Hundreds of interesting hostas are available to land-scapers; and this easy-to-grow plant thrives in shade. It is no wonder it has become such a significant factor in landscape designs. Only a few diseases bother hosta, but one or two of them can be quite damaging. It is important to identify diseases early to take quick ac-tion for preventing spread to valuable plants. Disease symptoms are starting to show now.
Anthracnose is a fungal leaf disease of hosta that has been prevalent for the last several years. Infection can result in loss of aesthetics, but the plant is not killed. 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. Little information is available about disease management, but a fungicide effective against leaf spots and having a general ornamental label should provide protection of new growth.
Fungicides are recommended on sites where this disease has been a problem. The thiophanate methyl fungicides are probably a good starting point if you are looking for fungicide help. Refer to the Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide. Read the product label to be certain it is regis-tered for your crop and there are no toxicity warnings.
Sclerotium blight has become a serious disease of hostas in isolated situations. Initially, lower leaves wilt and brown. In a short time, upper leaves also wilt; 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 sclerotia (fungal structures) the size of mustard seeds 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 ex-change and popularity of hostas. It was thought that the fungus would not overwinter in our cold climate, but that too is false. The fungus overwinters when protected under mulch and snow in mild winters. Research at Iowa State University is investigating resistant cultivars. Differences in susceptibility have been found, but nothing with high levels of resistance.
Carefully inspect any hostas 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 mulchfree area near the crown.
Once the fungus has invaded your planting bed, do not move soil or plants from that area to other beds. You can plant nonhost species in this area, but wash tools of soil before working other beds. Fungicides may be used to suppress Sclerotium rolfsii, but they will not eradicate it. PCNB, available as Terraclor, Defend, Pennstar, and Revere, is labeled for ornamentals and is available to commercial growers. It may cause some plant injury, so it is advised to treat only small areas of the bed to check your plants' reaction. A newer fungicide, flutolanil (Contrast), is another option. PCNB is not available to home growers. Prevention is still our best recommendation.
Foliar nematodes on hosta are also fairly new here and have potential to be very harmful. We are not see-ing a big problem, but the possibility is real; it will likely be shipped in from warmer climates. Nematodes are microscopic roundworms that cause disease--pathogens much like a fungus or bacterium but requiring moisture to infect; they live within the plant. 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, especially where protected by mulch. You cannot see the nematode with the naked eye, so watch for brown areas between veins. Foliar nematodes may occur on other perennial hosts, including anemone, creeping phlox, ground ivy, windflower, and heuchera. 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. It is questionable whether this nematode can survive the cold winters of Illinois, but it is possible that overwintering may occur in mild winters with protected locations that are mulched. The past winter may have provided those conditions.