What Disease is on my Mango Plant? Are there any remedies to cure it?

Question

I'm growing a mango tree and discovered a few days ago some oily dots on the top of a lot of the leaves and on the back of those specific leaves were black patches scattered around. A few isolated cases have arose with a white patch and another black patch right next to it shown on the top of the leaf.

My mango sapling is currently in a sandy soil I made for it with 2 parts "Miracle Grow Soil for Trees, Bushes, and Shrubs" and 1 part coarse sand. I've consistently stuck to my schedule of watering it every 4 days. It has been only in temperatures of 68F - 75F (20C - 24C) ever since germination. it gets 10 hours of light under my 2000W grow light in a 4ft by 4ft grow tent (the space is not occupied with any other plants except 2 avocados in cups of water that I'm trying to germinate) with two oscillating ground fans and has responded great to it.

From the research I've done, it looks like it could be an early formation of powdery mildew or anthronose, maybe both or something else too? What is my mango tree infected with? What can I do to treat/cure my tree so it can recover?

Answer 1

The appearance of the leaves which show intermittent patches of light and dark greens hints at a nutrient issue. Mixing the prepared soil with a significant proportion of sand, while improving drainage, does dilute the nutrient that the rest of the soil offers. Juvenile plants are adequately nourished by the large fatty seeds, but once that has been exhausted the soil must take over. A nutrient issue might explain some of the spots; many of the minor elements (in quantity) are nevertheless essential for leaf health.

I would imagine that the humidity in the tent is quite high; this is good, but with generous watering comes sweating of the plant (guttation) and any sugars expressed by the leaves may attract fungi which can colour the spots but otherwise do no damage to the plant since the surface cells are still intact and able to defend themselves.

A weak feed with a broad spectrum fertilizer now and then combined with a more relaxed watering schedule might be all that is required.