Horticulture

Apple Scab and Flowering Crabapples

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.


Prepared by Richard Jauron, Department of Horticulture
Iowa State University, Ames, Iowa.
This information subject to a usage policy.
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Last updated 12/31/97 by John VanDyk
http://www.ipm.iastate.edu/ipm/hortnews/1991/6-5-1991/scab.html