Bud rot found on one cola — is the whole plant ruined?

Had this exact issue last grow. What fixed it for me was flushing with pH'd water and letting the soil dry out before feeding again at half strength.

Same thing happened to me when I was overfeeding. Backed off the nutrients by 50% and new growth came in healthy within a week.

Environment is key here. What's your temp and humidity? Sometimes the symptom looks like a nutrient issue but it's actually environmental.

Post a picture if you can — hard to diagnose without seeing the actual damage pattern. But from your description, I'd guess pH-related lockout.

Post a picture if you can — hard to diagnose without seeing the actual damage pattern. But from your description, I'd guess pH-related lockout.