The saguaro (Carnegiea gigantea) is an arid-loving cactus that grows in U.S. Department of Agriculture plant hardiness zones 9 through 11. This symbol of the Southwest can live up to 200 years old, soar over 60-foot tall and weigh up to 12 tons. Sick saguaros often have weakened arms or trunks that fall or topple, seriously injuring people or damaging property. Fortunately, saguaro isn't susceptible to serious disease problems, with the exception of bacterial necrosis. Learning how to treat a sick saguaro can help save your plant while keeping your family members safe from falling cactus arms.
Bacterial Necrosis Symptoms
Bacterial necrosis of saguaro is a potentially fatal disease that causes the plant tissue to deteriorate rapidly. Early signs include small, light spots with watery margins forming on the branches or trunk. This symptom often goes unnoticed until the infected tissue turns deep brown to purplish-black. The dark, soft spots often enlarge and split open to ooze a dark brown, smelly substance. Bacterial necrosis symptoms are most often noticed after rainy periods.
Causes of Bacterial Necrosis of Saguaro
The Erwinia cacticida bacterium causes bacterial necrosis of saguaro. The pathogens enter the cactus through various plant injuries on the branches, trunks or roots. Entry wounds can be caused by insects, small animals or cold damage. The disease pathogens can survive in decaying or dead saguaros as well as in the surrounding soils.
Bacterial necrosis pathogens easily spread via insect feeding activity. One species of night-flying moths (Cactobrosis fernaldialis) bear caterpillars that feed on the flower buds and tunnel directly into the cactus, which not only transfers the bacteria but also opens a wound that gives the pathogens easy access to the plant's interior.
Treating a Sick Saguaro
Removing the diseased plant tissue as soon as you notice the symptoms helps treat bacterial necrosis disease and limits the spread of its pathogens. Using a sharp, clean knife, cut out all blackened, dead or rotting plant tissue along with about 1/2-inch of surrounding healthy tissue. Dipping the knife into a solution of 1 part chlorine bleach and 9 parts water between each cut helps prevent spreading the infection. Angle the cuts so that water drains out of the carved-out pocket.
Kill off the remaining bacteria by using the rest of the bleach solution to rinse the treated area. Allow the cactus to dry naturally and leave the hole uncovered so the wounded tissue heals faster. Place the diseased plant tissue into a plastic bag, seal the bag and immediately discard in a covered trash can to prevent spreading the pathogens to any nearby saguaros.
A Few Considerations
The saguaro cactus bears fragrant, white blooms that appear on the arms in spring before giving way to edible, vibrant red fruits in the summer. The flowers attract bees and hummingbirds. Despite their attractive features, be prepared to remove a sick saguaro with girdled arms or trunks because they are potentially hazardous to family members and pets. Although you can usually treat small infected areas about 2 inches in diameter or less, some cacti are too diseased to save and should be removed from your landscape.