What could cause this??

[Rrains007]

Hey guys been growing for a long time and I’m kinda stumped on what could cause this. I’m week 3 in flower and all of a sudden only one of my varieties starting having their fan leaves bend inwards and curl down. I also noticed real tiny yellow spots on some of those leaves. The only difference I made was switching dirt from pro mix to the pro mix hpcc which has 30% coco and the rest peat based. I keep my room at 75, 71-72 degrees on the leaf temp, 50 percent humidity, ph at 6.0-6.1 and 1400ppm co2. Any idea what this could be?
EDIT. I just called Athena whose nutrient line I use. They had me flush one out. My ec was 4.0 but my ph was 6.1 at runoff. They think I don’t have enough runoff between waters causing this.

[SoOrbudgal]

The more i see folks using the pro mix HP the issues seem all the same. I do not like it or using peat compacts if it was on sale i'd put it outdoors in my raised beds. Coco coir not peat and a light fluffy no slow release nutes in the bag. I'm sorry for your struggles with it but i've seen it all to often i would not recommend it.

[Tobycat]

I’ve had a similar problem in the past and I was told that it was the promix being “too hot” causing the issues. I laid off the nutes for a bit when I had those issues and the plants recovered.

[3Berries]

I use ProMix BX with the myco and it has very little nutes. Basically soil less.

[Bluey]

I have 100% coco, typically no run off and have only flushed once at flip..and only a 50ltr flush in 50ltr pots.

Your problem is not lack of flushing.
EDIT. Running those temps with CO² is likely not at all beneficial.. Temps should be above 32⁰C for CO² enrichment but then you may have root problems if not managed properly.