The leaves on several of my hostas have turned yellow, then brown, and finally collapsed onto the ground. What is happe

The hostas may have petiole rot.  Petiole rot is a serious disease of hosta.  It is caused by the fungus Sclerotium rolfsii var. delphinii.  (Petiole rot can also affect daylily, peony, phlox, columbine, and other perennials.)  Disease symptoms on hosta start as marginal yellowing and browning of the lower leaves.  A soft, brown, mushy decay may be seen at the base of the petiole sometimes accompanied by white fungal threads.  As the disease progresses, the leaves discolor and wilt.  In the final stages of the disease, the bases of the petioles rot away and the leaves collapse and lay flat on the ground.  Collapsed leaves pull easily from the crown of the plant. 

The disease fungus produces small, round fungal fruiting structures about the size of mustard seeds that appear at the base of the infected petioles.  These structures, called sclerotia, are a cream color when young, but gradually turn to a dark, brick red color.  The sclerotia are long-term survival structures of the fungus.  Sclerotia germinate during warm, humid weather and fungal mycelium grows in the ground until it finds a susceptible nearby plant. 

Effective management requires a combination of strategies.  Before planting hostas in the garden, carefully inspect each plant.  Do not plant any suspect or symptomatic hostas. Remove and destroy infected plants.  Also, carefully remove the soil in the area to a depth of 6 to 8 inches and replace it with non-contaminated soil.  Avoid moving plants from contaminated beds to other parts of the garden.  A mulch-free zone of several inches at the base of susceptible plants might also help.  Common garden fungicides available at garden centers and other retailers are not effective.