Geranium rust

Systematic position and pathogen type: Puccinia pelargonii-zonalis is a member of the Fungi kingdom, Basidiomycota phylum, and Pucciniales order. It is an obligate biotrophic pathogen that relies exclusively on living tissue of its host plant to extract nutrients, forming specialized haustoria to invade the host cells.

Diseases and affected crops: The fungus causes geranium rust, primarily affecting Pelargonium zonale. The disease is characterized by the formation of small, circular, reddish-brown pustules (uredinia) on the underside of leaves, which rupture to release spores, eventually causing chlorosis, leaf curl, and severe defoliation.

Biology and life cycle: The pathogen's life cycle in temperate greenhouses is maintained by the continuous production of urediniospores. These spores are disseminated through air currents, water splashing during irrigation, or contaminated tools. The fungus survives the winter as mycelium within the tissue of living host plants, as it requires a living host to survive and produce subsequent generations of spores.

Development conditions and impact: Disease development is favored by high relative humidity (above 90%) and moderate temperatures between 18°C and 22°C. The economic impact is significant in greenhouse production, as the disease severely reduces the aesthetic value of ornamental plants, weakens the plant's overall health, and can lead to total crop loss if left uncontrolled.

Control and protection measures: Effective management of geranium rust requires an integrated approach to sanitation and chemical intervention:

• Sourcing healthy, pathogen-free cuttings for propagation;
• Improving greenhouse ventilation to reduce leaf moisture and relative humidity;
• Prompt removal and destruction of infected foliage and debris;
• Application of systemic fungicides from the triazole or strobilurin groups;
• Avoiding overhead irrigation to prevent the spread of spores by water droplets.