It is sometimes called qualitative resistance because plants are either resistant or susceptible, without intermediate levels. It usually is effective against all races of a pathogen. This type of resistance is often called quantitative because there are intermediate levels ranging from resistant to susceptible. Often, polygenic resistance does not give a plant as high a level of resistance as major-gene resistance. Tolerance also describes the reaction of a plant to a disease.
Tolerance has two meanings.
It may refer to a moderate level of polygenic resistance or to the ability of a plant to maintain yield when the plant is diseased. This characteristic is a separate from resistance to infection. The type of resistance available depends on the specific disease. Most hybrids and varieties have some level of polygenic resistance to the common diseases. This resistance is usually adequate, although some yield loss may occur.