Just like any other plant, your cactus can develop a fungal infection. Although some fungal infections do minor damage to cacti, others left untreated can lead to the death of the cactus. Therefore, treat your cacti at the first signs of infection for the best chances of success.
Types of Fungal Damage
The vast amount of cacti can only be outcompeted by the huge quantity of fungal varieties. Fungus spots on cactus pads are common. It is often quite impossible to diagnose which fungal organism is causing the spots, but often that is unimportant since treatments are generally the same.
A few fungi types damage the roots and, eventually, the whole plant, so once their visual damage is seen, it is too late for the plant. On the other hand, simple topical fungal spots are much easier to combat and are usually not life-threatening to the cactus, provided steps are taken to control the offending fungus.
Lesions on cacti may present in many different ways. They may be round, irregular, raised, flat, and any other shape. Many are discolored, but the tones can range from yellow to brown and black. Some are corky, while others are weepy. These may ooze brown, rusty, or black fluid, indicating severe infection.
The cacti most frequently plagued by fungal lesions are Opuntias. Fungal lesions on the cactus usually start as water spots or slight discolorations on the plant's epidermis. Over time, as the fungi mature and spread, the symptoms can broaden and even eat into the cambium as the surface skin cracks, allowing the pathogen to enter.
Causes of Fungal Lesions
Outdoor cactus can come in contact with fungal spores in various ways. Spores may be blown in from the wind, in soil, or contracted from splashing water. Plants with consistently wet pads or stems are the worst affected. Conditions where rain or high humidity combine with warm temperatures, promote the formation of fungal lesions.
Fungus spots on cactus pads are more prevalent in the springtime. They are also enhanced by overhead watering and in areas where humidity is high. Greenhouse specimens may be particularly susceptible unless there is adequate ventilation. Condensation adds to the ambient humidity and promotes spore growth.
Soil is another contributing factor. Many soils harbor fungal spores, persisting for years until the right conditions occur. Even purchased potting soil may be contaminated with fungal spores.
How to Treat Fungus
Once a fungus affects your cactus, stopping cannot be easy. If the damage is not severe, a fungicide spray can usually help. However, if the plant is rife with lesions, it may be best to find some uninfected healthy material and start a new plant with a cutting. Use a sterile knife to take the cutting and dust it with sulfur to kill any possible adhering spores.
Controlling cultural conditions with plenty of heat, under stem watering, sterile potting medium, and ventilation will halt many fungal outbreaks. Another way to save a plant is to cut out the infected tissue. This does not work with all fungi, but it may sometimes be effective. Again, sterilize your cutting implement and remove more tissue than appears to be affected to ensure all the pathogen is removed. Keep the area dry as it calluses, and watch carefully for signs of reinfection.
- Succupedia: Browse succulents by Scientific Name, Common Name, Genus, Family, USDA Hardiness Zone, Origin, or cacti by Genus