Double xl Watermelon Weddingcake Autoflowering

Up until now I thought everything was going well - and that was the first time... well, since the second to last watering, the right plant has been showing signs of rust on its leaves; I find it strange that the rust spots are running from top to bottom!? The plants had been given the same compost tea the day before as the last times. I'm a bit confused - I think it could be a fungus because of the twice-daily watering. I stopped watering in the morning yesterday - now I give each plant 1.5L of water with 3ml calmag every day instead of 1.5L 1h after sunrise and 1L before sunset. Have any of you ever had something like this? Could it be due to a one-off incorrect pH value? I mix the compost tea with osmosis water and do not regulate the pH value. From day 50 I only add water with CalMag at the movement.