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The Homeowners Column
Time to manage apple scab
State Master Gardener Coordinator
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 certain crabapple trees flower stunningly 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 usually start on the undersides of leaves. Spots start as 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 or dew) in spring. 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. Fungicides labeled for apple scab control on crabapples include: chlorothalanil (sold as Daconil) 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 (typically every 7-10 days) 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 have to be applied every year to protect the tree and 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 such as 'Adirondack', 'Beverly', 'Dolgo', 'Donald Wyman', 'Sargent' and 'Tina'. Unfortunately many of the older cultivars such as 'Hopa', 'Almey' and 'Eleyi' are susceptible to diseases. Many beautiful crabapple cultivars are resistant to apple scab as well as powdery mildew and fireblight so do your homework before purchasing crabapples.
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 crabapples. http://www.mortonarb.org/trees-plants/tree-plant-descriptions/crabapple-cultivars For more information on apple scab, UI Report on Plant Disease No. 803 http://web.aces.uiuc.edu/vista/803.pdf
Not all fungi are bad. Join Master Gardeners for "Fungus Among Us" presented by University of Illinois Extension Educator, Diane Plewa at two locations and dates: Tuesday April 8th at 6:30 PM at Crossroads Christian Church, 3613 N. Vermilion St. in Danville and repeated Tuesday April 15 at 7:00 PM at UI Extension auditorium 801 North Country Fair Drive in Champaign.
Discover different kinds of fungi, some historical uses of fungi and how we interact with them every day without even knowing it. Also enjoy samples featuring different fungal foods.
Please register at http://web.extension.illinois.edu/cfiv/ or phone UI Extension in Champaign PH: 217.333.7672 or Danville PH: 217.442.8615.