The dry weather pattern that we have experienced recently in the desert southwest has greatly reduced the progression of diseases such as downy mildew that thrive when relative humidity and free moisture are abundant. Unfortunately, the same dry weather pattern subjected crops in the region to freezing temperatures earlier this month. This damaged plant tissue can serve as colonization sites for some biological pathogens of these crops. Lettuce drop for example, caused by Sclerotinia minor and S. sclerotiorum, often begins on senescing lettuce tissue. It does not matter to the fungi whether the tissues are senescing due to natural old age or because of damage resulting from one or more freeze events. Additionally, microorganisms that normally would not be able to infect healthy vegetable crop tissue can sometimes invade this injured tissue and cause additional damage. Crops that have sustained freeze damage should be monitored carefully in the weeks ahead for the appearance of disease symptoms caused by these opportunistic microorganisms. In a broader sense, some plant diseases or disorders can be caused by nonbiological agents and as such do not require a living pathogen. Symptoms of these abiotic or noninfectious diseases can result from a variety of causes, including low or high temperatures, excessive soil moisture, low or high light intensity, air pollution, mineral deficiencies, mineral excesses, and excessively acid or alkaline soils. In most cases, these abiotic diseases can be followed by secondary invasion by a biological plant pathogen.

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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.