Leaves yellowing and narcrotic

Noticed a few days ago that some of my leaves starting to die. Cant really pin point what the problem is. The effected leaves are toward the top of the plants. The rusting starts in the middle of the leaves and works its way out and towards the tip. After changing color they curl upward and dry out to a near crisp. This all happens in the matter of a few days. Plants are 5 weeks into flower. Grow room conditions are pretty good. I was thinkin maybe a PH issue??? Can anyone clarify what this is?

What is the pH? I just went thru that type. My pH was good but had some build up. Flushed and gave calmag and P. Alls good now.

Im not sure of the exact PH. I only have one of those cheap General Hydro PH Up/Down liquid test kits. Add a few drops and match the color chart. With this method the PH looks to be in the right range but Im testing the runoff water, which has a slight color already after going thru the soil. Dont know the PH of the soil either. Its a newer brand (Empire Builder) and I couldnt find any info yet. Gonna invest in a nice Hanna tester soon. Just flushed a few weeks ago. Not using any nutes, so I dont think it would be salt but it nothing seems to surprise me with these plants. I thought about a Cal/Mg deficiency. I have some liquid Cal/Mg so maybe I will just throw some of that into my next watering.

Thanks for your two cents Coho. Its much appreciated.

Ur in the 5th week of flower. have u lowered the N and increased the P & K. Looks like a def in those two. Need a picture of the plant really.

Did the colour of the drops stay the same? is green/yellow?

Did you use the rec'd dose on the bottle of nutes?