Issue 17, September 28, 2009

Hosta Problems

Hostas are popular shade plants, easy to grow, and relatively disease free. Here are some problems you may see on your hostas and tips on what to do about it now.

If hostas are grown in full sun, especially in soil with low organic matter, they will develop yellow foliage with scorched margins. Under drought conditions leaves will become pale or dull and leaf margins scorched. This is not an infectious disease. Sometimes you will find anthracnose in these scorched areas. Watering the soil early in the day will help temporarily, but planting in a semi-shaded or shaded site in high organic matter soil is the best long range management.

The only disease that might be confused with environmental scorch is anthracnose. This disease is caused by a species of the Colletotrichum fungus. It thrives in warm, wet weather, so it may appear on irrigated hostas. I would not expect to see it in drought conditions where plants have not been irrigated. Symptoms include large irregular spots with darker borders. The centers of spots often fall out and leaves become tattered and torn. Fruiting bodies are small (pin-head sized) masses of clear spores with small, black, hair-like structures (setae) sticking out of the spore mass. Obviously the symptoms are a bit different than scorch symptoms. Scorch is not caused by a pathogen and does not have spores or fruiting bodies.

Fusarium root and crown rot is a problem we have seen a few times, especially in production areas. This disease causes leaf yellowing, stunting, rotting of roots, and death of crown tissues. It can be distinguished from scorch by inspecting roots. Plants with environmental scorch will have white roots or roots with white growing tips.

Sclerotium blight has become a serious disease of hostas because it is persistent and causes collapse of the foliage. 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. 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. Reseach at Iowa State University has shown that this fungus will survive over winter in the crown of mulched plants. If you see this disease, remove all plant debris and pull back the mulch before winter. An Iowa web site with very good images of this disease can be found at (Adobe PDF). That publication refers to the disease as a crown rot, but Iowa research has since shown the disease to rot petioles.

Foliar nematodes on hosta are also relatively new to our area. 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. On hosta, the nematode feeds in the leaf, causing brown areas between veins. The brown areas in the foliage may take on various shapes, usually limited by veins as seen in the image. Scorch may also occur between veins, but unlike foliar nematodes, it will also involve leaf edges and leaf tips.

Viruses that infect hostas cause mottling of the foliage, crinkling, stunting, and some necrosis. An article on hosta viruses can be found in 2008, issue 5 of this newsletter. --Nancy Pataky

Nancy Pataky

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