circulation.
- Space trees appropriately to allow air flow.
- Applications of Cambistat can increase tree resistance to pathogens.
Spraying for apple scab is difficult. If timing is incorrect, or if rain washes away fungicide applications control will be lower. In perfect conditions control may be aesthetically pleasing, but the disease will always persist. The focus must be on suppression, not eradication.
Signs of Damage
- Brown to olive irregular-shaped spots on the leaves in late spring
- Spots become dark and velvety in appearance
- Heavily infected leaves may become yellow and result in premature defoliation from late spring through late summer
- Fruit may also be infected and display similar symptoms to those on leaves
- The velvety appearance of the leaf spots is the result of spore production on the leaf surface
Photo: Microscope image of an apple scab lesion
Trees At Risk
- Malus species
- The pathogen overwinters in infected leaves on the ground from the previous year.
- In spring, overwintered spores (ascospores) mature and are discharged over a period of 5 to 9 weeks.
- Wind and splashing rain carry spores from infected leaves to new growth on nearby trees where new infections begin.
- In late summer or early fall the primary infections produce secondary spores and create new infections, which can continue through the growing season during wet periods.
- Infections occur most rapidly between 55 and 75 degrees when leaves or fruit remain wet for a continuous 9 hours.
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.