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Prickly Pear Cactus is growing what looks like rot (shown in the attached pictures) where it's getting a yellowish color in the place these things appear.

Is this normal? Is it dangerous?

What is this and what can I do about it? Are the infected plant's fruits edible?

Please help.

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Looks like a longstanding and heavy scale infestation, evidenced by the fact that some of the deposits have actually gone black, which only happens after a goodly time. Opuntia is prone to infection by Dactylopius coccus, commonly known as cochineal scale. Light infestations don't kill the plant, but a heavy one like this might. Caught in its early stages, it's possible to use a hose to damage the white cottony coating, allowing the insects to die, but its a bit late for that now. You could try neem oil in winter, or use dormant oil with some insecticide and insecticidal soap, but don't apply this on a sunny day, choose an overcast day. More info here https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cochineal-scale-on-cactus.htm

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My guess is that you have a fairly severe Chochineal insect infestation. If you pop one of the white blobs off and mash it and it is brilliant red, you've got Cocheneal instects.

Are you located in the SW United States/Mexico? That is the natural range for these insects.

Extreme infestations can kill a plant over time. I've never dealt with this particular pest and haven't found any instructions for getting rid of it online. I don't think this would affect the fruit though.

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I agree with the above two answers. Prickly pear cactus is susceptible to cochineal insects and I have seen this infestation on wild plants here in Florida. An excellent read about it plus some interesting historical information comes from George Rogers, a professor at Palm Beach State College.

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