Light are too close, causing too much heat. Make sure there is goo air flow between the top of the canopy and the light bulb.
Get a Tablespoon of kelp meal, alfalfa, yucca and/or aloe, steep it in water, drain, and foliar your plant with them. This will make them more heat resistant and less prone to react when things get hot.
Also may want to check into some potassium silicate products(protect, armor si, agsil) this is the bees knees when it comes to stress/heat/mildew resistance.