With a manganese deficiency, the yellowing will begin at the base of the leaves and move outwards towards the tips

If you suspect your growing cannabis plant has a manganese deficiency, flush your system with clean, pH’d water that contains a regular dose of cannabis-friendly nutrients that includes manganese. This will remove any extra iron or nutrient salts that may be affected the uptake of manganese, it will help restore pH to the proper levels, and will supply the plant with any missing nutrients.

NOTE: After a manganese deficiency is cleared up, the problem (brown spots and yellowing leaves) will stop spreading to other growth within a week. Leaves which have been damaged by a manganese deficiency will not recover or turn green, pay attention to other growth for signs of recovery.