Saguaros are iconic cacti, often portrayed in Western movies as lonely silhouettes with the red desert sun peeking over one shoulder. In reality, saguaros are often used as landscape plants on both urban and rural desert properties. While rugged and hardy in general, a disease commonly known as bacterial soft rot or bacterial necrosis can cause these magnificent cacti to leak fluid.
Bacterial necrosis in saguaro cacti is a significant problem and should be taken seriously when symptoms appear. It is caused by bacteria in the family Erwinia, the same bacteria that cause fire blight in other plants. When the bacteria infects the saguaro through an open wound or after being brought in through the roots, it quickly begins to destroy the plant's flesh.
Unfortunately, the Edwinia bacterium can survive for a long time in the soil without causing any problems. It can even infect plants and not spread explosively. However, when a cactus is weakened by damage, other disease, stressful environmental conditions or transplant, Edwinia takes advantage. High humidity often triggers Edwinia proliferation, and Edwinia may be controlled by lowering humidity when possible.
The symptoms of bacterial necrosis of saguaro cacti are similar to other bacterial rot diseases of plants. Affected tissues develop an area that appears to be water-marked. The spot will progressively deteriorate, often turning black or developing a persistently oozy sore. The dark liquid that comes from the sore is noxious. After the sores develop, the plant's external tissues will rot completely and expose the cactus skeleton.
Control and Treatment
Several methods of control are recommended if a saguaro cactus develops bacterial necrosis. A spot that is less than 2 inches in diameter can be removed with a sharp knife. Be sure to remove a 1/2-inch margin of healthy tissue as well to ensure all diseased tissue has been destroyed. Be sure the cut is made so that water will not pool inside. Clean the injured area with a bleach solution -- one part bleach, nine parts water and 1 tsp. of liquid detergent per gallon. Allow the cut to scab after removal. Larger damage may require removal of large parts of the affected plant, both to slow the spread of the disease and to prevent injury should the plant become so weak it suddenly toppled.
- University of Arizona Cooperative Extension; Disease of Saguaro in Arizona -- Bacterial Necrosis; July 2011
- Texas A&M University Cooperative Extension: Texas Plant Disease Handbook
- Paradise Valley Community College; What Causes Unhealthy Saguaro?; John J. Dill II; May 2008
- University of Arizona Cooperative Extension; Problems and Pests of Agave, Aloe, Cactus and Yucca; Jack Kelly, et al.; March 2011
- University of Arizona Cooperative Extension; Diseases of Urban Plants; Mary Olsen; May 1999
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