Diseases and Insects of Shrubs and Small Trees

First in a series of articles

Azalea/Rhododendron - Rhododendron spp.

Botrytis/gray mold –Botrytis cinerea is a fungus that attacks a wide range of woody and herbaceous plants. The fungus can attack leaf buds, flower buds, leaves, flowers, stems and fruit. On most woody plants, Botrytis is most often noticed in the open flowers. It also causes buds to die (bud blast). Plants growing in stressful conditions such as damp moist areas are most prone to infection. The disease can over winter in dead as well as live plant material. On Rhododendrons/azaleas, the disease can cause flower, twig, and seedling blight. Brown lesions/spots in open flowers are a good indication that Botrytis has infected the flower. The more petals a flower has, the more likely it will get Botrytis following rainfall or overhead watering. The fungus can infect and grow between 32°F and 80°F. Good air circulation and other good cultural practices can help reduce Botrytis infection.

Dieback of rhododendron - Phytophthora spp. & Botryosphaeria dothidea attack both leaves and woody stems. Spores of the Phytophthora pathogen in the soil are splashed by rain onto the plant. Warm wet weather helps infection to occur in a few days. The pathogen survives in the soil on infected dead leaves. Phythophthora is a water mold and needs plenty of moisture to cause infection and to kill. The infected plant tissue is often invaded by Botryosphaeria dothidea – a canker causing fungus. Botryosphaeria dothidea is usually a very opportunistic canker organism. Phytophthora can also cause collar rot and root rot of rhododendrons & azaleas. To reduce the chance of these plants getting dieback, plant in soils with good drainage, let soils dry between watering, use an organic mulch so soil can not be splashed onto the plant, and remove all fallen leaves.

Black vine weevil - Otiorhynchus sulcatus (sometimes called the taxus weevil) larvae feed on feeder roots of plants. Large numbers can eat enough roots to cause the plants to decline and even die. However, in most cases the plant is able to grow new feeder roots faster than the black vine weevils eat them. The adults, which are all females (there are no males needed for reproduction), emerge in Northern Illinois during mid to late June. They emerge earlier in the rest of the state. The wings are fused so they cannot fly. They are often moved from one location to another on infested plant material or in the soil around the plant’s root system. The adults feed on plant foliage into the fall. The feeding damage appears as notches along the edge of the leaves. (On yews, the black vine weevil adults not only notch the needles, but sometimes eat the tip end of the needle off.) Adults feeding on plants close to a building often migrate into buildings as temperatures drop. Inside the home, the insects are just a nuisance. Black vine weevils feed on a wide range of plants including herbaceous plants such as strawberries, hostas and lily-of-the-valley. In addition, the strawberry weevil may feed on azaleas/rhododendrons causing similar damage. The black vine weevil feeds at night so insecticides should be applied just before dusk. The insect hides during the day – usually in plant litter or mulch around the base of the plant. This adult weevil is black with “gold” flecks scattered on its fused wings. The black vine weevils begin to lay eggs about two weeks after emerging from the soil as adults. Insecticidal control relies on eliminating the adults before they lay eggs. They can lay eggs throughout the summer months. After the eggs hatch the larvae start feeding on feeder roots. The weevil often over-winters as partially grown larva. Some adults may over-winter, if they are in a heated building.

Two-spotted spider mite - Tetranychus urticae is green to greenish yellow with two dark spots on its back. The females will overwinter in leaf litter or under the bark of several different trees and shrubs. In addition to attacking azaleas and rhododendrons, this mite attacks many other woody plants as well as numerous herbaceous plants. Reproduction can occur throughout the growing season as long as the weather is warm enough. Beneficial mites may be present at the same time as the two-spotted spider mite. Plant feeding mites such as the two-spotted spider mite move slowly and if crushed – leave a greenish stain. Beneficial mites move fairly rapid and when crushed leave a reddish stain. Check for mites by vigorously shaking/tapping a branch onto a piece of white paper. Watch closely. The mites will move but the debris will not. Do not treat if beneficial mites are present since they are natural control for plant mites. Treat if only the plant mites are present.

August/September 2004: Pet Waste and Water Quality | Diseases and Insects of Shrubs and Small Trees | Fall Garden Wrap-Up | Managing Thatch in Home Lawns