Rust spots first, days later yellowing starts

1) PICTURE OF PLANT *** VERY IMPORTANT ***
2) Growing indoor or outdoors
3) Watering schedule
4) Growing Medium
5) What stage of growth

Hey everyone. I found a picture of a very interesting problem with a plant I was hoping to learn the cure.

The plant is 1 of 3 Strawberry Diesel seedlings. The other two are very happy and are in the same exact conditions from day one. Same water supply lighting temps air you name it nothing different.

This plant in particular however shows fatter indica type leaves while the other two are more sativaish so that may play a part in the variation, different needs.

The problem started in the lower leaves. It stayed very green and health (this plant was the one that grew the fastest and was larger than all the others for awhile (now catching up). Anyways, the rust spots started but no yellowing just freckled leaves.

The freckled/rust spots started to move up to the next larger set of leaves at the very end tips. This has been going on for about a week and is very slowly spreading. Only a few days ago did the yellowing start a long side the rust.

The plants are fed a General Hydroponic 3 part schedule. 1/4 tsp per gallon as stated on the back for germination/seedlingts. They are currently manually watered but will soon be on trip irrigation. Manually for the purpose of lowering the ammount of nutes they would get. Small amount of nute/water mix on one day, then plain tap the next. So confused because the other two show no signs of anything similar and are VERY healthy and happy as far as the eye can see.

If anyone has any ideas please let me know I will post a pic below. It seems to be similar to a lot of the Plant MD faq/pics I see but not quite the same. It shows similarities from what i can tell to potassium, manganese and magnisium deficiencies.

The ph is adjusted from tap water (left out a few days for chlorine evaporation) to about 5.8. Ambient temps are about 70-75. RH ranges around 35-45.

If you have any other further questions let me know. Any help would be appreciated.

Looks like mg. Could be potassium also if the calcium in ur tap water is too high. Before I can tell u what to do to fix ur problem we have to figure out what is causing ur deficiency. U should be using some better water than tap (like distilled or RO, or have ur tap water tested) so u know whats in it. Chlorine is not the only harmful thing in tap water.. Too much of any nutrient can cause adverse effects also. If u want to try something cheap for a shot in the dark u can add epsom salts. That is a cheap mg supplement. Add it then re ph ur solution to where u need it. If the yellow turns green in 2-3 days u are good, the brown will not come back, it will die off..