Images of saguaro cacti come to mind when most people think of a cactus; their familiar U-shape growth pattern has been an ornament in countless westerns, cartoons and postcards of desert landscapes. If you plan to grow this iconic cactus in your home or garden, it is always worthwhile, just as it is when growing any other plants, to make yourself aware of all the most common diseases that may infect that plant.
Saguaro cacti are slightly susceptible to a number of diseases common among the entire succulent group of plants, but in general are regarded as one of the most disease-resistant succulent plants. Really, the only disease of any importance to saguaro growers is bacterial necrosis of saguaro, caused by the bacterium Erwinia cacticida. The pathogenic bacterium can be spread by insects or can persist in soil that has previously housed an infected saguaro. Infection generally occurs through tissue wounds on the saguaro.
The earliest symptom of bacterial necrosis saguaro is so subtle that it often goes unnoticed. A small, light-colored, water-soaked margin appears on the trunk or branches of the saguaro. As the diseases progresses, the tissue underneath the infection site turn black and often cracks open, oozing out a dark brown liquid. The disease continues to break down the saguaro's tissue if left untreated, eventually revealing the woody skeleton on the inside of the saguaro.
Preventative steps, such as never planting a saguaro in soil that is known to have previously held an infected saguaro, and treating tissue wounds can prevent bacterial necrosis from ever threatening your saguaro. Once infected, you can remove the infection site using a clean, sharp knife so long as it is still 2 inches in diameter or less. Unfortunately, if necrosis advances beyond 2 inches, the condition is untreatable, and your only recourse is to remove the saguaro and avoid planting any new saguaros in the infected soil.
According to the U.S. National Parks Service, there was a flood of reports during the 1950s of saguaros infected with disease. Upon investigation, experts found that the saguaros had actually been damaged by harsh weather conditions and were disease-free. Many of the tell-tale signs of environmental stress such as frost damage are quite similar to symptoms of diseases that affect succulent plants. If you notice symptoms that are inconsistent with a bacterial necrosis diagnosis, the most likely explanation is that the saguaro may have been damaged by excessively cold temperatures.