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“Hosta” la Vista?

June 20, 2001

Hasta la vista literally means “until later.” This play on words was suggested by a Plant Clinic visitor who escaped before we could get his name. Hostas generally grow wonderfully in Illinois if you provide shade and water. Until a few years ago, it was rare to see a hosta at the clinic. I assumed the species did not have many problems in Illinois. I still put this plant high on my list of plants with few problems; but problems do occur, and the major ones are discussed here.

Anthracnose is a fungal leaf disease of hosta that has been prevalent for 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 contributes to their aesthetic decline. Little 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. The thiophanate methyl fungicides would probably be a good starting point. Refer to the Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide for chemicals. Always read the label to be certain a treatment is registered for your crop and there are no toxicity warnings.

Sclerotium blight is a serious disease of isolated hostas. Initially, lower leaves wilt and brown. Shortly, the upper leaves 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 South but not here. 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 climate, but that is false. The fungus overwinters when protected under mulch and snow in mild winters.

Current research at Iowa State University is inves-tigating possibile resistance. Susceptibility varies, but nothing with high levels of resistance has been found.

Carefully inspect hostas planted into your gardens. Do not plant any with disease symptoms. This applies to other perennials as well because this organism can also infect ajuga, anemone, daylily, impatiens, peony, vinca, and others. 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.

Once the fungus invades your planting bed, do not move the soil or plants 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 do not eradicate it. PCNB sold as Terraclor, Defend, Pennstar, Revere, and PCNB is labeled for ornamentals and is available to commercial growers. It may cause some phytotoxicity, so treat only small areas 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 relatively new to our area. We do not see a big problem here yet, 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 a striped appearance. Foliar nematodes may occur on other perennials, including anemone, creeping phlox, ground ivy, windflower, and heuchera.

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, mulched locations.

Author: Nancy Pataky


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