The frequent rains this spring have created ideal conditions for the development of apple scab on crabapples. Apple scab is a serious problem on susceptible crabapple varieties. Scab appears on leaves as roughly circular, velvety, olive-green spots on both the upper and lower leaf surfaces. The spots eventually turn dark-green to brown. Margins of these spots are feathery rather than distinct. Heavily infected leaves may curl up, become distorted in shape, turn yellow and fall off. Highly susceptible crabapple varieties may lose almost all their leaves by midsummer. The premature leaf drop may weaken the trees somewhat, but it will not kill them. The damage is mainly aesthetic. Heavily defoliated trees are unattractive.
Apple scab may be prevented by the applications of fungicides, such as captan, starting as soon as leaf growth appears until about the middle of June. A much easier way to deal with apple scab is to plant scab resistant varieties. There are numerous crabapples that are resistant to apple scab and need no spraying for disease control.
When selecting crabapples, gardeners may want to avoid the following varieties. All of these varieties are highly susceptible to apple scab. Heavy defoliation can be expected by midsummer after a rainy spring.
Crabapple Varieties Susceptible to Apple Scab Almey Hopa Red Silver Van Eseltine American Beauty Pink Perfection Royal Ruby Vanguard Barbara Ann Radiant Royalty Dorothea Red Jade Spring Snow
The crabapple varieties listed on the following page possess good to excellent resistance to apple scab. These varieties should require no spraying for disease control. Flowering and fruiting characteristics, tree shape, and size are also provided to assist gardeners when selecting crabapples for the landscape.
Crabapple Apple Varieties Resistant to Apple Scab Cultivar Flower color Fruit color, size, Tree shape1) Retention and size Adams Red to pink Dark red, Small, R Excellent 15' H 20' W Bob White White Yellow, Small, Excellent R 20' H 20' W Centurion Rose-red Glossy red, Small, Good Upr-V 25' H Christmas White Bright red, Small, Spr Holly Excellent 10-12' H Donald Wyman White Bright red, Small, R Excellent 15-20' H Harvest Gold White Gold/yellow, Small, Spr Excellent 20' H 20' W Liset Dark red Maroon-red, Small, Good R 15' H Madonna White Gold, Small, Good Upr 18-20' H 10-15' W Molten Lava White Red, Small, Excellent Spr-W 10' H 10-15' W Prairifire Purplish-red Dark red, Small, Upr-Spr Excellent 20' H Robinson Deep pink Dark red, Small, Good Upr-Spr 25' H Sentinel Pale pink Bright red, Small, Col-Upr Excellent 15-20' H Snowdrift White Orange-red, Small, R Excellent 20' H Sugar Tyme White Red, Medium, Excellent Pyr-Upr 25' H _________________________________________________________________ 1)Tree shape symbols: Col = Columnar; Pyr = Pyramidal; R = Round; Spr = Spreading; Upr = Upright; W = Weeping; V = Vase.
This article originally appeared in the June 5, 1991 issue, pp. 100-101.