Japanese andromeda has relatively few problems caused by pests or disease and most can be avoided by soil that drains well and protection from strong sun. Keep in mind that a single distressed plant may have multiple problems simultaneously, and display symptoms of diseases, insects and of poor care and conditions.
This soil-borne disease is caused by any of five Phytophthora water molds that affect Pieris japonica in poorly draining soils. In wet, warm weather, Phytophthora is propelled from soil to leaves by rain or overhead irrigation, killing leaves and twiggy tissue.
Properly draining soil conditions are the best tool of prevention. If plants are affected, remove them and do not replant the area until drainage conditions are remedied.
Phytophthora can also cause root rot of Japanese andromeda and other ericaceous plants, infecting the fine roots and spreading from the roots to the base of the plant and sometimes up the woody stems. The infection may not be noticeable until weather becomes warm and stressful for the plant.
Roots become soft and brown. The infection can cause a blockage of water and nutrients through the stem and is usually detected when leaves begin to turn pale, droop and wilt.
Properly draining soil conditions are the best tool of prevention. Your county's Cooperative Extension office can recommend a diagnostic lab to confirm the disease before you decide on a management plan. If plants are severely affected, remove them and do not replant the area until drainage conditions are remedied.
The stunt nematode (Tylenchorhynchus claytoni) is a worm-like, parasitic animal that requires damp conditions to propel itself. It can occasionally cause damage to the roots of Japanese andromeda plants, stunting growth and discoloring leaves. Symptoms are apt to be noticed only after much root damage has occurred.
This pest, Stephanitis takeyai, is a problem specific to Pieris japonica though other lace bug species affect rhododendron, azalea, willow and other landscape shrubs. Originating in Japan, the insect was introduced in the eastern United States in the 1940's. It can be hard to detect early as the insect spends most of its life on the underside of the leaves. Eggs are laid in the midrib on leaf bottom. Up to four generations per year hatch in our area from May to September. Infestations are more likely to be severe in hot, sunny locations.
Nymphs and mature insects feed on leaves by sucking fluids from them, creating a speckled appearance. A home gardener is most apt to notice the problem in May, on new leaves. If the insect population is dense, the speckles begin to merge and entire leaves turn yellow and drop off. Plants may look dull and ragged, lacking strength. You may also notice the black excretion (frass) of the insects on the underside of leaves.
Lace bug can cause severe damage if unchecked to the point of significant leaf loss. The plant will decline in vigor and can die in extreme cases. The egg stage of the insect's development is protected so horticultural oils and insecticidal soaps will have an impact only if the mature insects or nymphs are targeted and thoroughly covered. Give particular attention to spraying the underside of leaves where insects are most populous. If you maintain a chemical-free environment, diverse in plant life and beneficial insects, the lace bugs are typically controlled by natural predators.
Dirr's Encyclopedia of Trees and Shrubs
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