,,rust" spots advice needed

Organoman over feeding with what pray tell ?

yep P,Ca deficiency

Cal mag and the problem is gone ! :)

could also be low P or something locking P out. coco-mix? is that soilless? If Ca ppm have been 100+ in your fertigations, i'd lean P ... P is fine around 40-60 ppm -- again, this assumes soilless method of growing and fertigating every irrigation with 10% runoff. if it is soilless and not religiously stickign to the 10% runoff minimum, then i'd look into lockout. Download an image of "mulder's chart" to see all the possible causes of lockout due to levels of other nutrients. The clarity of dealing with problems of a proper soilless or hydro grow makes things easy. More fog of war if you have to consider what amendments are adding to the mix, too. Calcium should be part of the full and complete diet and not just added here or there. Your tap water may impact exactly how much you need to add to avoid deficiencies and toxicities....

It could definitely be CalMag-related – the rust spots are a classic sign. But before upping the dose, check the pH of your runoff water to make sure your plant can actually absorb CalMag. In soil, aim for a runoff pH between 6.2 and 6.5. Even if you’re feeding the right amount, a pH imbalance can block uptake. Also worth checking: salt buildup in the soil or excess potassium can interfere with calcium/magnesium absorption too. If in doubt, a light flush and recalibrating your pH and feeding might help! You’re on the right track – just dial in the details and your girl will thank you.