I have had previous problems with Mango #1 and posted here a while ago, link here
I have 3 mango trees I am growing from seed that are approximately 6 months old now. They all have the same watering schedule of approximately 5-7 days per watering. They've only been in 68F (20C) - 74F (24C) their entire life in a grow tent with a 2000W grow light approximately 38-44 inches away from foliage. There are 2 auto-rotating fans in the tent. The soil composition has been 6 parts Miracle-Gro soil for Trees, Shrubs, and Bushes to 1 part coarse sand.
Mango #1 has displayed these grayish-orange spots for about 2 months already and have been getting worse over time. I've thought to this day that it is a fungal issue so I bought "Serenade Garden AGRSER32 Disease Control Effective Organic Fungicide" and been applying to foliage with a sprayer as directed by the bottle for about a month and a half. I haven't noticed much happening but at the same time I don't know what I should expect to see to know "the disease has been controlled" as I wouldn't expect the affected foliage spots to recover.
A month and a half ago, I gave 2 tablespoons of "BGI CitrusFood" fertilizer to all three of my mango trees. For some stupid reason I thought it would be a good idea to give an extra tablespoon to each of my mangoes 2 weeks after the first fertilization. I noticed a few weeks later that on mango #1 and #3 some of the tips started to blacken and it slowly grew. From some research online it looked like it was salt toxicity (or so I think). To try to solve the issue I tried to heavily water them when it came time to water them on their schedule to drain the soil so salt wouldn't sit. That wasn't working. I then tried to give a distilled water flush but that didn't seem to change the problem either as the problem kept progressing as the days went on.
A few days ago I decided I had enough and I transplanted only #1 and #3 into brand new soil which is 6 parts "Miracle-Gro Moisture Control Potting Mix", 6 parts "Kellogg Organic Plus Plating Soil for Trees, Shrubs, and Roses", and 1.5 parts desert sand.
As of a few days ago #2 has started to show one little black tip on a leaf and another leaf indicated by the red circle showing a larger blackening effect. I'm thinking of transplanting #2 into new soil as well as I don't want to increase the severity of what I believe is a salt toxicity problem in the soil.
Mango tree #3 had 5 leaves prior to the second fertilizing round in the old soil and 2 brand new leaves. (Sorry I don't have pictures for this) Typically when new foliage grows, it starts off as a deep purplish-red color and slowly progresses into a mature green foliage however these two new leaves didn't have any purple and grew into a similar sized foliage as the remaining two on the stem in the picture album. The tips started to curl and brown/blacken until the two new grown leaves fell off the stem. The entire leaf was a very light greenish yellow with all the veins of the leaf having a healthy green color. There were no abnormalities with the vein sizes. The leaves prior to the new foliage falling off was starting with the oldest leaves first if that helps. I believe it is the same "salt toxicity" problem in the soil but would really like some reassurance that I'm on the right path with saving #3 since it is in the most vulnerable position.
~Does #1 have a fungal disease besides the blackening tip problem or do I have it completely wrong and it is a nutritional problem instead?
~Are the blackening tip/curling leaves problem with all three of my mango trees a salt toxicity problem or is it another problem after all?
~Anything I can do to increase the chance of Mango #3's survival?