yellow spot on ascocentrum ampullaceum/ and black tip roots

[mollyminiveg]

Hi,

My Ascocentrum ampullaceum "Mr. Del Rei" has been having 2 problems.

1. A yellow spot has developed on one leaf in the last 3 days, and I'm wondering why? Conditions have stayed the same. 75 degrees F during the day/ 50 F at night. About 80% humidity. Full sun from 3-6 pm (when it's sunny). I fertilize lightly once a week and use a drop of superthrive with the fertilizer. I water with rain water usually every day. Sometimes I mist during the day so maybe it's a sun burnt spot???

2. I have two new roots that will grow well for a while then get a black tip. Then it will start to grow again with a healthy green root, and then turn black again. Right now a healthy one is continuing from the black root tip, but I'm wondering why this cycle keeps happening.

Any Ideas?

Thanks