Hostas have become popular landscape plants, so questions about disease problems have increased in recent years. Here is rundown of possible infectious problems.
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. 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. Not much information is available about disease management, but a fungicide effective against leaf spots and having a general ornamental label should protect 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 thiophanate methyl fungicides would 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 has no toxicity warnings.
Sclerotium blight has become a serious disease of hostas. Initially, the 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. The fungus, Sclerotium rolfsii, appears as a fluffy, white mass of mycelium on the petioles and surrounding soil. Tiny, tan, mustard seed--sized sclerotia (fungal structures) can be seen in this mycelium and on the soil.
This disease historically has 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 is false. The fungus overwinters when protected under mulch and snow in mild winters. Research at Iowa State University is investigating the possibility of resistant cultivars, finding differences in susceptibility but nothing yet with high resistance.
Carefully inspect any hostas that are planted into your gardens. Do not plant those with disease symptoms. This applies to other perennials as well because this organism can also infect ajuga, anemone, daylily, impatiens, peony, vinca, and other species. Because the fungus can overwinter under mulch on the plant stems, pull the mulch back from the crown a few inches and leave a mulch-free area near the crown.
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 fungi or bacteria; 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. Foliar nematodes may occur on other perennial hosts, including anemone, creeping phlox, ground ivy, windflower, and heuchera. The brown areas in the foliage may take on various shapes, usually limited by veins.
Disease management is not easy. Inspect new plants for symptoms, avoid close plantings, avoid excessively wet foliage, and discard contaminated stock. These nematodes can survive even the cold temperatures of Minnesota, so Illinois winters are not a threat.
A few “newer” problems have been found in the last few years in Illinois. Fusarium root and crown rot is a problem we have seen a few times. This disease causes leaf yellowing, stunting, rotting of roots, and death of crown tissues. In addition, several hosta viruses have been reported in scientific literature. These include hosta virus X, tomato ringspot, impatiens necrotic spot, and an unnamed virus in the tobra-virus group. The U of I Plant Clinic can positively identify all of the problems discussed except viruses. If symptoms suggest viruses, a private lab offering serological tests is suggested.