El Nino sure has given plant pathologists plenty of business. The fungal diseases that we see only in very wet seasons have been causing concern among gardeners, commercial growers, and consultants. In most years, hostas are a sure bet to give a good show in shady locations. This year we have seen tattered holes from slug feeding, edge burn from scorch or scald, and tattered leaves with brown spots as a result of anthracnose.
The anthracnose fungus that infects hosta is a Colletotrichum species. If you have a hand lens or microscope, look for the fruiting bodies, called acervuli. They have little black hairs, called setae, sticking out of the center of the structure. The wet or humid conditions we've had lately mean that you don't have to put the plant material in a humidity chamber to see these diagnostic structures. Look for irregularly shaped tan to brown spots on the leaves. The spots often lose their centers and leaves become tattered. The setae can be seen along the brown edges of the tattered spots.
There are some fungicides that can be used to protect healthy foliage. The copper fungicides are registered for this use. Be certain to read the label carefully to be certain that hosta is listed as a host that can be sprayed. (Often the concern is phytotoxicity to the host plant.) The anthracnose fungus can overwinter on the dead leaves, so be sure to remove dead leaves from the garden in the fall. Crowded plants provide ideal conditions for growth of fungi because of the high humidity in this microclimate. Water plants early in the day so that the leaves will dry more quickly. Also try to water the soil rather than the foliage.