Plant pathogens are similar to other living organisms in that they contain a degree of genetic variability within their genes that govern physical structure and internal biochemical activities. Any selection pressure imposed on a population of an organism can result in visible and invisible changes within that population. Selective breeding is a tool used to express the genetic diversity within a population of an organism, as demonstrated by the proliferation of dog breeds or varieties of agricultural crops when compared to their original ancestral forms. Other selection pressures can result in unwanted changes within a population, such as the development of resistance to antibiotics used to treat human and animal diseases and to plant protection chemistries used to manage plant diseases. In our desert vegetable production area, plant protection products are used mainly against diseases caused by fungi and closely related organisms. Specific recommendations have been established by an organization called the Fungicide Resistance Action Committee to manage the development of fungicide resistance within a target plant pathogen population. Major resistance management strategies include the following. 
1.) Do not use a single mode of action in isolation. Instead, apply the material as a mixture or in alternation with one or more fungicides with different modes of action within a treatment program. 
2.) Restrict the number of applications of a particular mode-of-action within a season and only make applications when necessary. 
3.) Do not apply less than the manufacture’s recommended dose. 
4.) Target fungicide applications for disease prevention and not eradication. 
5.) Use an integrated approach to disease management. By employing these resistance management strategies as well as using disease-resistant cultivars, biological control agents, crop rotation, and other beneficial cultural practices, the end result can be a high level of disease control, lower amounts of total fungicides needed, and decreased selection of fungicide-resistant components within the pathogen population.