Sick Pieris japonica needs help
I have a large shrub, a pieris japonica, in a shady spot. It has been on the property at least 40 years. It is losing leaves. Some branches are now bare. One observer suggested it might have root rot, but I don't know how to address or confirm this. How can I bring it back to its usual healthy state? Thank you, Donna Scarboro Takoma Park
Montgomery County Maryland
A degree of root rot is a possibility since Pieris and many of their relatives dislike soggy soil and poor drainage. It appears that the nearby downspout empties into the root zone of the Pieris (it's hard to see clearly in the photo). If so, the excess moisture may have flooded the root zone enough that roots were deprived of oxygen too often and have since died back or succumbed to opportunistic infection. The springs of 2018 and 2019 were excessively wet, so this very well could have contributed to some root dieback. While root rot is untreatable, it can be partly avoided in the future if the extra water is directed elsewhere. The plant would have to regrow healthy roots in order to recover and replace leaves, which it could do if it has enough vigor left. Root loss is one possible reason for the branches that have lost their leaves.
You can use a "scrape test" to determine if the leafless twigs and branches are alive or dead; if dead, trim them all off. Sapwood lies just underneath the bark and on living wood should be green. If you nick the bark or scrape off a small section, look for green tissue just underneath. If instead it is dry and white/brown, then the twig/branch is dead and can be removed.
Increasing shade as the plant ages is another possible cause of leaf loss. This isn't correctable without pruning away leafy branches to let in more light, which is not something we would recommend on an ailing plant that needs all the foliage it can keep. Pruning the large mature tree above it is also probably impractical. You can check for signs of lace bug damage on the leaves, as heavily-damaged leaves will be shed by the plant earlier than it normally would for healthy leaves. Stippling is the classic sign to look for on the upper side of the leaf; the insects themselves, or their detritus, will be on the underside of the leaf. Here are a few pictures as it appears on Azaleas and Rhododendrons, which they also feed on: https://extension.umd.edu/hgic/topics/lace-bugs-trees-shrubs
If you are unsure of the presence of lace bugs, you can send us close-up photos of the leaves - preferably the upper and lower surfaces. They do look off-color and paler than they should be, but we cannot tell if this is an artificial artifact of the camera/photo, due to lace bugs, due to ailing roots, or due to nutrient deficiency. Nutrient deficiency in the foliage can result from soil deficiencies, but more commonly is a result of unhealthy roots unable to absorb them. An organic granular fertilizer formulated for acid-loving plants may help if the soil is the issue, though spring may be a better time to apply it than autumn.