Phytophthora root rot of chrysanthemum

Phytophthora root rot is a devastating fungal-like disease caused by the oomycete Phytophthora chrysanthemi. This pathogen primarily attacks the root system and the crown of the chrysanthemum plant, disrupting water and nutrient transport, which inevitably leads to the decline and death of the plant.

The primary host for this pathogen is the Chrysanthemum species. While it primarily affects commercial greenhouses, it can also manifest in home gardens. The disease spreads rapidly through contaminated soil, reused containers, irrigation water, and infected cuttings introduced from external sources.

The clinical signs of the infection include wilting, chlorosis (yellowing of foliage), and overall stunting. Infected plants often show signs of water stress despite moist soil. Upon closer inspection, the roots appear soft, discolored, and necrotic. A longitudinal cut through the stem usually reveals dark, brownish streaks extending upward from the soil level.

The development of Phytophthora chrysanthemi is heavily favored by cool to moderate temperatures combined with waterlogged conditions. The pathogen produces zoospores that move freely in the water film, allowing it to rapidly colonize neighboring plants within a greenhouse setting or a flower bed.

Effective management requires an integrated approach to break the disease cycle and create an environment unsuitable for the pathogen:

• Implement strict sanitation practices for all pots, benches, and tools.
• Ensure excellent drainage and avoid overwatering the growing medium.
• Remove and destroy any infected plant material immediately to stop the spread.
• Use fungicides containing ingredients like metalaxyl or fosetyl-aluminum as a preventive measure.
• Source plant material only from certified disease-free propagators.