I grow a lot of Trichocereus cacti from seed, I also send a lot out each week to customers. This involves plucking them out of their pots, washing the soil off and packaging them up. Sometimes there are a few in each tray that develop the classic fungal/bacterial red spotting, which generally we attribute to climate effects; so relative humidity, the time that dew clings to the plants in the morning or night, temperatures encouraging growth of the disease. Whilst it is rarely terminal, it can devalue the seedlings significantly. There have been comparisons between nutrient status, calcium:magnesium balance, presence of organic material in the potting media and other factors, to the overall hence and resilience of Trichocereus to this disease.
Recently I noticed a pattern emerging in seedlings I was pulling out and keeping aside, as they were developing several spots (seedlings are always free of this disease unlike cacti grown from cuttings). This starts at the base of the cactus and then slowly spreads upwards over time. Always I noticed the presence of root mealybugs (Rhizoecus spp.). Healthy cacti I chose to send were always free of root mealybugs. Mealybugs are known to be vectors of fungal disease on other species, as they tap into a plants roots and suck out sugars, which invariably attracts fungi. So could this be the cause of this disease and could control of mealybugs be an effective treatment, if not a cure all, for some of these spotting diseases? I doubt it would control it in cacti like Trichocereus bridgesii monstrose which is highly susceptible to humidity related diseases.
Root mealybugs are active all year but more active in warmer temperatures. The adult lays eggs covered in a white powdery substance and these hatch into crawlers. The crawlers are especially active when you water plants and it is often when you are watering that they are washed out of one pot and move to neighbouring plants. Crawlers also wander about on their own, mostly at night. I have noticed less problems when the plants are kept on benches for example, rather than on concrete or gravel on the ground.
A possible treatment I will be trialling is neem as a root drench (I have never recommended Neem as a spray for Trichocereus cacti, as it tends to encourage disease as it is usually slightly oily and trichocereus do not tolerate oil sprays. As a root drench this should work fine. One method to rid plants of root mealybugs is submerging potted plants in 49ºC water until the internal root ball temperature reaches 46ºC and is 100 percent effective in killing root mealybugs and does not significantly affect the potted plants. Insecticidal soaps are also touted as a treatment along with other non-organic solutions, which I will steer clear of.
I think it is worth researching this further as a cause and we can instead use a targeted approach for root mealybugs, as opposed to the use of fungicides and copper sprays for controlling the symptoms.