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To find out why your pieris is losing its leaves, you need to do some detective work. The most usual causes of pieris leaf loss are pests, disease, lack of nutrients/alkaline soil, and sunlight.
The main bug that’s going to cause your pieris to lose its leaves is the Pieris Lacebug. This is a winged insect that’s named after the pieris shrub as it usually feeds on that. (Well, it does hang out on rhododendrons a bit too.)
Look for your fallen and falling leaves to be pale with a bleached out colour. Turn the leaves over and on the underside you’ll probably find the Lacebugs. My article Pieris leaf problems goes into more details about this and how to eradicate these bugs.
Fungal leaf spot is the most usual disease that causes a pieris to lose its leaves. Once again, check the leaves closely. You’re looking for brown spots that get larger and larger over time. Eventually, the leaves become completely brown and fall off. I give instructions on what exactly to look for and how to treat this fungal infection in the article Pieris Forest Flame Diseases.
Lack of nutrients
Your pieris plant needs nutrients to produce enough energy to fuel its health and growth. If the leaves turn a sickly yellow colour before falling off, this could indicate that it’s not getting enough nutrients. Add some ericaceous compost to introduce the minerals and compounds that the soil is lacking.
One reason for the lack of nutrients could be that your soil is too alkaline for the pieris to grow well in. You need an acidic soil. Even if you improved the acid level of an alkaline soil by adding an acidic compost, over time all those acid nutrients become used up and the soil revert to being alkaline. If you do start with an alkaline soil, you need to regularly (perhaps yearly) add acidic/ericaceous compost to keep the acid level to the right amount.
If your Pieris is grown in a pot, ensure it is planted in ericaceous compost as this is acid and a mist for lime hating plants such as pieris.
Amount of sunlight
Your pieris plant needs a sheltered location in partial, or preferably dappled, shade. If it receives too much direct, strong sunlight, the leaves turn yellow and the pieris starts losing its leaves. Check how much shade and sunlight your pieris shrub receives, especially in the peak hours around noontime.