Fungicidal Management of Pathogen Populations ,

Fungicides are the only means of controlling disease epidemics in a susceptible crop when conditions favor a virulent fungal pathogen that produces air‐borne spores rapidly and abundantly. Most available fungicides are surface protectants that act externally on the host to prevent infection by the pathogen. A high percentage of the plant tissue must be protected because large amounts of inoculum are produced by small numbers of infections. Maintenance of a protective chemical deposit effective throughout the vulnerable period of crop development requires frequent fungicidal spray applications of relatively high dosages. In contrast, systemic fungicides now under development act within the host and control disease with one or two applications at low dosages. Many of these materials are highly selective; they are effective against specific groups of pathogens. Some systemic fungicides suppress epidemics of rust and mildew diseases when applied as seed or soil treatments at planting. Such fungicides have great potential for preventing severe disease epidemics in field crops threatened by virulent, aggressive fungal pathogens.