Fortunately, there only a handful of fungal diseases that affect hostas. With the possible exception of Southern Blight, they are very rare in the home landscape.

Fungal diseases are by far the most common type of disease of landscape plants. The one thing they all have in common is the need for moisture in order to thrive, grow and reproduce. If the leaf or soil around the roots is kept relatively dry, the potential for fungal diseases will be limited.

Fungicides commonly used in the home landscape are preventative. This means that they must be in place before the fungal spores land. So, before even thinking about using a fungicide on your hostas, be sure to have a proper diagnosis from a university or other plant diagnostic laboratory.

Southern Blight a.k.a. Petiole Rot, Crown Rot


Southern blight of hosta also called white mold, petiole roty and crown rot is caused by the fungus Sclerotium rolfsii. In addition to hosta it is also an infection daylily, astilbe, peony, phlox, ajuga, delphinium and potato.

The disease commonly attacks the plant at or just below the soil line. The first symptom is a yellowing and wilting of the foliage. The fungus produces a large amount of cottony white, thread like material called mycelium, which can grow up the stems of plants and also spread out across the soil to infect other plants.

Although this is called Southern blight, it does occur in northern gardens too but is fairly uncommon. It is a difficult disease to control and, for the home hosta gardener, the key is sanitation. If you have this disease (diagnosed by a reputable plant lab), be especially careful in moving plants around the garden and transporting soil from one location to another on tools.

In Southern states, the process of solarization my kill the fungus in the soil. This involves covering the area with clear plastic sheets and letting the sun "cook" the soil. Unfortunately, this does not seem to work as well in northern areas.

If you have a serious problem that cannot be controlled by sanitation or solarization, you might need to hire a commercial pesticide applicator. Only they have access to the soil sterilants that are supposed to be effective on this disease. They are not available to the home grower.

Here is a great publication by Iowa State University on this subject. 
 

Fusarium Root and Crown Rot


Fusarium is a common fungal root rot organism that affects a wide variety of plants in the landscape and vegetable garden. It has been reported on hostas primarily in plant nurseries in the Southeast United States but rarely in home landscapes.

Since it attacks the roots of the plant, it ultimately leads to severe wilting and death of the plant. The crown may be covered with brown pockets and the roots will be brown or black instead of whitish as healthy roots appear. In less severe cases, the plant may be stunted, have low vigor and does not emerge well in the spring.

This is a soil borne fungus so, IF you have hostas that are diagnosed by a university or other plant laboratory, you should not replace the plant with others that are susceptible to it.
 

Anthracnose


This is a fungal leaf disease that is common to many types of plants in the landscape or garden from oak and maple trees to tomatoes. It is generally a disease of the early spring especially during cool, wet weather. Fortunately, it is not a fatal disease but does cause the plant to loose some vigor and may degrade the aesthetic value of the hosta.

Do not assume that brown or black blotches on the hosta leaves are anthracnose. Be sure to have it properly diagnosed at a university or other plant laboratory. IF it is anthracnose, they will prescribe possible treatment alternatives.

 

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