How to Care for a Magnolia Tree with Fungus

Magnolias flourish in the southern part of the United States.

Magnolia trees have waxy, evergreen leaves and delicate flowers. They are often planted as prized ornamental trees in parts of the country where mild winters are common. As with many plants, they are susceptible to fungal infections. Common magnolia fungal infections include powdery mildew, leaf spots, cankers and wood rots. Powdery mildew is a white to gray, powder-like fungus that grows on leaves, buds and flowers. A number of different fungi cause discolored spots on leaves. Nectria canker affects bark and causes oval-shaped scars in branches and bark. Wood rots are usually diagnosed by the appearance of mushrooms growing out of the trunk.

Instructions

Powdery Mildew

1. Prune out as many of the infected leaves, buds, flowers and twigs as possible. Infected plant material should be burned or put in a trashcan bound for a landfill. Do not compost infected plant material.

2. Avoid watering late in the evening and don't wet the foliage when you water. Splashing water can spread the fungus so it's best to keep water off the tree.

3. Clean up any debris that falls from the tree as soon as you can and remove or destroy it. Powdery mildews can survive on dead tissue and can reinfect the tree.

4. Prune to open up the tree's canopy and ensure proper airflow and light penetration. It may also be necessary to thin overcrowded plantings. If you have several trees planted close together, powdery mildews may persist.

5. Apply foliar sprays of fungicide if cultural controls are ineffective. Use products containing the chemicals sulfur, neem oil, triforine or potassium bicarbonate. Read the manufacturer's label carefully to ensure the product you buy is appropriate for use on magnolias and follow all directions carefully.

Leaf Spots

1. Fertilize trees in early spring if they suffered from fungus-related defoliation during the previous years. Fertilize with ammonium nitrate or urea and follow the manufacturer's application directions for the size and age of your tree.

2. Prune your tree to remove dead or diseased growth and to thin out the canopy by removing any weak, crossing or rubbing branches. Pruning helps improve air flow, keep foliage dry and promote growth.

3. Apply fungicides only if cultural methods of control are unsuccessful and your tree is in danger of dying due to several successive years of infection. Use a foliar application of copper fungicide and always read and follow manufacturer's directions carefully.

Perennial Nectria Canker and Wood Rot

1. Prune to remove infected branches, dead or weak growth and any branches that are crossing or rubbing during the winter. Cankers are oval-shaped lesions on the bark that develop a target pattern as successive years of infection injure the bark. Burn or throw away the infected branches to avoid reinfecting the tree. Cut mushrooms off the trunk as close to the wood as you can and burn or destroy them.

2. Water your magnolia as needed to avoid drought stress. Soil should be kept moist but not soaking.

3. Fertilize to encourage growth and help keep the tree healthy. The University of Minnesota recommends using a balanced fertilizer with equal parts of nitrogen, phosphorous and potassium. Fertilize in the spring with a slow-release granular formula.

4. Apply a 2- to 3-inch layer of mulch. Cover as much of the root zone as you can.

5. Remove the tree if cankers or wood rots have caused enough damage to make the tree structurally unsound. Cankers can girdle trunks, and wood rots cause the tree to rot from the inside. Both of these conditions can lead to weak trees that are a danger to humans and buildings.

References

• The Morton Arboretum: Powdery Mildews
• Colorado State University Extension; Powdery Mildews; B. Edmunds, et al.; May 2009
• University of Illinois Extension: Fungal Leaf Spot Diseases of Shade and Ornamental Trees in the Midwest
• UC IPM Online; Wood Decay Fungi in Landscape Trees; G.W. Hickman, et al.
• UMass Extension; Perennial Nectria Canker; Daniel H. Gillman
• University of Minnesota Extension Service; Magnolias for Minnesota; John Eustice