We have two large crabapple trees whose leaves have become yellow with brown spots, wilty and with the occasional hole in the leaf. The leaves are falling off the tree in droves and the bark of the tree has the appearance of coming off. Last year, we had lovely crabapples at this time and this year it seems the tree is dying. What might be wrong and how do we treat it? We abhor losing the tree as it is a major focal point and shade source for our backyard landscape.
From what you are describing, it sounds like you have a fungal disease on your crabapple tree. It is likely Apple Scab which is very common after a wet humid spring like we have had. It begins with olive green spots that turn black and midsummer leaves turn yellow and drop off. The fungus overwinters in infected leaves, dead bark or fruit on the ground. The spores from the fungus then start developing in late winter or early spring and are released into the air by rain, wind or insects. It then germinates on the leaves when the surface is wet/humid and begins creating the spots. Heavy infections cause the leaves to turn yellow and drop off at this time of year. If you have any cankers in your tree bark, then it is possible you have a black rot fungus which causes leaves and fruit to have brown spots with concentric rings, turn yellow and drop off. All the fungal diseases that affect the tree overwinter in the same dead tissue so it is advisable to always clean up any leaves, dead branches or fruit in the fall as a usual practice and to do the same thing right now. Put all the waste material in the garbage and do not compost any of it. Prune off any dead wood from the tree or any cankers on the branches if there are any. Remove all infected fruit in the tree as well and you could purchase a sulphur based fungicide to spray the tree with. It will probably be susceptible to getting the same infection next year so spraying should begin when the buds break all the way until 3-4 weeks after the flower petals fall off. Leaves and flowers must be covered with the spray. If it rains, then the fungicide must be re-applied. The tree is definately salvageable but it will take a bit of maintainance. Best of luck in tackling the disease!