As the seemingly unending days of summer heat in the desert persist, we can seek
refuge in air-conditioned vehicles or buildings and can obtain food from any number
of sources at any time. Not so for plant pathogens, which 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 other structures that
will allow the organisms to survive hostile environments in a dormant state. The
visible dark-colored sclerotia produced by the lettuce Sclerotinia pathogens are
examples of such structures. Much smaller sclerotia and thick-walled spores facilitate
long-term survival of the soil-borne pathogens
Rhizoctonia and
Fusarium,
respectively. On the other hand, 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 vectors or living plants. 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 capacity
of these plant pathogens.
Sclerotinia sclerotiorum
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To contact Mike Matheron go to: matheron@ag.arizona.edu.