I am looking for a Malus (crabapple), not necessarily native, but is decorative in terms of blooms and foliage. I am also interested in plant diseases. I am hoping for a tree that will mature to about 20 feet with a 20 foot spread. Growing conditions are half shade, half sun, behind a semi-dense fence. We live in the San Juan Islands where the soil is not great and the tree will not get much water past establishment.
Here is what I found about the culture of flowering crabapples from the book Flowering Crabapples, the Genus Malus, by Fr. John L. Fiala (1994), p. 45:
“Crabapple trees luxuriate in full sunlight in deep rich soils that are well drained. Soils with a pH range of 5.0 to 7.5 suit crabapples well, but the ideal pH range is from 5.5 to 6.5. Even if gardeners are fortunate to have ideal soil conditions, they may not be able to allocate the best part of the garden to crabapples. Flowering crabapples, however, are not greedy and will accept almost any soil that is not waterlogged or overly dry. As long as the soil has a reasonable amount of nutrients and water, crabapples manage to do very well.
“Like most plants, crabapples prefer rich sandy loams, but even in heavier clay soils they do better than many other trees and shrubs and seem to bloom well once they are established. They will accept slightly wetter soils than lilacs, for example, but in these heavier soils they should have excellent drainage as they will not grow in waterlogged, swampy areas nor in soils inundated for long periods of time.”
Regarding particular trees you might like that would be disease-free, I found a couple of crabapples that were listed in The Pacific Northwest Gardener’s Book of Lists,by Ray and Jan McNeilan (1997). This is from page 24:
1. Malus ‘Prairiefire’ has red foliage when young that matures to deep green, has bright pink/red blossoms and deep purple-red fruit. It grows to 20 ft x 20 ft and has excellent resistance to scab and mildew (Pacific NW scourges).
2. From the book Flowering Crabapples, the Genus Malus, by Fr. John L. Fiala (1994), p. 147:
Malus sieboldii ‘Calocarpa’ (trade name, Redbud crabapple) is a dense, upright to spreading tree, 15 ft high and as wide… buds deep red, opening to single, white to pink-white flowers 1.4 in across; fruit 0.4 in diameter, bright red to red-orange… A reliable, abundant, annual bloomer… One of the most beautiful of all the ornamental crabapples both in bloom and in fruit. Birds relish the small fruit which never is messy.
From The Pacific Northwest Gardener’s Book of Lists, p. 25, I found that this tree is rated ‘excellent’ in terms of resistance to both mildew and scab.
3. Malus ‘Strawberry Parfait’ is a “vase-shaped, spreading tree 18 ft high and 20 ft wide; leaves red-purple, turning green with maturity; buds red, opening to single, pink flowers in clusters; fruit yellow with red blush, 0.4 inch in diameter. Excellent disease rating but not rated for fire blight [bacterial disease]. Not very ornamental.”
From The Pacific Northwest Gardener’s Book of Lists, p. 25, I found that this tree is rated ‘excellent’ in terms of resistance to both mildew and scab.
[Note: fire blight appears to be more the issue in the midwest and eastern U.S.]