Go easy on the Cal-Mag supplement at this point in flowering. I will explain why at the bottom of this post . . .
You have a cal-mag deficiency for sure. Middle leaves (semi-mobile nutrient deficiency), intervenal chlorosis (Mg) and rust spots (Ca).
There are four possible causes, two of which have been listed already:
* Not enough transpiration: calcium uptake is controlled by transpiration in a feedback loop – evaporation at the leaf surface draws calcium-rich nutrient into the plant, and calcium controls stomatal opening, which in turn regulates transpiration. As others have pointed out, LEDs have less IR which means lower leaf temps, which can lead to lower transpiration if ambient temperatures are also low.
Solution: increase ambient temperatures
* Not enough alround nutrient: LED has a better spectrum than HID which can lead to faster growth, meaning more nutrient requirements. Lower transpiration also means higher ECs are required for the same nutrient uptake. Cooler weather (less transpiration) also requires higher ECs.
Solution: raise EC levels (hydro) or organic fertilisers
* Low pH will lock out Ca and Mg. Generally you want a higher pH during veg for this reason, as the plant uses more Cal-Mag in relation to PK, which is facilitated by a slightly lower pH. However, soil is a much better pH buffer than hydroponic media (coco is somewhere in the middle), so pH problems are rarer in soil than hydro, but not uncommon in coco.
Solution: if your pH is too low, raise it slightly. Add lime to organics to raise pH and boost calcium and magnesium to the mix
* Cal-Mag lockout during flowering: Some growers overdo the PK too early in flower. Potassium (K) competes with calcium (and vice versa). Plants still need calcium, magnesium and nitrogen – especially during the stretch period – before the full onset of flowering. In fact, plants tend to need more calcium during the initial stages of stretch because they are building a lot of cells (leaf and stem mass) to create bud sites. Nitrogen deficiency is also common during this period, and nitrogen facilitates magnesium uptake, which can lead to a deficiency.
Solution: Maintain Ca, Mg and N levels in early flower. Maintain slightly higher pH until after the stretch. Do not boost with PK untikl the full onset of flowering (around the beginning of week 5 in 8-week strains). Pretty much all Cal-Mag supplements contain nitrogen (calcium-nitrate and magnesium nitrate being the common ingredients), so if you hit your plants with a Cal-Mag supplement at the begining of flowering, that will also take care of the nitrogen requirements.
I am leaning towards the last cause as being your issue. Especially if you have any coco coir in the mix.
Take heart, even though you have a deficiency, it could be a lot worse! So I'm guessing you only need to add a little more Cal-Mag at the right time (or change you nutrient regime and/or organic mix) to get back on track. Your plants don't look too bad.
HOWEVER, you are now in the middle of flowering, so if you add too much Cal-Mag at this stage, you will lock out potassium (K) which will affect flower development, which will impact yields.
Best to learn from this grow and improve the next.
