I saw the same post about canna rust. Underside of leaves look normal. Think it must have something to do with bringing lights closer. Lower leaves show no black, Middle leaves - barely any black, Very Top leaves - maybe 60% of leaves are 20-30% black on topside. Plant looks healthy and green.

What is your experience level? Amateur

.1) Type and wattage of lights. HPS 2x250W
.2) Distance from tops? 15"-16"
.3) Reflector type? Bulb, Hood: aluminum sheet metal painted white & area surrounded w/thermal reflective sheets.
.4) Is there a consistent fresh air supply? Yes
.5) Do you have an exhaust fan and a circulation fan? Yes
.6) What are the bulb wattages, kelvin ratings, and schedule? 2x250W, 12/12

.7) Specific brand and type of soil. Foxfarm soil w/some manure, perlite, sand added
.8) Size of container? 3-4 gallon
.9) Did you use peat pucks (or similar) to root clones or germinate seedlings? No

10) Source of water? Distilled (bottled)
11) Method of checking water ph. Pen
12) Method of adjusting water ph. add Ca (dolomite lime) to raise (Haven't needed to adjust pH until this week.)
13) Specific brand and N-P-K ratio for each bottle. Foxfarm nutrients per feeding schedule.
14) How often are you watering between feedings, and how much per watering? Water once between feeding, water until runoff.
15) Any additives or tea's? (Superthrive, CalMag, molasses, Mother's Earth...) No
16) Are your ph levels stable, or do they fluctuate? Stable, around 6.4-6.6
17) What is your ingoing water's ph? ...your runoff ph? 7.0, Runoff - never check.
18) Do you foliar feed? If so, with what, how often, and at what time do you spray? No

19) Indoors or outdoors? Indoors
20) What size of closet, room or hut? 10-11 sq ft
21) What are the temps and humidity levels? DAY: 70 deg, 30-50% Humidity, NIGHT: 62-64 deg, 30-50% Humidity.
22) Have you seen signs of insects in the growroom? No

23) What strain are you growing? (Indica dominate or Sativa dom?) White widow
24) From seeds or clones? Seeds
25) Is this an autoflower strain? No