I’ve got 3 White Widows about to 4½ weeks in veg and overall they look great except for a very few leaves that look like these on 2 of the 3 plants. I plan to begin flowering over the weekend and am wondering what advice the experts might offer to insure I don’t do any more harm.
They’re in a small tent (36" x 20") under a 400 watts of MH light. I’m using Fox Farm soil and nutrients. Last week I bumped up the nutrient ratio up from about half strength to about 3/4, and I’m hoping that might be my problem. The temperature has been pretty steadily in the mid-70’s and mid 60’s during the 6 hours of darkness. Humidity has been a running around 40%. When I go to flower, I’ll change over to HPS light and start the Tiger Bloom. When I fill the my hot tub, I know the water comes out of my well at a ph of around 7.2, and I have not adjusted anything to water these plants.
Any words of wisdom from those who have done this before is greatly appreciated! 
Yes, it could be bad pH, nutrients burn and disease. You need to monitor your well water regularly as it will vary greatly from time to time depending on environmental conditions in your area. What is most important is the pH at the roots themselves. You can check this with a soil probe or by testing the pH of the run off after watering with a digital pH pen to have a better idea of what is happening at your roots. You need to be sure what the pH of each watering is or what your nutrient mix is before you send it to your roots. You should be monitoring the run off for nutrient concentration as well, testing its EC/PPM with a digital TDS pen or again you can get cheap soil probes to check this as well to be sure nutrient salts haven’t built up to too high of levels in your soil. If EC/PPM levels are very high you need to be using lower strength mixes and/or feeding less often between the regular watering. You also need to be sure you are not over watering as this can cause nutrient imbalances similar to the way pH can cause an imbalance, lockout or toxicity. Also over watering, keeping the soil too wet, too often, and for too long, can cause disease. Cannabis roots like lots of airflow or oxygen at the roots. You should be letting the soil nearly totally dry out between feeding or plain waterings. Get used to the feel of the weight of the plant and its container with totally saturated water and also almost totally dry to have a better idea of when next to water. Keep an eye on the leaves, they will tell you quickly if it is getting too dry by getting kinda droopy, this is not bad for the plant as long as you don’t let it get too droopy or stay like this for too long. And before you know it you will not have any problems with overwatering or underwatering and you’ll be able to tell what your plants need by the look of their leaves and be able to know it’s time to water without an average person noticing any sign of leaf wilt.