Downy mildew of chrysanthemum

The causative agent is the oomycete Paraperonospora chrysanthemi-coronarii, responsible for downy mildew in various Asteraceae species. This pathogen is a significant threat to greenhouse production due to its ability to proliferate rapidly under controlled, high-humidity environments. It is classified as an obligate biotroph, meaning it relies on living host tissue to complete its life cycle.

This disease primarily impacts chrysanthemum cultivars, including the crown daisy (Glebionis coronaria). High-density plantings are particularly vulnerable, as the pathogen can spread through water droplets and wind-dispersed spores. Commercial flower producers often suffer heavy financial losses when this pathogen compromises plant health and aesthetic value.

Symptoms typically manifest as chlorotic lesions on the upper leaf surface, which later turn brown and necrotic. On the underside of the leaves, a diagnostic grayish-purple sporulation layer appears, especially under high humidity. Severe infections lead to leaf curling, premature senescence, and overall stunted growth of the entire plant.

Development is driven by prolonged high humidity (often above 85%) and moderate temperatures between 15°C and 22°C. The pathogen thrives in poorly ventilated greenhouses where moisture remains on the leaf surface for extended periods. Water splash from irrigation is the primary driver of transmission between neighboring plants.

Effective management requires a combination of cultural and chemical strategies to mitigate risk and control outbreaks. Key practices include:

• Reducing leaf wetness through optimized irrigation timing and improved greenhouse ventilation.
• Immediate sanitation by removing and destroying all infected plant debris.
• Maintaining proper plant spacing to ensure airflow and minimize humidity accumulation.
• Applying preventive fungicide treatments, particularly during periods of high humidity and cool temperatures.