Most definitely is.
You’ll have to trim it out and give a spray to the rest with some 3% peroxide and water to kill any stray spores.
looks like it
This are really dense buds and we have had a lot of rain. Sooo 3% peroxide? Like what we use and mix with how much water? I haven’t seen any on my other plants yet but I guess I’ll have to go through them carefully. Thanks for confirming what I didn’t want to hear lol. 
I use 1 part h2o2 and 2 parts distilled water.
I’d make sure to treat the whole area by cleaning thoroughly. Also treat the soil if you aren’t already using a beneficial microbe treatment.
- H2O2 foliar spray, add 3 mL of H2O2 per gallon of H2O.
- For a maintenance foliar spray, add 1.5 ml of H2O2 per gallon of H2O.
- To treat root rot, add 15 mL of H2O2 per gallon of H2O daily until the problem is resolved.
- Standard and Warm Water Tank H2O2 Application - add 5 ml of H2O2 per gallon of H20
- For maintenance, add 2.5 ml h202 per gallon of H2O.
Yes, that is bud rot!. Peel the rot out and look for caterpillar(s) inside the bud and around it. Check the entire plant for more rot. You may have only one or two caterpillars around or maybe more. Keep a close eye. Check your plants daily. Don’t spray during flower. If you see a pillar, pluck it out and terminate the bugger.
WELCOME aboard 
…@Manuhi …
Yes, welcome, like Al G said.
What they said 
Welcome aboard. Wealth of knowledge here, ask and you shall recieve!