Fungal issues plague almost every form of plant. The sheer number of fungal organisms is staggering and most survive by being dormant for long periods of time. Fungal lesions on cactus may be caused by any number of fungal types, but the important things to note are what causes them to colonize and how to prevent their damage. In this way, the gardener is armed with knowledge on how to treat fungus on cacti should any disease symptoms develop. Some fungal diseases simply cause cosmetic damage while others can develop into rots that completely eat the cactus from the inside out.
Types of Fungal Damage in Cactus
The vast number of cacti species can only be outcompeted by the huge quantity of fungal varieties. Fungus spots on cactus pads are common, as in the case of Phyllosticta pad spot. 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 fungus types damage the roots and eventually the whole plant, so once their visual damage is seen, it is too late for the plant. 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, again, the tones can range from yellow to brown and all the way to black. Some are corky, while others are weepy. These may ooze brown, rusty, or black fluid, evidence of severe infection.
The cacti most frequently plagued by fungal lesions are Opuntia and Agave. Fungal lesions on cactus usually start as water spots or slight discolorations on the plant’s epidermis. Overtime, as the fungi mature and spread, the symptoms can broaden and even eat into the cambium as the surface skin cracks and allows the pathogen to enter.
Causes of Fungal Lesions on Cactus
Outdoor cactus can come in contact with fungal spores in various ways. Spores may be blown in from wind, 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, which can persist for years until the right set of conditions occur. Even purchased potting soil may be contaminated with fungal spores.
How to Treat Fungus on Cactus
Once there is a fungus affecting your cactus, it can be difficult to stop. If damage isn’t severe, a fungicide spray can usually help. 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 doesn’t work with all fungi, but it may be effective at times. 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.