Hello, I’ve had this (what I think to be an optunia) cactus for several years. I’ve been able to propagate several times, but the original one has progressively looked worse. It began with whitish-gray circular spotting around the spines on several cladodes and has since spread... The spots had always been ‘firm’ and if wiped, look reddish brown underneath. They have since begun to crack in spots. I’m thinking sunscald. If this is the case; how do I prevent it from occurring on future propagations? (These were taken from the wild. I’m in Seattle, WA. Indoor setting. Homemade cactus mix that has worked great for all my cacti/succulents/agave.)
You are correct in identification. Unfortunately, this is not sunscald. These are used to bright sunlight. What this appears to be is a fungal infection. This far, in the original, it may be too far gone, and unable to treat. You can still try, with wettable sulfur, or peroxide, or even both! Use peroxide the day before using sulfur so that it does not oxidize the sulfur. Then the next day, water in as instructed with the sulfur.
Your offshoots may survive just fine, but it could be too late for the original. This is most often caused by overwatering, or using infected soil. Just be sure to let this dry out fully between waterings.
Here is an article for more information on their care: https://www.gardeningknowhow.com/ornamental/cacti-succulents/prickly-pear/growing-prickly-pear.htm