In April I wrote about hawthorns (Crataegus sp.), the good and the bad. The photo above is an example of the bad. Really, do you want this in your yard? If you have one of these unfortunate trees, is there any reasonable thing that you can do to keep it from defoliating? Not really.
Certain hawthorns, most notably ‘Paul’s Scarlet,’ are affected by a leaf-spotting fungus called Entomosporium mespili. This same fungus affects quite a list of other members of the apple family (apples, crabapples, pears, photinia, and 50+ more species). The spores overwinter in old leaf litter, on bud scales and twigs. You can remove the fallen leaves but that doesn’t help with the spores that are still on the tree so cultural controls don’t work. Chemical controls are problematical.
The WSU Extension Bulletin (EB1279) on this fungus says,
“Proper timing of spray applications is important. Infection can begin when new leaves emerge in spring and can continue as long as the weather is moist. During this period, new leaf tissue is being produced, and fungicides are weathered and washed off. Thus, several applications are usually necessary to provide adequate protection.”
There are few, if any, trees that I’d go to that kind of trouble for and certainly not a hawthorn. If it is too ugly, just take it out and put in something else, something resistant to E. mespili, just to be safe.
If you don’t have a hawthorn but wanted to buy one, avoid the English hawthorn, Crataegus laevigata, which is highly susceptible to the fungus. According the the Oregon State University Extension service, other species “appear resistant.”