Expression of disease resistance in Brassica-Hyaloperonospora host-patho system- A review

Genetic resistance is one of the most important attribute of the host defense against pathogens. host resistance provides an economical, environmentally benign, and widely accepted method of managing disease especially downy mildew in crucifers. Present day's emphasis on organic agriculture relies mainly on large scale deployment of disease resistant crop cultivars in addition to the use of techniques such as cultural practices (sowing time, soil solarization, crop rotation, use of organic manures, plant population, clean cultivation, soil and plant moisture regulation), and biological disease control. Hyaloperonospora-Brassica-interaction can be classified into two types, compatible and incompatible interaction, leading to the phenotypes of susceptibility, and resistance to certain pathogens. Obviously, the incompatible interaction has been extensively exploited by the Brassica breeders to develop resistant cultivars for increasing crop production. Large scale deployment of elite cultivars carrying an r gene (monoculture) imposes higher selection pressure on the pathogen carrying the cognate AVR gene to survive, resulting in the modification or deletion of the recognized AVR gene or generation of novel effector genes that can escape the recognition of the old ‘R’ gene. this R-AVR interactive co-evolution explains why many resistant cultivars in the field lose their resistance in a relatively short period of time. Hence, knowledge on the molecular mechanisms of Hyaloperonospora-Brassica i.e., host-pathogen interaction, and use of horizontal, durable and broad-spectrum resistance has become very important in disease resistance breeding programmes. the interplay of host resistance and pathogen virulence in latter system is likely to be quite different to major gene–based system. Major gene resistance in crucifers to downy mildew has been exploited for breeding resistant cultivars.