Powdery mildew of hydrangea

The causal agent of this disease is the fungus Erysiphe hydrangeae, an obligate parasite that specifically targets members of the Hydrangea genus. This pathogen colonizes the surface of plant tissues, forming a mycelial network that absorbs nutrients directly from the host cells, thereby undermining the plant's physiological processes.

This disease primarily affects hydrangea species, such as Hydrangea macrophylla, particularly when grown in environments with restricted airflow. While the fungus is most commonly associated with leaves, it can also spread to stems and developing buds, leading to stunted growth and poor flower quality in both garden and greenhouse settings.

Symptoms initially appear as discrete, small white patches on the underside of leaves. As the infection progresses, these spots expand and coalesce, eventually forming a thick, powdery white or grayish coating that covers both surfaces of the leaves. In severe cases, the foliage becomes discolored, distorted, and may drop prematurely, significantly impacting the plant's energy reserves.

The development of Erysiphe hydrangeae is driven by specific climatic triggers, most notably high humidity and moderate temperatures ranging from 18°C to 25°C. Poor ventilation, crowded planting sites, and periods of prolonged leaf wetness create an ideal environment for spore germination and the rapid secondary spread of the fungal infection throughout the garden.

The damage caused by the fungus is significant, resulting in weakened plant health, reduced vigor, and loss of ornamental value. Chronic infections often lead to the failure of stems to harden off before winter, making the plant more susceptible to frost damage and further opportunistic secondary infections by other pests and pathogens.

Managing the disease requires an integrated approach:

• promoting air circulation through proper pruning and spacing;
• applying appropriate fungicides containing sulfur or systemic active ingredients at the first signs of infestation;
• avoiding excessive overhead watering that leaves foliage damp;
• removing and destroying severely infected plant material to reduce the total inoculum load in the landscape.