PH Problems?

Is this a pH issue? The strain is mango, using 1/4 strength grow solution (400 ppm), and 1 tsp/gal vinegar for pH. 2 plants have these spots. The problem also appeard 10 days in when feeding the plants with pure distilled water. I realize i need to buy a pH tester anyway, but any suggestions?

looks like ph problem but otherwise looks good.

So I did some math, and 1tsp/gal vinegar in distilled water will give you a pH of 3.9. Oops.

Now mixed 1tsp vinegar in 500mL of water and then took 1tsp/gal of that solution and added it to the water for the plants. Should result in a pH of 5.1