Hostas have recently shown some leaf necrosis. The Plant Clinic received one such sample from the Chicago area and has received reports of two from the central part of the state. Leaves show necrosis at margins or scattered in irregular blotches on the leaves. These symptoms are most likely caused by an environmental stress such as cold injury or possibly wind damage to tender foliage. Anthracnose would be a common secondary invader.
Anthracnose is a fungal leaf disease of hosta that has been prevalent for the last several years. The pathogen is a Colletotrichum species that thrives in warm, wet weather. We have seen it this year, so the warm spell of late has sufficed to allow this fungus to infect. Symptoms include large irregular spots with darker borders. The centers of spots often fall out and leaves become tattered and torn. I have not seen this disease kill plants, but it certainly has contributed to their aesthetic decline. There is not a great deal of information available about disease management, but a fungicide effective against leaf spots and having a general ornamental label should provide protection of new growth. Fungicides would be recommended on sites where this has been a problem, although I do not treat my plants that have anthracnose. The disease usually follows a stress. Often, plants exposed to sun and sunscald become infected with anthracnose. The thiophanate methyl fungicides would probably be a good starting point if you are looking for fungicide help. Read the label to be certain it is registered for your crop and to be certain there are no toxicity warnings.
Sclerotium blight has become a serious disease of hostas. Initially, lower leaves wilt and brown. In a short time, the upper leaves also wilt; and close inspection shows a soft, brown rot of the base of petioles. This disease is much different than anthracnose. In this case, the entire leaf collapses. The fungus, Sclerotium rolfsii, appears as a fluffy, white mass of mycelium on the petioles and surrounding soil. Tiny, mustardseed-sized, tan sclerotia (fungal structures) can be seen in this mycelium and on the soil. This disease has historically been a problem in the southern states, but not in Illinois. It has invaded our state, probably on transplants and with the open exchange and popularity of hostas. It was thought that the fungus would not overwinter in our cold climate, but that too has been shown to be false. The fungus does overwinter when protected under mulch and snow in mild winters. Pull mulch back from the base of plants before winter. Current research at Iowa State University is investigating the possibility of resistant hosta cultivars. There are differences in levels of susceptibility but nothing with high levels of resistance yet. Iowa State University has a very good publication on Sclerotium blight available at http://www.extension.iastate.edu/Publications/SUL8pdf. I suspect we will see this disease in June if warm temperatures and rains continue.
Foliar nematodes on hosta are also relatively new to our area. At this time, we are not seeing a big problem in Illinois but the possibility is real. Nematodes are microscopic roundworms that cause disease. They are pathogens much like a fungus or bacterium, but they require moisture to infect; and they live within the plant. The foliar nematodes are in the genus Aphelenchoides. On hosta, the nematode feeds in the leaf, producing brown areas between veins. It is thought to overwinter in the crown. You cannot see the nematode with the naked eye, so watch for brown areas between veins, giving the plant a striped appearance. The brown areas in the foliage may take on various shapes, usually limited by veins. This problem does not cross veins like anthracnose. Go to this site for some good photos of foliar nematodes on hosta: http://www.hostalibrary.org/diseases/nematodes.html. Foliar nematodes may occur on other perennial hosts, including anemone, creeping phlox, ground ivy, windflower, heuchera, and others.