I`ve got the same going on 1 out of 4 of Red Poison auto`s...had it early on in flowering and initially thought Rust which auto`s can be prone to but it wasn`t spreading and there were other indicators. Looked like a CalMag issue but my water is high in both and I have to treat for that and eventually found it was a Ph issue with the drainage Ph coming out much higher than it went in due to the method I used to initially lower the Ph not being very stable. Got a good Ph Down and no issues from then on though it took a couple of weeks to get the soil ph right. Here`s the Red, she`s at the end so leaves only hint at what I`m talking about but you can see some of the markings....pleased with this strain as only grown under 12/12 as I grew them alongside some photo`s. So mate, reckon it could be a Ph in and out job....possibly. Take care mate