Flowering Crabapple - Leaf Apple Scab (Venturia inaequalis)
Does you Crab Apple flower beautifully every spring and then by mid to late summer , the tree has lost half its leaves and the rest look awful. If that's the case you likely have Apple Scab. This is a very common disease and is caused by a fungus called Venturia inaequalis. The fungus slowly infects the leaves over the spring and summer and if it infects enough of the leaf, the tree will drop the leaf. This disease is almost never fatal to the tree but the tree can look so stressed , many landowners would wish the tree would die.
Apple Scab damage visible on a flowering crab leaves (September, 2022 - Union, Ontario)
The fungus emergence is very much tied to rainfall and temperature. Dry , cool springs can limit how much it shows up later that year. Frequent late spring rain and moderate temperature means you are looking at a apple scab year. This is one fungus that can be effectively controlled by an arborist/landowner\applied fungicide such as non lime sulfur however, multiple applications may be required (often after every solid spring rain) and it may be tough for the arborist to get there enough for good control. Raking and removing fallen leaves does help but doesn't guarantee good control. If you are a Crab apple nut, its best to purchase a resistant variety. resistant varieties can easily be found on the web and most nurseries carry at least one resistant variety.
Less than perfect canopy foliage on a heavily infected crab apple . August , 2022. Union, Ontario