Answer: Leaf spots are a common problem, but they are usually not caused by a disease. For example, leaf scalds occur when water droplets on the leaves act as lenses and focus excessive light in one spot, bleaching the chlorophyll and killing the underlying tissue. Spots with patterns are signs of a disease, including a tan center, dark borders, and/or light-colored borders called ?halos.? Dark structures may be present; these contain a means of dispersal called spores.
If the spots were caused by sunscald, the injured area will callous over all on its own. To prevent a repeat, be sure to keep water droplets off the top of your plant.
If the spots are caused by disease, removal of the diseased tissue is the most practical way of limiting the spread of the infection within the plant. This procedure must be done quickly, though, because the rot can spread rapidly and reach the point where treatment may not be possible. Use a sharp knife or a spoon that has sharpened edges to gently remove the affected tissue. If the affected area is small, remove all of the rotting material and about one-half inch of the surrounding healthy tissue to make sure that all of the diseased tissue is removed. This process will result in callouses over the treated area. It won't be attractive, but you will have saved your plant from a slow death.
Best wishes with your cactus!
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