This could be any one or a combination of leaf spot diseases. Older varieties of crabapples are susceptible to many leaf spot and scab problems. Leaf spot diseases like this cause the leaves to release ethylene, which causes even healthy leaves to drop. To prevent this, you would have to spray the trees as if you were managing ornamental apple trees. You could follow the Virginia Tech home fruit spray schedule. This would require repeated spraying, starting in early spring before symptoms even appear. There is nothing curative. Another option would be to work with a certified arborist. Many of the major arborist companies have labs to test for tree pathogens and they can provide a plan for managing the trees (or removing/replacing ones that are not treatable). You can find a certified arborist near you using this website from the arborist association. http://www.treesaregood.org/

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