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I have several cacti growing in my yard and one (t. pachanoi, if it matters) has developed a white layer across the top of the plant. It is very fine and looks almost sandy, like perhaps a mould or fungus. It does brush off fairly easily though.

What is this white stuff? Is it dangerous to the plant or its neighbours? How can I treat it?

EDIT: Now with picture. I've brushed most of it off but you can see the white dots across the top. Before my attempt to clear it it was covering the entire surface

close up of white growth

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We really need a picture, but it could be the cochineal scale insect that is responsible? This is very common on wild opuntia (prickly pear) here in Texas.

The insect hides in a white growth that does look like a fungus. Get a pen knife and find one of the insects and crush it. A strong red dye will be produced. This was a major product from the New World, and even today is still used as a food coloring (usually under an anonymous euphemism: people usually don't like to know that their candy/sweets are coloured using crushed bugs!)

http://en.wikipedia.org/wiki/Cochineal

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    (after picture has been posted) You've brushed it off so I suspect it isn't. This implies it is fluffy (like some moulds) - which the cochineal stuff isn't. Are there any scale insects left? If so try crushing one - the crimson dye is unmistakable!
    – winwaed
    Commented Jun 10, 2011 at 15:59

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