What is the reason why the high-temperature leaves of Lagerstroemia indica will flush white cloth in summer?

At the beginning of the disease, white spots appeared on the leaves, which were reticulate after unfolding. The plants affected by powdery mildew will become shorter, the tender leaves will be twisted and withered, the leaves will become smaller without spreading, and the branches will be deformed. If it is serious, the whole plant will die. Small packages of pesticide treatment can be bought in the flower and bird market.