Crabapple scab, a leaf fungal infection, is prevalent on older crabs and we see it especially on the reddish leafed crabs. It often causes significant defoliation in late summer. Sometimes the defoliation is so severe that homeowners have the tree removed because they think it is dead.
Some newer cultivars of crabapple trees are bred for fungal resistance, although we do see that resistance wear off in many cases.
In years with very cool, wet spring weather, the problem is worse.
Early spring sprays with proper fungicides and proper technique can result in significant reduction in the prevalence and severity of disease. The first spray should be applied as buds are swelling, with another spray just before the blossom and one after the blossoms fall.
When sprays are not feasible or have failed in the past, trunk injected fungicides can be used. They are applied in the fall, so the fungicide will be present in the tree in early spring. The best timing is just before leaf abscission, so that most of the fungicide stays in the tree, rather than moving to the leaves and falling with the leaves to the ground.
We have had good success using propiconazole injected in late September every two years.
Trees that have significant defoliation and that have not been previously injected warrant a somewhat more intensive regimen: injection of Phosphojet in mid-September followed by injection of propiconazole in late September, possibly supplemented with soil drench or bark spray of Agrifos.