Phytophthora root rot of Zantedeschia

Phytophthora root rot, caused by the oomycete Phytophthora richardiae, is a destructive disease primarily affecting calla lilies (Zantedeschia spp.). This pathogen targets the root system and rhizomes, leading to severe decay and potential collapse of the entire plant. It is widely recognized as a significant threat in both commercial production and home gardening.

The disease host range is largely confined to the Araceae family, with calla lilies showing high susceptibility. The pathogen invades the subterranean parts of the plant, disrupting water and nutrient uptake. Over time, the vascular system becomes compromised, leading to systemic wilting that cannot be corrected by simple watering.

Visible symptoms often start with the yellowing and chlorosis of lower leaves, which eventually become necrotic. As the root system decays, the plant loses its vitality. If the roots are inspected, they typically show signs of water-soaking, softening, and browning. In advanced stages, the decay extends to the crown, resulting in the separation of the leaf stalks from the rhizome.

Conditions favoring the development of Phytophthora richardiae include poor drainage, waterlogged soil, and temperatures ranging from 18 to 25°C. The pathogen produces resilient oospores, allowing it to persist in the soil and contaminated irrigation water for extended periods. Once established, the pathogen spreads rapidly through wet soil, especially during heavy rainfall or over-irrigation.

Management strategies focus on strict sanitation and environmental control. Growers should ensure that all planting material is disease-free and planted in well-draining, sterilized substrates. Fungicides specifically targeting oomycetes, such as metalaxyl or fosetyl-aluminum, can be used for preventive and therapeutic control when applied according to the manufacturer's instructions to protect the root zone.

• Ensuring excellent soil porosity and drainage.
• Culling and destroying infected plant material immediately.
• Rotating planting sites or refreshing soil in containers.
• Sterilizing tools to prevent cross-contamination.