Terms used in UKCPVS Reports - explanatation of terms used to describe resisrance and virulence Specific resistance and specific virulence Resistance is the ability of a host cultivar to defend itself against infection by a pathogen isolate. Conversely, virulence is the ability of a pathogen isolate to infect a host cultivar. Some cultivars possess resistance that is more effective against some isolates than others and this is termed "specific resistance'. Similarly, some isolates are more able to infect some cultivars than others and this is termed "'specific' virulence". The terms "'specific resistance factor"' and "specific virulence factor" are used to describe unidentified genes in host and pathogen which interact with one another. Specific resistance factors are numbered R1, R2 .... Rn and specific virulences are numbered V1, V2 ... Vn. Each individual specific resistance factor is effective against all isolates except those possessing the corresponding virulence factor. Hence a cultivar possessing R4 has effective resistance against all isolates except those possessing V4. Cultivars lacking specific resistance are classified as RO and isolates lacking specific virulence are classified VO. Specific resistances and virulences relating to particular cereal diseases are described by additional prefixes for crop (W = wheat, B = barley and 0= oats) and disease (M = mildew, Y = yellow rust, B = brown rust, C = crown rust, R = Rhynchosporium), hence WYR 2 and BMV5. Terms describing resistance at different growth stage Resistances may also be classified according to the growth stages at which they are effective
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