Extension Educator, Horticulture
In horticulture there are some sure things. Compost happens. Spring will get here, eventually. The rabbits will find the $25 tulips before they find the 25-cent tulips. And some crabapple trees will flower beautifully in the spring, but will be barren of leaves by August.
Unfortunately some crabapple and apple trees are susceptible to a fungal disease called apple scab. Symptoms of apple scab usually start on the undersides of leaves. Spots, at first, are small, irregular lesions that are light brown to olive green in color. As infection continues, lesions become more circular and velvety olive green to black in color. Leaves may curl and scorch at the margins. Leaves usually turn yellow and fall off in mid-summer. If the fruit stems become infected, fruits may drop early. The apple fruits may develop scabby lesions.
Infections occur during moist conditions (rain, dew or constant irrigation). The temperature affects the severity of infections. In order for infection to occur in cool weather, the plants must remain wet relatively longer than in warm weather.
With apple scab management you have three options:
With existing crabapple trees you can do nothing and let the tree defoliate each summer. Apple scab is generally not life threatening for the plant, but certainly lessens its ornamental appeal unless you are really into bare branches. As with other diseases, try to keep plants healthy by watering during drought and fertilizing periodically. Following good sanitation practices may help. Remove and destroy infected leaves, flowers, and fruit as soon as possible.
Second option is a fungicide program. There are several fungicides labeled for apple scab control such as captan and chlorothalanil, often sold as Daconil. Be sure to read and follow all label directions and precautions.
The battle against scab is won or lost during late April through early June (from bud break to fruit set). Begin fungicide spraying as leaves develop and continue according to label intervals until frequent wetting by rain has lessened, usually by July 1. If some spray intervals are missed, apple scab would still be lessened but complete control may be lost.
Remember fungicide sprays are predominantly protectants against infection so new leaves have to be sprayed before infection occurs. Thorough and uniform covering of all leaves and developing fruits is required for control.
In addition fungicide sprays would have to be applied every year to protect the tree. Remember once leaves start to yellow and fall off the tree that it is too late to spray fungicide for control during the current growing season.
The third option would be to prune horizontally at the soil line (cut the tree down). The best way to avoid apple scab is to select scab resistant varieties of crabapples. Unfortunately many of the older cultivars such as 'Hopa', 'Almey' and 'Eleyi' are susceptible to diseases.
There are many beautiful crabapple cultivars that are resistant to apple scab as well as powdery mildew and fireblight. One of my favorites is 'Snowdrift.' The red flower buds open into single white flowers which cover the tree from late April into early May. The flowers are followed by small orange-red fruits that persist into winter until the cardinals snack on them. Any fruits left in spring quickly get snatched up by the robins. 'Snowdrift' shows good resistance to apple scab.
'Prairiefire' is an introduction from the University of Illinois that reaches 15 to 20 feet tall. The dark red flowers, shiny red bark, persistent red fruit and disease resistance make 'Prairiefire' a beautiful addition to the landscape.
For more information on crabapple selection, check with your U of I Extension office for the Horticulture Fact Sheet LH-3-80 "Crabapple trees recommended for landscape use in Illinois" and the Report on Plant Disease No. 803 which can also be accessed at www.ag.uiuc.edu/~vista/rpd.html