Grey mold of zonal geranium

The causative agent of the disease is the necrotrophic fungus Botrytis cinerea, which infects Pelargonium zonale tissues under high humidity conditions. A specific symptom on the leaves is the appearance of yellow V-shaped spots, which develop from the leaf margin toward the base, delimited by veins; these spots eventually turn brown and become covered with a greyish-brown sporulation layer.

The infection also manifests as small spots on the petals, leading to the rotting of inflorescences, and affects stems at sites of mechanical injury or cuttings, which can result in complete girdling and death of the shoot. Resistance of geranium to the pathogen is a quantitative trait dependent on the morphological characteristics of the cultivar and tissue density, as no specific resistance genes have been identified in this crop.