Symptoms and Control Methods of Common Leaf Spot Diseases in Geraniums

Geranium leaf spot disease is a common disease in spring and summer. It is most likely to occur in hot and rainy seasons, often causing most of the infected leaves to die and drop off. When the stem is infected, it turns black, wilts and rots; cutting open the base reveals brown or black vascular bundles. Affected cuttings fail to root, with rot starting from the stem and developing upward, causing leaves to wilt and die.

Symptoms: Geranium leaf spot disease includes two types: black spot disease and brown spot disease.

Black spot disease: initial water-soaked spots appear on leaves, semicircular to irregular in shape, later becoming circular, 2 to 3 mm in diameter, with a slightly sunken necrotic spot in the center. Larger lesions are 6 to 10 mm in diameter. They are often restricted by leaf veins and appear irregular, dark brown, with concentric rings, bearing a sparse black mold layer. Lesions on leaves are scattered or merge together; in severe cases, leaves turn black, shrivel and die. This disease is more common on older leaves in the lower part of the plant, but upper leaves are also susceptible.

Brown spot disease: lesions on leaves are circular or oval, 1 to 4 mm in diameter, light brown, grayish-white to brick-red, with a narrow dark brown margin. Lesions on the underside of leaves are sometimes slightly raised, with scattered black granules or dark gray mold-like material on the surface.

Pathogen and disease characteristics: The pathogens of both leaf spot diseases are fungi of the Deuteromycotina. Irregular brown spots are caused by infection with Colletotrichum. Nearly circular gray-brown spots are caused by infection with Cercospora, Phyllosticta, and Ascochyta. The above pathogens all spread via conidia through wind and rain in spring and summer for primary and secondary infection. The disease is especially likely to occur in hot and rainy seasons. It is often carried to leaves by watering, wind and rain, and water droplet splashing. It mainly spreads through contact with gardening tools such as knives, invading through wounds. Damp environments and overly dense plant growth predispose plants to the disease.

Control methods: Create conditions for ventilation and light penetration, avoid overly dense plants or branches, reduce humidity, and prevent excessive vegetative growth of branches and leaves.

Avoid overhead irrigation when watering to prevent water from spreading the pathogen. Water should slowly seep in along the edge of the pot. Avoid overwatering and water accumulation in the pot, which can cause root rot.

When diseased leaves are found, they must be removed and destroyed promptly.

Do not apply excessive fertilizer. Too much nitrogen fertilizer can easily cause excessive vegetative growth of stems and leaves, resulting in no flowering or few flowers. In early spring or early autumn, appropriately apply more phosphorus and potassium fertilizer.

Before the disease occurs, spray with 50% benomyl wettable powder at 2500 times dilution, or 1:1:100 Bordeaux mixture. At the initial stage of disease, spray with 50% thiophanate-methyl plus 80% mancozeb (1:1) at 1000 times dilution, or 70% chlorothalonil plus 70% thiophanate (1:1) at 1000 times dilution, or 50% prochloraz wettable powder at 1000 times dilution. Alternate the above treatments for 3 to 4 sprays, with intervals of 7 to 15 days, closer intervals at first and wider later, for good control results.