Looks like a P deficiency to me. P deficiencies tend to show at the end of the leaves, which clued me in.
I pulled the below off of one of the many plant problem identification sites, there are a bunch with the same info, but
this is the one I got it from. Maybe that site can help in the future.
GS
Figure 12 shows Phosphorus (P) deficiency during vegatative growth. Many people mistaken this for a fungus, but look for the damage to occur near the end of leave, and leaves the color dull greyish with a very brittle texture.
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