Apple Scab
Description
Apple scab is a common landscape disease that infects trees like Crab Apples and apple trees, know by the genus Malus. The disease is actually a fungus (venturia inaequalis) that show up as a dull black or grayish-brown lesion on the trees leaves and fruit. The underside of the leaves is the most common place to find Apple Scab lesions on a tree, although, sometimes, but not often, the lesions can also be found on the woody parts of the tree. Apple Scab doesn’t usually kill the infected tree, but it can cause serious damage and stunt the growth of the infected tree. What most people recognize as Apple Scab disease is the curling or scorching of the leaves (yellowing) and the early leaf drop that leaves an almost-bare tree.
Life cycle
The cycle of infection starts in the springtime, when warmer temperatures and spring moisture promote the release of the disease ascospores from leaves that litter around the base of previously infected trees. These newly activated spores rise into the air, blow around, and land on the surface of a susceptible tree, where they germinate, infect the tree, and start the cycle all over again.
Control
There are a variety of chemicals that can offer control on the sometimes annual disease. Some chemicals have been effective for years, and others are being less used due to resistance build up issues, such as Benzimidazole fungicides. There are also contact fungicides that work well for controlling Apple Scab, and are not prone to resistance issues. Other treatments such as copper mixtures are less effective, and can cause russeting on the fruit.
Apple scab is best controlled from late April through early June, which in Illinois and Indiana is the time from bud break to fruit setting. This is the time of year that the disease is airborne and can infect your trees. During this early time of year, Suburban Landscaping should be spraying your Crabapple trees for apple scab disease. Depending upon many factors, generally one to four applications are required to protect your trees.
What Does Apple Scab Treatment Cost?
Generally speaking, $30 to $90, depending upon how many trees you have. Most condominiums pay around the $30-$40 mark, while homeowners with one or two trees may pay around $80 - $90 per tree.
When Do You Apply Treatment For Apple Scab?
The fungicide treatment should start at the first sign of bud break, which is usually late April. Repeated applications should come every 7-10 days, if required.
Can I Treat Apple Scab In The Summer?
Not usually, but there are some new fungicides that show some promise. Treatment after the leaves have fallen off of the tree is usually too late.
Homeowner Tips:
Rake up and burn the fallen leaves from your infected crab apple every fall. This will prevent a reoccurrence of the disease from the same leaves. Also, dormant prune your crab apple trees in the winter to allow an open and airy canopy that allows leaves to dry and chemical treatments to penetrate.
Are All Crab Apples Susceptible to Apple Scab?
No. Below is a list of the Malus cultivars that are resistant to Apple Scab disease.
High Resistance:
- Bob White
- Louisa
- Prairie Maid
- Prairifire
- Redbud
- Red Jewel
- Sargent
- Silver Moon
- Sugar Tyme
- White Angel
Medium Resistance:
- Centurion
- David
- Donald Wyman
- Doubloons
- Harvest Gold
- Jewelberry
- Mary Potter
- Sentinel
Stay away from the following varieties that have a high susceptibility to Apple Scab disease.
- Adams
- Brandywine
- Indian Magic
- Indian Summer
- Profusion
- Robinson
- Snowdrift
- Velvet Pillar
