The dry weather pattern that we are currently experiencing has greatly reduced the incidence of diseases caused by fungi and bacteria on vegetable crops in the desert southwest. This is especially true for most foliar diseases caused by these pathogens, which require moisture and high humidity for infection and disease symptom expression. On the other hand, plant diseases or disorders caused by nonbiological agents are not dependent on a living pathogen and can occur whenever circumstances favoring their development are present. Symptoms of these abiotic or noninfectious diseases can result from a variety of causes, such as low or high temperatures, excessive soil moisture, low or high light intensity, air pollution, mineral deficiencies, mineral excesses, and excessively acid or alkaline soils. At this time of year, low or freezing temperatures can be a concern. Depending on the crop and its location, freeze damage can occur. This damaged plant tissue can then serve as colonization sites for some biological pathogens of these crops. Furthermore, microorganisms that normally would not be able to infect healthy tissue can sometimes invade this injured tissue and cause additional damage. Crops that have sustained freeze damage should be monitored carefully for disease symptoms caused by these opportunistic microorganisms.

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.