Disease resistance refers to the ability of a plant to limit the growth and development of a pathogen or insect that attacks it. There are several types of disease resistance in plants:
Passive resistance involves physical or chemical barriers that prevent infection, such as waxy layers on leaves, bark on stems and branches, or production of toxins. These barriers are always present.
Active resistance involves recognition of a pathogen and mobilization of complex biochemical defenses only when a pathogen attacks. This resistance may be:
Nonhost resistance: The plant is resistant to all genetic variants of a pathogen. This resistance is very durable.
Race-specific resistance: The plant recognizes specific races or strains of a pathogen via gene-for-gene interactions. This leads to a hypersensitive response, stopping pathogen growth. However, races of the pathogen not recognized can still infect the plant.
Quantitative/horizontal resistance: The plant slows but does not completely stop growth of all genetic variants of the pathogen. Many minor genes control this type of resistance. It is generally more durable than race-specific resistance.
Breeding for disease resistance involves identifying good sources of resistance genes and introducing them into elite crop lines by conventional breeding or genetic engineering. Combining different resistance genes into a single crop variety provides more durable resistance against multiple pathogen variants or races.
Some key considerations for breeders when developing disease resistant varieties include:
Effectiveness and durability of identified resistance under field conditions
Possible negative impacts on agronomic performance
Combining resistance with other desirable traits
Regional differences in pathogen populations
Changes in pathogen races over time
I hope this overview on what disease resistance encompasses and how it functions in plants was helpful!