Powdery mildew is a widespread fungal disease affecting numerous plants, including roses. It is caused by various fungi, with Podosphaera pannosa var. rosae being the culprit behind rose powdery mildew. This pathogen thrives in environments with high humidity, cool nights, and warm days, but unlike other fungi, it can grow on dry surfaces. This disease can quickly complete its life cycle in just a few days under optimal conditions.
Powdery Mildew Management in Rose
The Disease cycle of Powdery Mildew
The powdery mildew disease cycle starts with the fungus overwintering as dormant mycelium in the bud scales and rudimentary leaves of rose bushes. Infected buds break and open in the spring and develop into infected shoots, on which the fungus sporulates, producing large microscopic spores called conidia. These conidia are carried by wind or other means to healthy rose tissue, infecting the upper and lower leaf surfaces and initiating a new disease cycle.
Factors Favoring Population Increase in Powdery Mildew
- Moderate temperatures: Powdery mildew thrives in moderate temperatures between 68°F and 86°F, allowing optimal growth and reproduction.
- High relative humidity: Humid conditions above 95% create a favorable environment for the powdery mildew fungus to grow and reproduce.
Symptoms of Powdery Mildew
- Dwarfed and distorted new shoots in the spring.
- Whitish-gray mildew growth on new shoots and expanding leaves.
- Light green to reddish, slightly raised blister-like areas on the upper leaf surface.
- Dense, powdery white growth (mycelium, conidiophores, and spores) of the mildew fungus on infected leaves.
- Severely infected young leaves become curled or irregularly twisted and covered with enlarged, whitish-gray, powdery, mealy, or felt-like patches of the fungus.
- Infected leaves often turn reddish-purple, then yellow, dry, and drop prematurely.
- Older infected leaves develop round-to-irregular areas covered with flour-like mildew growth.
- Buds, young stems (canes), thorns, peduncles, fruit sepals, and even flower petals may become infected and entirely covered with the typically dense, flour-like growth.
- Flower petals may be discolored, dwarfed, and fail to open properly, causing the flowers to die early.
- Severe stunting of plants if they are heavily infected early in the growing season.
- Resistant rose cultivars may show a hypersensitive reaction where invaded dead cells appear as black-to-rusty specks on the leaf surface, with little evidence of mildew growth.
Powdery Mildew Management in Rose by Cultural method
- Remove and dispose of infected plant parts: Prune out and dispose of any infected leaves, stems, or flowers as soon as you notice them. This can help reduce the spread of the disease.
- Improve air circulation: As you mentioned, good air circulation can help reduce humidity and limit the spread of powdery mildew. This can be achieved by properly spacing plants, pruning to open the canopy, and removing any weeds or debris obstructing airflow.
- Water early in the day: Watering early in the day allows leaves to dry off quickly, which can help reduce humidity and limit the spread of the disease.