Answer: If you've been successfully caring for your cactus for 20 years, you certainly know its growing requirements and the care it needs. It sounds unusual for it to suddenly become unhealthy, unless something has changed in the growing conditions or care it has recently received. It may be that what you used to rid the plant of spider mites has caused some damage. Not all pesticides/miticides are safe to use on all types of plants. To be on the safe side, you should always read the label and make sure that both the plant and the targeted pest are listed on the label. Aside from waiting to see if your cactus will recover, there's really not much else you can do at this point. I wouldn't stress it any further by repotting or changing the watering schedule. Let's hope for the best!
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