We can escape the desert heat by finding refuge in air-conditioned buildings or can obtain food from any number of sources at any time. On the other hand, plant pathogens must survive high temperatures and lack of food by employing other tactics. A few plant pathogens can thrive at temperatures common in the desert during the summer and cause disease on plants growing at that time; however, most others cannot function at temperatures much above 90°F. To survive inhospitable temperatures or lack of a host on which to feed, fungal pathogens often produce thick-walled durable spores or some other structures that will allow the organism to survive hostile environments in a dormant state. The visible dark-colored sclerotia produced by the lettuce Sclerotinia pathogens are such structures. Much smaller sclerotia and thick-walled spores facilitate long-term survival of the soil-borne pathogens Rhizoctonia and Fusarium, respectively. In comparison, bacterial plant pathogens do not have recognized survival structures, but can subsist for some time in a reduced metabolic state on, in or near living or dead plant tissue. Virus pathogens also cannot make resistant structures, so survival usually occurs in living plants or vectors. These plants can include weeds or cultivated crops that do not express disease symptoms, but serve as sources of virus to visiting insect vectors. Finally, nematode survival stages can include eggs and certain larval forms. Many of the cultural disease management methods that we employ are effective because they disrupt the normal survival capability of these plant pathogens.

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To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.