By Mike Matheron, UA Extension Plant Pathologist

Plant pathogens are similar to other living organisms which contain a degree of genetic variability within the 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 the original ancestral forms.

Other selection pressures can result in unwanted changes within a population, including the development of resistance to antibiotics used to treat animal diseases and to plant health chemistries used to treat plant diseases.

In the Yuma County area, plant health products are used primarily against diseases caused by fungi. Specific recommendations have been established by an organization called the Fungicide Resistance Action Committee or FRAC to manage the development of fungicide resistance within a target plant pathogen population.

These resistance management strategies include the following points:

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 manufacturer's recommended dose.

4 - Target fungicide applications for disease prevention and not eradication.

5 - Use an integrated approach to disease management. By utilizing as many resistance management strategies as possible plus using disease-resistant cultivars, biological control agents, crop rotation, and other beneficial cultural practices, the end result can be reduced disease incidence and the total fungicides needed, which in turn can decrease the selection of fungicide-resistant components of the pathogen population.

More information on this topic, the publication entitled "Fungicide resistance in crop pathogens: how can it be managed" is available online at

Contact Matheron: (928) 726-6856 or