There is white mold powder on the tender leaves of Lagerstroemia indica. What happened?

There is white mildew powder on the tender leaves of Lagerstroemia indica, which should be powdery mildew of Lagerstroemia indica.

Powdery mildew is a fungal disease, which mainly harms leaves. Young leaves are more susceptible to infection than old leaves. The disease also harms branches, shoots, buds and buds. At the early stage of the disease, small white speckles appeared on the leaves, and after expansion, they became round or irregular faded spots, covered with a layer of white powdery mildew, and the layer of white powdery mildew turned gray in the later stage. After the flowers are infected, the surface is covered with white powder, and the flower spikes are deformed, losing ornamental value. 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.

The whole plant can be sprayed with 70% thiophanate-methyl 1000 times solution, 20% triadimefon EC 1500 times solution or 800 times solution of 50% carbendazim wettable powder. Once every 7 days, 2-3 times in a row.