Wet soils, heavy rains and powdery mildew can result in yellowing foliage. Cool wet soils can limit a plants ability to take up needed nutrients. In addition, heavy rains can leach (wash) the nitrogen through the soil. You should see some improvement as the weather warms and dries. Try fertilizing plants with yellow or pale green leaves in early August. Use a low nitrogen slow release fertilizer to avoid damaging the stressed roots. Northern gardeners should fertilizing all perennials, trees, and shrubs in early August. Late season fertilization promotes late season growth that can be winter killed.