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Most commonly found on opuntia. Rusty coloured, corky areas appear on the stems. Cells of the epidermis break open and curl up.
The scab can cover the entire plant, except new growth at the tips.
Scab is thought to be a form of edema, resulting from overwatering, low temperatures and poor ventilation. Possibly also, too much nitrogen in the feed.
Can be confused with spider mite attack.