Here summer is hot and humid: high 90's in the day around 80 at night. Every morning starts out near 100% humidity and drops slightly as the temperature rises. For whatever reason I don't have powdery mildew at all, nor do I have Japanese Beetles or even aphids (on the roses - they prefer Hibiscus it seems). I do however have plenty of black spot! I do no spraying or other pest/disease prevention, and since I have concentrated on the old garden type of rose, they all seem to survive and bloom well enough to make me happy.
