Rhododendron is a family of plants with a wide array of characteristics, from dwarf plants to plants the size of trees. The plant most people call azalea is known scientifically as a rhododendron. They are mostly evergreen but there are some deciduous varieties. The leaves can be a minute /2-inch long to nearly 2 feet, and can be leathery or delicately soft. They can be dark green, pale green and even reddish, and can have a smooth or fuzzy underside.
The most common insects found on rhododendrons are either black vine weevils or strawberry root weevils. They live in the soil and come up to feed on the leaves at night, causing notching. Mites and thrips will cause stippling, but thrip activity creates a more silvery appearance. Leaf miners will feed between the top and bottom part of the leaves, leaving little trails. Distorted areas on the leaves called galls may be the result of the rhododendron gall midge or a fungal disease. Tiny moth-like insects called whiteflies suck juices from the leaves.
The most frequent problems of rhododendrons mostly fall within nutritional imbalances or insect problems. However, there are a few fungal diseases that crop up. If leaves have brownish raised areas on the underside of the leaf, it is often a fungal disease called rust. Necrotic ring spot will cause reddish brown spots or rings on leaves. Powdery mildew will look like a white powdery substance. Leaves that are distorted with white fleshy areas are suffering from leaf gall or gall midge.
Animals generally don't forage on rhododendrons. Rhododendrons are poisonous to many animals, and animals tend to avoid eating them if there are other plants to forage. The leaves have a chemical called grayanotoxin that can cause anything from gastrointestinal distress to death. However, deer seem to graze on young plants or small-leaf varieties of both rhododendron and azaleas without negative results. They avoid only those plants with large leathery leaves or leaves with indumentum (a fuzzy underside).
Leaves that are yellow between the leaf veins are showing symptoms of a pH problem, or an iron, calcium or magnesium deficiency. These problems may be interconnected, as a pH imbalance may prevent uptake of certain nutrients. An overall yellowing of the whole plant may be a nitrogen deficiency. Young leaves that are reduced in size, slightly cupped and have burnt tips are often suffering from copper deficiency. If leaves are very dark green and reddish on the lower side, phosphorus deficiency may be the problem.
Heat or cold stress causes damage to rhododendron leaves. During long periods of dry weather, windy conditions, or bright sunshine leaves may take on a dry, crispy appearance around the edges. This is called leaf scorch, and depending on the severity, the leaves may drop prematurely. During cold stress, leaves curl and droop downward. Afterward, depending on the extent of the damage, leaves eventually die or show irregular brown dry spots.