What are the control methods for Geranium leaf spot disease?

Symptoms mainly affect the leaves. Initially, small water-soaked spots appear, which later become circular, 2-3 mm in diameter, slightly sunken in the center, with a small brown center and a yellow spreading margin around it. Old lesions can reach 6-10 mm in diameter and, restricted by leaf veins, become irregular in shape, dark brown, with concentric rings, and bear a sparse pale black mold layer on the surface. When there are many lesions on a leaf, they can merge with each other, often causing the leaf to turn black, shrivel and die.

The disease conditions mainly occur on older, weakened tissue, so it is most often seen first on the lower old leaves. Rainy weather and excessive watering favor the occurrence of the disease.

The pathogen Alternaria pelargonii Ell. Et Ev. belongs to the Fungi Imperfecti, Hyphomycetes, genus Alternaria, Geranium Alternaria. Conidiophores are solitary or in bundles, olive-brown, paler at the apex, with the basal cell swollen, unbranched, with 0-2 geniculate flexures, 2-8 septa, 24-80 x 3-5 microns. Conidia are solitary, olive-brown, obclavate, with a slightly long, pale beak, the spore body having 3-7 transverse septa, 0-2 longitudinal septa, constricted at the septa, 16-54 x 8-13 microns. Diseased plant residues carry the fungus. The pathogen produces few spores and mainly spreads through diseased cuttings.

For control methods, refer to the control of chrysanthemum black spot disease.