SCORCH is a disease of unknown origin that makes an iris extremely sick. Generally the first apparent symptom is the brownish-yellow color of the center leaf in the fan. If this leaf resists being pulled out and the scorched appearance spreads to adjacent leaves, chances are that the plant has scorch. If the scorched plant is dug, the root system will appear to have disintegrated. Scorch occurs most often after mild winters in cold climates. It seems to be the result of stress that occurs when plants grow when they shouldn't.
There are few preventative measures and no known measure is fully certain. In cold climates, mulch to keep the soil around the roots frozen during winter thaws. Time the application of fertilizer so that it is available to the iris during its maximum growth periods rather than when the plant should be dormant.
-- Main.RPries - 2010-10-13