The Sweet Wild crab (Malus coronaria, Zone 4) grows from southern Ontario down to Missouri, forming dense thickets that are late to come into bloom but, when they do, burst into a froth of white. Its common name is a misnomer: the green, slow-ripening fruit is never sweet, and even birds turn up their beaks until the apples are softened by frost. Selection from this species gave us early hybrids such as ‘Charlottae', a double-flowered form with good bronze fall colour that was discovered in 1902 and is still available. Like other double-flowered cultivars, it usually sets less fruit than single-flowered types.
The West Coast is home to the Oregon crab (M. fusca, Zone 7). It's native from the Aleutian Islands in Alaska, through British Columbia and into northern California. Pink or white flowers in spring give way to pale green, pleasantly scented, somewhat dry fruit that's good for making jellies. But its crowning glory is its orange and scarlet foliage in fall.
Many crab varieties that were popular in the past and are still available are highly susceptible to certain diseases. Here are some you should steer clear of: ‘Almey', ‘Hopa', ‘Klem's Improved Bechtel', ‘Makamik' (shown above), ‘Radiant', ‘Royalty', ‘Snowcloud' and Malus ionesis ‘Plena'.
Like regular apples, crabapples can suffer from several diseases, though they're seldom much of a problem. In my 27 years as curator of the Dominion Arboretum in Ottawa, there was rarely a serious outbreak among the dozens of crabs growing there. However, disease resistance depends on which cultivar you choose (see above).
For best results, plant disease-resistant varieties (see “The Best Crabs”), and keep an eye out for the following problems.
Cedar-apple rust A fungus that causes yellow spots on leaves but needs junipers or cedars nearby to complete its life cycle. Inspect conifers in early spring and prune off any orange galls.
Apple scab A fungus that causes dark spots on leaves and corky patches on fruit, but the birds will still eat them. Apple scab overwinters in bark crevices and on fallen leaves. Good sanitation and spraying with lime sulfur in late winter help control it. Severe attacks can weaken the tree over a few years and eventually kill it.
Fire blight A bacteria that, true to its name, makes a branch look like it's been burnt with a blow torch. It can spread rapidly, so remove infected wood as soon as you see symptoms; prune back to where there's no brown discoloration on the exposed end of the cutting. Be sure to disinfect pruning tools between cuts with a 50/50 bleach-water solution or undiluted rubbing alcohol to avoid reinfecting the wound.
Mildew Another fungus, it covers the leaves with a white coat; it's likely to occur in humid regions. Repeated attacks can defoliate and weaken the tree.