you I believe are in nutrient lockout, you have other issues I see other than the brown patches, potassium , magnesium & nitrogen are all beginning to show signs, if I am correct the brown patches may be attributed to a phosphorus def in the vegatative state ( which is a different looking pattern in flowering cycle). you need PH test equipment, chemical PH test or digital PH meter also PHup & Down Correcting Solutions....for emergency you can use vinegar to go down and baking soda to correct up.... Complete a soil runoff PH test....... with PH corrected water (6.5-6.8PH ) pour enough water in the pots so the water runs out the bottom of the pot ...test the PH of this water.... If the PH reading is out of optimum range 6.5-6.8 PH, you need to flush your plant. Flush with PH corrected water, till in optimum PH range, 3 times the volume of the pots rule of thumb but may require much more flushing if the rootball is thick & sour EG: 1 gal pot = 3 gal. flush
I think you may have mold or some other fungi. I'd need better pictures to be sure. If it is mold or the likes, spray some epsom salts on it and get your environmentals right. Molds/fungi do not like the Sulphur in it.