Are rust spots a concern for harvest?

This looks like an issue with deficiencies, not necessarily because the nutrients ISNT in the substrate but because the roots aren't in the right environment to uptake them properly. Try watering with PH-balanced water (COCO & perlite id run low 6PH) test the outflow, if it's within the range you want, don't do anything. Again, make sure you have 100% saturation. Some comments are right about fade, but this isn't that. This is an issue with your rootzone. To answer your question about damage yield and potency. At this point, yes, but don't get flustered, you can still get some great flower out of this plant! If my theory is right about the rootzone environment not being ideal (lack of water to dilute and allow filtration of the nutrients), and let's say you have a lot of nutrients in the substrate, there are multiple studies that show that a plant (especially cannabis) will yield lower but have a more "Premium" flower.

Rust spots on fan leaves during the later stages of flowering can be a natural part of the plant's aging process, commonly referred to as the "fade" or senescence. This is when the plant redirects nutrients from older leaves to support bud development. Given that you're in the 5th week of flowering and using organic dry amendments with a pH of 6.3, it's reasonable to assume this could be a normal to happen. If the rust spots are uniform and not spreading rapidly, it might be a part of the natural aging process. Generally, it shouldn't significantly impact yield or potency, especially if it's occurring closer to harvest. Monitor the overall health of your plants, and if the symptoms worsen or spread rapidly, consider a closer inspection and potentially light adjustments to your nutrient regimen. Always observe closely as harvest approaches to ensure the best possible outcome.

Completely normal part of fade, as nutrients become less available you will start to show signs of deficiency. If you plan on stretching your girls to weeks 13 14 or 15 then you might want to think about adding a top dressing. But if these gals will be done within the next two or three weeks then i would continue as normal and harvest without worrying. All my fan leaves turned yellow brown and eventually crumbles Completely dry when I harvest. I run in coco and go 7 to 10 days water only. Before I even start the water only most leaves are already yellow. So it's normal part of fade. Please consider selecting me as the answer so I can be entered for a chance to win grower of the month contest 🙏 please and thank you 🙏 😊