The most likely problem is iris borer, the worst pest of bearded iris. The borers are larvae of moths that lay eggs on dead iris foliage in autumn. The eggs hatch in spring, and the borers briefly feed on the leaves before heading to the roots. To get the problem under control, dig up your iris this summer, cut off all the rotted rhizomes, and put the rotted material in the garbage. Soak the good pieces in a mixture of 1/2 cup bleach per gallon of water, then replant them in fresh soil. In late fall, gather up the dead leaves and rake the bed as clean as you can. Next spring, at about the time forsythias and tulips bloom, check your iris for ragged leaves or dark streaks in the foliage. If you see either condition, treat the infested plants with insecticide, or douse them with beneficial nematodes. Although adding nematodes to a garden may seem strange, studies show that beneficial nematodes are more effective for controlling iris borers than insecticides. The nematodes enter the borers bodies and kill them, and continue to work for 4-5 weeks. Soft, mushy rhizomes in iris may also be caused by bacterial soft rot. You can easily distinguish bacterial soft rot from iris borer because plants affected by soft rot will smell bad. Dig up and destroy affected areas of the plant. Replant healthy portions in a new area.