Problems are more likely to occur when a plant is stressed by its general condition. A plant that receives inadequate moisture or too much sun is less able to put up a resistance to the onslaught of insects. And those that are planted in too shady a spot, are planted too closely together or get watered in late evening are more likely to suffer from fungi. To minimize problems with fungi, make sure the plants are mulched, have plenty of air circulation and are watered in morning. Try to keep them outside the drip lines of nearby trees and remove the severely damaged twigs and foliage.
Many insects feed primarily on just one genus, or even species, of plant.
For instance, the rhododendron lace bug, Stephantis rhododendri , feeds almost exclusively on rhododendron. While its cousin, S. pyrioides , feeds mostly on azaleas. Large, single-species groupings of plants, so popular with landscape designers, are a sure way to create problems. Within a few weeks they will have a permanent footing and you will be locked into periodic sprayings of insecticides. By diversifying your plantings you minimize these risks.