How to prevent and control leaf spot disease in geraniums? A detailed guide to practical methods
[Symptoms] In the early stage of the disease, chlorotic spots appear on infected leaves, and the spots gradually expand afterward, forming relatively large round lesions. The lesions are dark brown with concentric rings. In the late stage of the disease, a dark green mold layer appears on the lesions, which consists of the conidia of the pathogen.
[Occurrence pattern] The pathogen overwinters as mycelium in diseased residues or diseased tissues. When environmental conditions are suitable, it produces conidia. The conidia are spread by wind and rain to cause infection and damage. During the growing season, the conidia can repeatedly infect plants. The pathogen can directly invade through stomata, lenticels, or the epidermis, forming a primary infection. After an incubation period of 2 to 3 days, lesions appear, and after 3 to 4 days, conidia are produced, and multiple repeated infections occur through airflow and rainwater.
When geraniums enter the vigorous growth and flowering stage, the basal leaves begin to age, and the pathogen accumulates on the plants. At this time, when there is continuous rainy weather with relative humidity greater than 70%, the disease begins to occur and spread. Therefore, how early the rainy season arrives, the number of rainy days, and the amount and distribution of rainfall all affect the outbreak and spread of the disease.
[Control methods] 1. Reduce sources of infection. Promptly prune diseased leaves when found, and destroy them together with the diseased leaves in a centralized manner; 2. Chemical control. In the early stage of the disease, spray a 1:1:200 Bordeaux mixture, or 50% Tuzet wettable powder, or 65% zineb wettable powder at 600 to 800 times dilution, once every 10 to 15 days, spraying 2 to 3 times.
Control methods for geranium leaf spot disease
Distribution and damage
Geranium leaf spot disease is a common disease in spring and summer. Leaf spot disease is most likely to occur in hot and rainy seasons, often causing most infected leaves to die and fall off. If the stem is infected, it turns black, wilts, and rots; when the base is cut open, the vascular bundles can be seen turning brown and black. If cuttings are affected, they cannot take root, rot starts from the stem, and spreads upward, causing leaves to wilt and die.
Symptoms
Geranium leaf spot disease includes two types: black brown disease and brown spot disease.
Black spot disease: Water-soaked spots first appear on the leaves, semicircular to irregular in shape, later becoming round, 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, and a relatively sparse black mold layer grows on them. The lesions on the leaf spots are scattered or merge, and in severe cases the leaves turn black, shrink, and die. This disease is more common on older leaves in the lower part of the plant, but upper leaves are also susceptible to infection.
Brown spot disease: The lesions on the leaves are round or oval, 1 to 4 mm in diameter, light brown, grayish white to brick red, with a narrow dark brown edge. The lesions on the underside of the leaves are sometimes slightly raised, and black granules or dark gray mold-like substances are scattered on the surface of the spots.
Pathogen
The pathogens of the above two leaf spot diseases are both fungi of the subphylum Deuteromycotina. Irregular brown spots are caused by infection by Colletotrichum. Nearly round grayish brown spots are caused by infection by Cercospora, Phyllosticta, and Ascochyta.
Occurrence pattern
The above pathogens all spread as conidia in spring and summer by wind and rain to cause primary and secondary infections. Disease is especially likely to occur in hot and rainy seasons. They are often carried to the leaves by watering, wind and rain, and splashing water droplets. They are mainly spread through contact with gardening tools such as knives, invading through wounds. A humid environment and overly dense plant growth make disease more likely.
Control methods for geranium leaf spot disease
Distribution and damageGeranium leaf spot disease is a common disease in spring and summer. Leaf spot disease is most likely to occur in hot and rainy seasons, often causing most infected leaves to die and fall off. If the stem is infected, it turns black, wilts, and rots; when the base is cut open, the vascular bundles can be seen turning brown and black. If cuttings are affected, they cannot take root, rot starts from the stem, and spreads upward, causing leaves to wilt and die.
Symptoms
Geranium leaf spot disease includes two types: black brown disease and brown spot disease.
Black spot disease: Water-soaked spots first appear on the leaves, semicircular to irregular in shape, later becoming round, 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, and a relatively sparse black mold layer grows on them. The lesions on the leaf spots are scattered or merge, and in severe cases the leaves turn black, shrink, and die. This disease is more common on older leaves in the lower part of the plant, but upper leaves are also susceptible to infection.
Brown spot disease: The lesions on the leaves are round or oval, 1 to 4 mm in diameter, light brown, grayish white to brick red, with a narrow dark brown edge. The lesions on the underside of the leaves are sometimes slightly raised, and black granules or dark gray mold-like substances are scattered on the surface of the spots.
Pathogen
The pathogens of the above two leaf spot diseases are both fungi of the subphylum Deuteromycotina. Irregular brown spots are caused by infection by Colletotrichum. Nearly round grayish brown spots are caused by infection by Cercospora, Phyllosticta, and Ascochyta.
Occurrence pattern
The above pathogens all spread as conidia in spring and summer by wind and rain to cause primary and secondary infections. Disease is especially likely to occur in hot and rainy seasons. They are often carried to the leaves by watering, wind and rain, and splashing water droplets. They are mainly spread through contact with gardening tools such as knives, invading through wounds. A humid environment and overly dense plant growth make disease more likely.
Control methods
1. Create conditions for ventilation and light penetration, avoid overly dense plants or branches, reduce humidity, and prevent excessive growth of branches and leaves.
2. Avoid sprinkler irrigation when watering to prevent the spread of pathogens through water splashing. Water should slowly seep in along the edge of the pot, and excessive watering should be prevented, as water accumulation in the pot can cause root rot.
3. When diseased leaves are found, they must be promptly removed and destroyed.
4. Fertilization should not be excessive. Applying too much nitrogen fertilizer can easily cause excessive growth of stems and leaves, resulting in no flowering or few flowers. In early spring or early autumn, appropriately apply more phosphorus and potassium fertilizers.
5. Before the disease occurs, spray 50% benomyl wettable powder at 2500 times dilution, or a 1:1:100 Bordeaux mixture. At the beginning of the disease, spray 50% carbendazim thiram plus 80% mancozeb (1:1) at 1000 times dilution, or 70% chlorothalonil plus 70% thiophanate-methyl (1:1) at 1000 times dilution, or 50% prochloraz wettable powder at 1000 times dilution. Alternate the above chemicals and spray 3 to 4 times, once every 7 to 15 days, with shorter intervals at first and longer intervals later, which gives good control results.