Deciduous Bonsai trees or nursery stock sometimes suffer from burnt leaves starting in mid-summer when temperatures are are between 90-100 degrees. Here is a great article that discusses how to prevent burnt leaves where you live.

The Bonsai Art of Japan – Episode 42  by Bjorvala Bonsai Studio  Great video on how a Japanese Bonsai nursery protects their trees during the hottest hours of the day.

Crataegus Bonsai by Michael Hagedorn: How to Avoid Burned Leaves on Deciduous Bonsai

Bonsai4me: Heat and High Temperature Damage to Bonsai