I collected a small ponderosa pine in August 2019 and it has done well overall. I wintered it on a heat bed and there are roots appearing from the bottom of the container. However, very recently, in the last few weeks, I noticed some browning of the foliage on a lower section of the tree. The apex, and other parts of the lower section are still looking healthy. Does anyone have an idea of what this is, how I can fix it, or at least prevent it from spreading to other sections? Thank you!