Resistant cultivars — Planting disease-resistant cultivars is one of the most effective ways to manage apple scab in the landscape. Disease-resistant cultivars are less susceptible to apple scab and, therefore, do not require chemical control to prevent infection. Several resistant cultivars of crabapple are available (Table 1).
Cultural — Apple scab overwinters on infected leaves and fruit that have fallen off the tree. Raking and removal of this plant material prior to the first snowfall will eliminate locations where the fungus can survive, reducing the source of spores producing new infections the following spring.
Because dense foliage, nearby susceptible trees and protected sites favor development of apple scab, prune branches from the interior of trees to open the canopy and allow more air movement to promote an environment that reduces the risk of disease development.
Remove highly susceptible trees and avoid planting susceptible species or varieties in areas with multiple susceptible trees to reduce the likelihood of problems. Plant susceptible species in open sites (sunny, open to wind) and avoid directly wetting trees with lawn irrigation. Limit irrigation to the early morning.
Hardiness and suitability to a particular area are of greater importance than disease resistance when selecting an apple tree for North Dakota. Even resistant trees that are stressed can be susceptible to severe infection.
Chemical — Protectant fungicides, including chlorothalonil, propiconazole, thiophanate-methyl, captan, myclobutanil and mancozeb, can be used on susceptible cultivars during periods of prolonged wet weather. To control this disease effectively, you first must apply fungicide at the ½-inch green tip growth stage, when leaf buds are swollen and expanded with approximately ½ inch of visible new leaf tissue.
Fungicide application should continue at regular intervals according to the label directions. Wherever pesticides are selected as a management strategy, be sure to read, understand and follow the label instructions. It is the law.
Table 1. A list of flowering crabapple cultivars with resistance to apple scab and specific tree characteristics.
Cultivar/Trade Name
|
Flower Color
|
Fruit Color
|
Foliage Color
|
Tree Form
|
Height
(feet)
|
Width
(feet)
|
Zone
|
---|
Gladiator®*
|
Bright pink
|
Reddish-purple
|
Purple-bronze
|
Upright crown form
|
20
|
9
|
2
|
Purple Prince
|
Bright red
|
Maroon, ½ inch
|
Purple-bronze
|
Rounded
|
20
|
20
|
4
|
Emerald Spire®*
|
Pink
|
Sparse red fruit
|
Copper-green
|
Dwarf,
columnar habit
|
15
|
6
|
2
|
Marilee®*
|
Double, large white
|
Nearly fruitless
|
Medium green
|
Stiffly upright
|
24
|
10
|
4
|
Molten Lava®
|
White
|
Orange-red,
½ inch
|
Dark green
|
Horizontal weeper
|
15
|
12-15
|
4
|
Adirondack
|
White with pink edge
|
Red to orange-red, ½ inch
|
Dark green
|
Upright,
columnar
|
15-18
|
12-16
|
4
|
Donald Wyman
|
White
|
Red, 3/8 inch
|
Dark green
|
Upright in youth, rounded in maturity
|
20
|
25
|
4
|
Malus baccata ‘Jackii’
|
White
|
Purple or maroon-red, ½ inch
|
Dark green
|
Upright to rounded
|
30
|
|
3
|
Adams
|
Pink
|
Red, 5/8 inch
|
Green with reddish tint in spring
|
Dense, rounded
|
20-25
|
20
|
4
|
*Cultivars marked with an asterisk are newer releases and have not been evaluated for long-term disease resistance in North Dakota. |