With so few leaves left exhibiting symptoms, it's hard to diagnose at this point, but numerous diseases can infect apples and their crabapple kin, especially if that particular variety has low resistance. Our Apple and Pear diseases page lists some of the most common. Several crabapple cultivars have decent disease resistance, but none are immune or highly resistant to every possible disease they can get, so there is still some vulnerability. With dozens or hundreds of crabapple cultivars being grown, we can't tell which cultivar this tree may be, though it doesn't appear to be very resistant.

Apple Scab is our best guess as to the main cause for this tree's defoliation since these are typical symptoms and it's a common disease. No treatment is practical now, and if you want to use fungicides, you'll need to start applications early next year and continue them throughout most of the growing season. Be advised that fungicides might have negative side effects for pollinators if the tree is sprayed while in bloom, and we do not know if any residues will impact birds or other wildlife feeding on its fruits. A product with the active ingredient of myclobutanil is probably the best choice if you choose to spray. Getting good spray coverage of the tree's entire canopy, which is essential for good efficacy, may be difficult for a tree this size unless you hire a professional pesticide applicator who will have more application tools at their disposal.

Since fungicides are not foolproof measures and they can still fail to protect a plant if the weather is uncooperative, this approach may not be worth the expense or effort. If the tree has survived this long and still leafs-out well each spring, it may continue to do so for many years into the future, even if not treated. Repeated defoliations sometimes weaken a tree over time, depending on how early or late in the summer they occur, but some trees are more tolerant of this than others. The only other alternative to leaving the tree to its own devices is to replace the tree with a more resistant variety or a different species entirely. If you do, we suggest you ring the base of its trunk in mulch so the lawn does not grow right up to the bark, as injuries from mower or weed-whacker equipment risk causing dieback or allowing wood decay over time. (Don't mound mulch against the trunk itself, just cover a level area several feet out from the trunk with a flat two-inch layer or so to keep both weeds and the lawn at bay.)

The growths on the twigs and/or bark are lichen, which are harmless organisms; they don't take anything from the tree, nor do they cause infection. They are a special type of fungus (unrelated to the scab) that relies on algae living inside its body for photosynthesis, and can actually be a good sign of decent air quality since they are intolerant of too much pollution.

Miri