Use of Environmental Factors in Screening for Disease Resistance

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Abstract
In the case of resistance to Fusarium vascular diseases, control of the temperature of the growing medium is most important. Wounding of the tissue at the site of invasion when exposed to the inoculum usually improves the assay. In the case of root-rotting fungi, other factors are also important, among which are seed quality, level of inoculum, depth of planting, and moisture content of the growing medium. When the pathogen or virus concerned is an aerial invader, control of temperature again is generally most important. When resistance is mono-genic, environmental conditions, which are much more extreme and applied at much earlier stages of host growth than those which occur in nature, may enhance the rapidity of the assay. When multigenic resistance is involved, suboptlmum conditions of temperature and inoculum level may be desirable. In the use of controlled environment for the assay of plant populations for resistance to a given causal a-gent, several advantages may be gained. Comparable results from time to time and from progeny to progeny may be assured. The time required to complete the assay may be greatly shortened. When numerous pathogenic races exist, a distinction between their host-parasite interactions may be made. When 2 genetic types of resistance occur in the host, they may sometimes be separated by manipulation of the environment. In other cases, shift in environment may result in a useful change in degree of penetrance or in a complete reversal of dominance. There is a bibliography of 72 references.