Alternaria leaf and stem blight of Ginseng

Alternaria panax

Cause. The fungus Alternaria panax, which is specific to ginseng (this is not the same fungus that infects potatoes or tomatoes). The fungus was found in Oregon and Washington for the first time in 2000, although it had been in British Columbia for many years. This disease affects both seedlings and established plants. The fungus will be a problem anywhere temperatures are moderate and relative humidity is at or above 95% in the canopy. The disease is worse in beds that are thickly planted and subject to prolonged moisture retention.
The fungus survives the winter on infected stems or leaves in the ginseng bed. Infection may first appear in the spring on young shoots shortly after they emerge from the soil or mulch. Wind, water splash, tools, or people moving through the beds can disperse the spores from stem or leaf lesions. During damp weather or high humidity, these spores will land on plant surfaces and initiate new infections. Sporulation, dissemination, and infection can occur several times within a single growing season.
Symptoms. The fungus causes an elongated brown lesion on the stem, which becomes velvety with spores as the disease progresses. Stems may become girdled by the lesion, which causes the top of the plant to wilt or topple over. Leaves may also become infected. Leaf lesions begin as watersoaked or oily looking spots or, depending on the environmental conditions, yellow spots that later enlarge and dry out in the center. Leaf spots are usually noticed when the centers of the lesions are tan or straw color and are bordered by a thin brown margin, which is surrounded by yellow tissue. Roots are not normally affected.

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