Geranium rust

Geranium rust is a destructive fungal disease caused by the pathogen Puccinia pelargonii-zonalis. It is an obligate parasite that specifically targets geranium species, severely affecting their aesthetic value and overall physiological health by disrupting photosynthetic processes.

The primary host for this fungus is Pelargonium zonale. The disease is particularly problematic in greenhouse environments where high plant density and controlled climatic conditions can lead to rapid spread of the pathogen throughout the entire collection, causing significant economic damage to nurseries and growers.

Symptoms initially appear as small yellow or chlorotic spots on the upper leaf surface. Soon after, distinctive circular, reddish-brown pustules erupt on the underside of the leaves. As the infection progresses, these pustules release spores, leading to widespread leaf yellowing, necrosis, and premature defoliation of the geranium plant.

The development and dissemination of the disease are heavily dependent on environmental conditions. High humidity (typically above 70%), lack of proper air circulation, and the presence of moisture on foliage facilitate spore germination and infection. The pathogen can spread rapidly through splashing water during irrigation, contaminated hands, or air currents.

Effective management and prevention of geranium rust involve:

• Maintaining low humidity and providing excellent ventilation in the growing area.
• Regularly scouting plants for symptoms and removing any infected leaves immediately.
• Avoiding overhead watering to keep leaf surfaces as dry as possible.
• Applying systemic fungicides according to local recommendations once the disease is detected.
• Maintaining a quarantine period for any new plant material added to an existing collection.