Certified Professional Horticulturist (CPH) Practice Exam

A client seeks advice because their rhododendrons have yellowing newest leaves. What nutrient deficiency might this indicate?

1. Iron

2. Manganese

3. Boron

4. Chromium

The correct answer is: Iron

The yellowing of the newest leaves on rhododendrons is commonly a symptom of iron deficiency. This condition is especially prevalent in acidic soils, which are favored by rhododendrons, where the availability of iron can become limited under certain pH conditions. Iron is essential for chlorophyll production; when there is insufficient iron, the plants cannot produce enough chlorophyll, leading to interveinal chlorosis, where the veins remain green while the leaf tissue turns yellow. While manganese deficiency can also lead to yellowing of leaves, it typically presents differently and might not be as specifically associated with the newest leaves as iron deficiency. Boron is crucial for plant growth and development but is less commonly linked to leaf yellowing in rhododendrons. Chromium is not an essential nutrient for plants and does not play a role in the symptoms described. Therefore, when diagnosing the issue with the client's rhododendrons, recognizing the significance of iron is key to understanding the yellowing of the newest leaves.