Yellow leaves is a common plant symptom, and could be a sign of many things. Yellowing can be a sign of over or under watering, a nutrient deficiency, a soil pH issue, a plant pathogen, or natural die off in the plant. Looking at the photo, it looks like it is the lower older leaves that are yellowing and that the new leaves are green. The photo is a little blurry, so I can’t tell if the leaf veins are green or also yellow, or if there are any leaf spots, blotches, or other symptoms.

Daphne’s do not like soggy soil, so make sure you are not overwatering. The soil should be moist, but not soggy. Daphne’s prefer a soil pH between 5.5 and 7.0. When the soil pH is out of the desired range, then some nutrients are no longer available to the plant, and you start to see deficiency symptoms. If the leaves are yellow with green veins, then this is a sign of iron chlorosis and means that iron is not being taken up by the plant. The way to fix this is by adjusting the pH of the soil so that the iron can be taken up by the plant. Yellowing of old leaves can also be a sign of a nitrogen or nutrient deficiency. Consider a soil test so that you can analyze the soil pH and nutrient levels. Daphne’s are also susceptible to some pathogens, including Phytophthora root rot, that can cause yellowing of leaves. Typically, with pathogens, you would see other symptoms such as wilting or leaf spots. I do not see any other symptoms in the photo, but the photo is a little blurry. Finally, this could be a case of the older leaves dying off (in other words, this is natural, and no cause for concern).

Check the watering situation and consider a soil test, if there are no issues there then you can rule those out as possible causes. Keep an eye on the plant, and if you see the yellowing spread throughout the plant, or new symptoms appear, then send me a few more photos and I will take another look at the case. The final thing I will note is that nutrients can be easily washed out of potted plants. If you have not fertilized the plant this year, apply a little of a balanced fertilizer (ex. 10-10-10). In general, you should apply some fertilizer in the spring before it starts to put out new growth, and again after the flowers fade.