Black Spots on Prickly Pear Cactus

We usually think of native cactus species as being tolerant of desert conditions and trouble-free. However, there are two diseases that can cause lots of problems to prickly pear and saguaro cactus.

The first is something called dry rot of prickly pear. It is a fungus disease that causes the pads, or sections, of the cactus to become spotted with black, usually quarter size patches. These black circles may be few or numerous. When numerous they merge causing irregular shaped blackened areas to occur. These infected areas may drop out leaving round holes. Severe infections may result in the complete decay of the entire plant.

There is always some occurrence of dry rot on prickly pear throughout the desert. It is not an epidemic and does not threaten the overall population of prickly pear in the natural environment. However, if you have an infection of dry rot occur in your landscape it can threaten your individual plants.

The pronounced black coloration on infected pads are large masses of microscopic spores. These dust-like spores are released when mature, and are carried by wind, splashing rain, and insects to other prickly pears. If conditions are right the spores will germinate and infect other prickly pear.

Studies conducted at the University of Arizona have indicated the optimum growth of the fungus occurs between 77 and 82 degrees F. Spore production was greatest between 82 and 91 degrees F. With temperatures on the rise, this means the disease will again become active as early as April.

Although there is no control as yet for this disease, there are step that can be taken to protect healthy plants and reduce further damage to diseased plants.

Infected areas on plants should be removed. If you notice large black spots on the upper pads, remove the entire pad back to the point of attachment. If lower pads are infected, cut out only the infected areas. Cuts should be made only in healthy tissues, at least 2 inch outward from the diseased area.

Apply a 10% solution of household bleach to each cut surface and disinfect tools in the same solution after each cut. Pruning should be done during warm, dry periods. Infected pads and plant parts should be removed and disposed of! Remember, these infected areas serve as the source of inoculum for future spread of the fungus disease. Another type of rot affects our native saguaro cactus. Unlike dry rot of prickly pear, this is a soft rot caused by a bacteria, not a fungus. The disease is referred to as bacterial necrosis of saguaro. The first evidence of the disease is the formation of circular shaped dark, soft tissue. In time, the infected area usually enlarges, becoming purplish-black, and splits open. A dark odorous material may leak from this opening, or the opening may dry revealing black inner tissue. The diseased portion may be walled off with callus tissue produced by the saguaro, or it may spread throughout the plant causing it to die.

Control for bacterial necrosis is similar to that of dry-rot on prickly-pear. Diseased portions must be cut out to prevent it's spread throughout the saguaro. Rotted tissue should be scooped out along with about 2 inch of the surrounding healthy tissue. Slope the bottom of the excavation so that water will drain out. The walls of the cleaned out cavity should be smooth. In removing the rotting material, be careful not to make "puncture-type" cuts into the remaining healthy tissue since the pathogen can survive in such sites.

Finally, thoroughly wash the cleaned out cavity with a 10 % household bleach solution to kill any remaining bacteria. Allow the cavity to stand open to hasten healing. Do not fill the the cavity with cement or other materials! This will only trap water and make it more likely for the disease to reoccur.

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material orifinally appeared in Arizona Dailt Stargardening column, on February 22, 1998
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