do you have that area completely covered? if so you need proper ventilation to let out excessive heat, also a fan would help. its either that or the roots cant breath for some reason, you might be watering it too much, let the surface dry out to where u can stick your finger in a about an inch and its dry, then rewater. what type of soil are you using? *edit* now that i look at it it looks like heat stress. oh and i love the basketball lolol in every pic the soil looks wet though, so i would ventilate and hold off on watering as well, its much more easy to overwater, then to underwater