Pieris Japonica is a low growing flowering shrub with a variety of cultivars. Some of these varieties have naturally pale leaves. However, if your Pieris japonica's leaves turn yellow, it is a sign that something is wrong. Pale leaves can be caused by a number of problems: insects, disease or inappropriate growing conditions. Pinpointing the exact cause of your plant's pale leaves will take a bit of troubleshooting.
Check the underside of your plant's leaves for insects. Pieris japonica is often afflicted by lace bugs. These tiny, flightless bugs have lacy wings, are about 1/8 of an inch long and white to pale brown in color. Their nymphs are spiny and darker than the adults. When lace bugs feed on Pieris japonica's leaves, they leave what looks like bleached spots behind. If present in large enough numbers, they can make Pieris japonica's leaves look pale. Control lacebugs by spraying the plant with a pesticide prescribed for use on these insects.
Test the soil's pH with a home pH testing kit. Pieris japonica needs acidic soil to thrive. Pieris japonica grown in alkaline soil often develops chlorosis, which reduces the amount of chlorophyll in its leaves, giving them a pale appearance. If the pH is above 6.5, you will have to lower it by adding sulfur to the soil. The amount of sulfur will depend on the type of soil that your Pieris japonica is growing in. Light soils (like sandy loam) require around 10 lbs. of sulfur per 1,000 square feet to lower them one pH point. Heavier soils will take between 15 (medium loam soil) and 20 (heavy clay loam) lbs. of sulfur to lower them one pH point. Or, consider replacing the soil with an acidic commercial potting soil like Acid-Loving Plant Mix.
Move your bush out of direct sunlight. Pieris japonica in native to forests where it benefits from the shade of surrounding trees. Too much sun may cause your bush's leaves to become pale. Partial sunlight is best for these plants.
Check your Pieris japonica for root or crown rot which can cause its leaves to become pale. Uproot the bush and examine its roots. If they are black, rotten or easily broken, your plant likely suffers from root or crown rot. The plant and the surrounding soil will have to be discarded. Before replanting in that area, improve the drainage of the soil by adding sand. Pieris japonica with wet feet is more susceptible to the fungus that causes the disease.
Water your Pieris japonica less frequently. Overwatering can cause pale leaves and root rot. Your bush should receive infrequent, deep waterings (water should be given slowly to allow it to penetrate about 2 feet into the soil. And the top 7 inches of the soil should be allowed to dry out between waterings.