I am going to guess that you still have an abundance of bacterial spores in the soil and the fact that you have a sprinkler system leads me to believe the soil is kept too moist for the impatiens, begonias and Japanese maple but the hosta, camellias and azaleas love moist soil so they are thriving. You can test my theory by removing some of the begonias now and planting one or two in a container of potting soil. Do not overwater but water when the soil is just turning dry to the touch. Now compare with the begonias in the ground that get water from your sprinkler system. If the fresh potting soil and less water make your begonias perk up then overwatering is the problem. Plants that are susceptible to mildew and leaf spot such as begonias and impatiens do not like overhead watering. Japanese maples do not like wet soil as it causes root rot. An easy answer could be plant more hosta and azaleas in this area or to turn on the sprinklers less often. Keep growing, Marianne Binetti