Question
Dear Greg,
I planted a Mexican fence post cactus in the fall. There are three different posts that are infected with black/brownish spots. It truly looks like cancer. Do you know what this is, if there is a cure and if it will infect other plants in my dessert landscape?

Kind regards,
rebecca

Answer
Rebecca,

Sorry for the delay in getting back to you. It is probably not a disease. It is more likely either damage from the sun or from insects, depending on the size of the spots.

If the spots are large, it is most likely caused by sunburn. Cacti grow a thicker skin on the south side of the plant to resist the intensity of the sun. Many times, when a cactus is planted, the nursery does not tell the purchaser which side of the plant should be directed south. This almost always results in sunburn.

If the spots are small, it may be caused by a insect which is virtually impossible to get rid of, short of squishing them manually. If this doesn't sound correct, you can send me a photo to [email protected] which will help me diagnose the problem.

Sincerely,

Greg