What Is Wrong With Yellow Leaves on a Hydrangea?

Identification

• Hydrangeas are susceptible to oak root fungus disease commonly known as armillaria root rot. The disease-causing fungus Armillaria mellea infects the plant through the roots. The fungus persists in soil for extended periods of time.

Damage

• Armillaria fungus damages the plant's cambial tissues responsible for conducting water and nutrients. This causes the death of major roots and lower trunk area. Early visible symptoms of disease include stunted foliage growth accompanied with yellowing, browning and leaf drop. Branches die starting from the top and the whole plant is slowly killed. Clusters of mushrooms are often seen at the plant base.

Management

• Try to use resistant varieties to minimize chances of infection and keep plants in good health with adequate fertilization and irrigation. When preparing the planting site, remove all old roots from the soil. The University of California Extension recommends air drying the soil prior to planting.