Extension Educator, Horticulture
Some things are sure bets. 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 have barren branches in 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 light brown to olive green lesions. As infection continues, lesions become more circular and velvety olive green to black. Leaves may curl and scorch at the margins. By mid-summer leaves usually turn yellow and drop. If fruit stems become infected, fruits may drop early. 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, plants must remain wet relatively longer than in warm weather.
Three options are available for apple scab management on crabapples:
First option - 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 enjoy naked branches in August. As with other diseases, try to keep plants healthy by watering during drought. Good sanitation practices may help. Remove and destroy any fallen leaves, flowers, and fruit as soon as possible.
Second option is a fungicide program. Several fungicides labeled for apple scab control include: mancozeb, chlorothalanil (sold as Daconil), calcium polysulfides (sold as Hi-yield Lime Sulfur and Ortho Lime-Sulfur), copper sulfate or potassium bicarbonate (sold as Bonide Remedy). 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 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 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. Remove the tree and replace with scab resistant crabapple cultivars. 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 gobbled up by the robins.
'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.
Remember resistance doesn't mean complete immunity to disease. During some exceptionally wet years resistant varieties may get apple scab, but to a lesser degree than susceptible cultivars. The Morton Arboretum has a useful list of desirable crabapples. PH: 630-968-0074 http://www.mortonarb.org/res/CLINIC_Sel_CrabapppleHomeLandscape.pdf or stop by our office. For more information on apple scab, Report on Plant Disease No. 803 www.ag.uiuc.edu/~vista/rpd.html