Plant pathologists have long been intrigued by the idea that plants like animals could be immunized to potential pathogens. One of the first critical reviews was by Chester (1933). At that time he could not find strong evidence for what he called acquired physiological immunity in plants. Since then many reports and reviews have been written (Hamilton, 1980; Goodman, 1980; Kuc, 1982a; 1982b; Matta, 1980; McIntyre, 1982; Ouchi, 1983; Sequeira, 1983; Suzuki, 1980). During recent years the pesticide industry has also become interested: could induced resistance have potential as a new method for controlling plant diseases?