Windowsill cactus problems

I have a cactus about 12 inches high that resides on my windowsill. I water it once a month. Now it has some sort of affliction - almost looks like someone splattered some brown mud on it. I'm thinking that perhaps it is some sort of fungus. How can I cure this? I lost another cactus earlier this year; I don't want to lose this one. Thanks.

I have discussed your question with Dr. Natalie Goldberg, NMSU Cooperative Extension Service Plant Pathologist. We both wonder if the problem really is a disease. According to Dr. Goldberg, fungal diseases which attack cacti tend to form sunken, black areas at the site of the lesion. Bacterial diseases would also produce different symptoms. Could the problem be scale insects? They should be relatively easy to scratch off the plant with the edge of a knife blade. There may be a yellowish area beneath the scale insect, or there may be no evidence of damage to the cactus at the point of scale insect attack if the insect were not there long. If it were a fungus, it would also scrape off but would leave a distinct blemish. On older parts of a cactus plant, corky, often light brown or tan areas form naturally. I wonder if that is what you are describing. Such corky areas can also form at the point of some physical injury. Insect feeding, scratching by the spines of another cactus or a wound due to having something dropped on the plant, pet damage, any other form of physical injury can cause the formation of such a corky scar. To be sure about this, however, I would suggest that you take the plant to your local Cooperative Extension Service office so that they can look at it and describe it to either an entomologist or plant pathologist. Once the problem is properly identified, then appropriate treatment may begin if necessary.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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