Disease control by plant breeding and selection
If plant pathogens were stable entities their control by cultivating varieties of crop plants resistant or, better, immune to them would in many ways be the ideal solution providing such varieties were agronomically and commercially suitable. Unfortunately, many plant pathogens are genetically variable and produce new races with different parasitic capabilities, to which resistant varieties of plants are in time likely to succumb. This is well exemplified in the breeding of varieties of wheat resistant to Puccinia graminis f.sp. tritici (black stem rust) in which several hundred races of the fungus have been identified, necessitating a permanent programme of plant breeding to keep ahead of new races of the pathogen as they arise, as described by Stakman and Harrar (1957). The ways in which new races are produced are considered elsewhere (p. 286).
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