Cactus care -- residue "rotting" plant?

Question

I have a couple of potted cactus plants. One gradually developed a layer of white particles / residue on the skin of the cactus. Additionally, the plant started "rotting" -- meaning areas of the cactus are dying and turning brown.

Originally, I naively thought the residue layer may be pollen. I initially wiped it away with a toothbrush. But after a couple of months it gradually came back on the same cacuts (others are fine).

Is this a mold or fungus? Any ideas on how I could save my cactus?

Answer 1

The name of it is "damping off fungus", and is do to too much water. You need to treat with a mild fungicide before it spreads to the other guys. As an avid cactus guy, I can tell you that most of the time this fungus is fatal to your seedlings, unless you catch it in time. Here's a website on growing cactus from seed hope it helps.

http://www.plot55.com/growing/general/ca...

Answer 2

It sounds like powdery mildew. The cactus is probably getting too much water. That would be why it is rotting.  

Answer 3

You are watering it too much.

Answer 4

Could be cochineal scale, which is a white fuzzy scaly substance caused by cochineal bugs.  Or it may be mealy bug residue.  There's lots of information online on both of these pests.