Answer: You're describing the symptoms of crown rot. This fungal disease, caused by Sclerotium rolfsii var. delphinii, can severely damage hostas within a week. Once established in an area, S. rolfsii var. delphinii is difficult to eradicate.
Symptoms begin to appear on hostas after prolonged hot, humid weather. The lower leaves turn yellow, then brown, and wilt from the margins back toward the base. The upper leaves may soon collapse, as well. Wilted leaves can be easily pulled from the crown, because they have been infected at the base of the petiole. The bases of these damaged petioles show a brown discoloration and mushy texture. With plants with less succulent stems, such as peony, stem bases may be girdled and then the leaves will become discolored and wilted, but the stems may not collapse. Ropy-textured white threads (mycelium) of the petiole rot fungus typically are present on the rotted tissue and surrounding soil.
There are no chemical controls for this problem so you may need to dig out the affected plants. Wish I had better new for you.
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