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Cactus tip
I have an ornamental cactus (no name on plant when purchased) about 3ft high. The pot is 8 in. in diameter and about 8 in. deep. Four ribs extend the length of the plant at right angles to each other, tapering from the bottom to the top. Tissue color is a brindle pattern with the green parts varying between light green to greenish gray. The rays of color extending from the center of the plant ribs out to the tip at a 45 degree angle. The plant has been in the same "soil" that it came in from the nursery which is mostly a volcanic small pea sized gravel. I water it twice a month during the summer and rarely between September and April. The water can drain freely through the bottom of the pot. The cactus sits in the south facing window and receives direct light for about 6 hours (depending on the time of year) and diffuse light until sunset. The humidity is low (we live in northern Colorado)and the temperature fluctuation within the house is minimal, a low of 65 degrees in winter to 80 in summer. It is not in a draft and shares the space with a poinsettia and a jade plant with good separation between the three. It has been there for the last 3=4 years. The damage is at the apex of the plant. It is black spotting that covers almost the entire top of the plant. The "rot" is soft and the spines are not stiff or sticky. The tissue below the damage is healthy and turgor seems good. No shrinkage is observed anywhere along the ribs. This damage has occurred in the past week. Nothing significant has changed in that time.

From your image, this area of the plant looks like a potential bacterial issue, especially if the tissue is mushy. For these diseases, best to keep the plant surfaces dry as possible.  This includes no misting or water to the leaves/stems.  I would suggest a bit of surgery to remove the offending portion.  Make the cut well away from the mushy area and dispose of the rotted area.  Be careful on the watering--making sure to avoid too much water, most cactus thrives on neglect. Lots of sun like you have it is good!



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