Black/Brown Dots

Nutrients are classified as either immobile or mobile depending on if it can be moved around the plant once it’s been fully absorbed. Mobile nutrients are stored in older leaves and can be moved around the plant wherever it’s needed. Immobile nutrients stay close to where they were originally deposited. Deficiencies & excess of mobile nutrients will appear in older growth and leaves first. Where as, deficiencies & excess of immobile nutrients will appear in new growth first usually at the top of the plant. Once you understand the difference and which nutrients are mobile and immobile, it makes it easier to diagnose & fix deficiencies. Whenever you suspect a deficiency, always check your pH first and ensure that it is in the correct range. If it isn’t within the correct range, it won’t be able to absorb some nutrients. So, you could be treating a deficiency, but if the pH is off, it won’t be able to absorb the nutrient still. If your pH is in the correct range, you can now try to figure out what’s going on. NPK the big 3 (& magnesium) are all mobile nutrients so the plant can take nutrients from old growth and use it for new growth if there is a deficency. Since growth is only affected up top it tells us it's an immobile nutrient. Does look like potassium but it would be all over if it was. It is crucial for cell integrity and growth. Calcium helps with the flow of nitrogen and sugars throughout the plant. Deficiency usually happens with hydroponics or outdoors in very wet, cool conditions with acidic soil. First sign is distortion and curling of lower leaves. This is followed by irregular yellow-brownish spots with brown borders that grow over time. Finally, root tips will wither and die resulting in a stunted plant with low yields.

Is it all over or just the top of canopy>?

Looks deficient phosphorus or potassium? Plants can get these problems nearer the end of flower, try flushing and giving a low dose of NPK plus a compost tea, or biosys microbe tea, usually works , hope it helps, happy growing ✌️🌱

Just calcium lockout because of too high N feeding ... Or ph issue, not in range, usually too low if leaves goes to yellow and has rusty spots .. Some people grow rot, do mistakes and offer wrong ways ... If you in soil - never flush to sort problems !

Usually if there's not a clear answer looking at leaves, it's pH or watering issue in my experience. Doesn't hurt to do a little flush and see how she responds!