Why does ASR usually appear on the lower leaves first?
The lower leaves experience longer periods of leaf wetness, higher humidity, less exposure to UV light, and cooler temperatures during the day and warmer temperatures during the night. All of these factors favor rust development. However, climatic conditions in Virginia make it likely that other areas of the plant will experience six to eight hours of leaf wetness and the optimal temperatures for ASR development. Rust also tends to develop more rapidly on older leaves. Therefore, leaves originating from the lower part of the main stem will likely show more disease than leaves from branches, regardless of their vertical position in the canopy. Therefore, scouting efforts should be focused on the lower leaves originating from the main stem.
