There are various viral diseases in Iris. Sometimes Irises may be infected with a virus but show no signs except slightly reduced vigor. But if stressed in other ways, such as wildly variable temperatures, the virus may express itself. Often this is seen as light streaks in the foliage or odd streaks of color in the flowers. Often plants may grow and flower with little sign of the virus load they carry. In nature old plants may die but produce seed that is virus free therefore carrying on the line. Since Iris are generally selected clones that are increased vegetatively this escape from virus is not an option for carrying on the clone. Since Viruses are often killed at a lower temperature than the plant tissue heat can sometimes restore a plant. But there is only a few degrees difference between virus death and tissue death. Today tissue culture can offer possibilities for restoring a plant to virus free status but usually is an impractical solution for Iris. Viruses are spread by sucking insects. Controlling the leafhoppers can help control virus. Most growers simply destroy plants that are showing obvious signs of virus so as the prevent further spread to other plants.
-- BobPries - 2014-06-06