What are the common pest and disease control methods for African violets?
I. Diseases
1. Ringspot disease This is a disease caused by a virus, generally spread by the pest thrips. After the leaves are infected with the virus, ring-shaped markings appear. Preventing the occurrence of viral disease by controlling thrips is the main measure for controlling ringspot disease. Plants infected with the virus generally cannot be cured with medicine; the only option is to pull them out and destroy them.
2. Leaf spot disease This is a leaf disease caused by fungi, first appearing on the lower leaves of the plant, especially leaves that are damaged or in contact with the growing medium. In the early stage of the disease, small necrotic spots appear on the leaves, after which the lesions expand rapidly; under suitable conditions, the lesions can reach 5 cm in diameter or even larger. Suitable fungicides include carbendazim, chlorothalonil, thiophanate-methyl, benomyl, etc. Spray in the early stage of the disease, once every 7-10 days, for 2-3 consecutive sprays.
3. Powdery mildew This occurs easily in the rainy season or under high temperature and high humidity, mostly affecting leaves, though flowers can also be affected. In the early stage on leaves, chlorotic yellow spots appear, gradually enlarge, and afterward a layer of white powdery substance grows on them; in severe cases the entire leaf is covered with a white powdery layer. Suitable fungicides include lime sulfur mixture, dinocap, sulfur-carbendazim mixtures, thiophanate-methyl, chlorothalonil, benomyl, etc.
4. Phytophthora blight This is also a fungal disease, and it can occur from the seedling stage to the mature plant stage. Damage begins on the leaves; the lesions are brown, water-soaked, and have obvious concentric rings. When moist, the edges are indistinct and the lesions spread extremely quickly, with sparse white mold-like growth on the lesions; when dry, the lesions stop expanding and the diseased tissue dries up. Suitable fungicides include metalaxyl, metalaxyl-mancozeb, fosetyl-aluminum, oxadixyl, copper hydroxide, etc.
5. Southern blight It mainly harms the stem base and roots. At first, water-soaked yellowish-brown small spots appear at the base near the soil and rapidly spread into the roots; in severe cases the affected area rots and the entire plant dies. At the diseased area, white silky fungal threads often grow, spreading in a radial pattern and extending over the surface of the growing medium. Later, the mycelium interlocks to form sclerotia that are initially white, gradually turn yellow, and finally become brown. To control southern blight, fungicides such as dinocap, carbendazim, flutolanil, Bordeaux mixture, and Baume lime sulfur mixture can be sprayed on the stem base and the growing medium.
II. Pests
1. The broad mite is a type of mite, light brown, extremely small, and difficult to observe with the naked eye. The female mite is about 0.23 mm long, and the male about 0.2 mm. It can harm leaves and flowers, but mostly hides in unopened tender leaves and buds to cause damage. It survives by piercing and abrading the leaf epidermis with its mouthparts and then sucking the sap. The damaged areas show chlorotic spots, deformity, curling, shriveling, etc. In severe cases, the tender leaves are difficult to unfold, and the petals become deformed and even fall off.
In addition to insecticides such as dimethoate, omethoate, dichlorvos, and malathion, there are also many specialized acaricides, such as dicofol, propargite, bromopropylate, fenbutatin oxide, benzomate, and fenpyroximate.
2. Scale insects The body is about 1-3 mm long, egg-shaped or oval. Scale insects pierce and suck the sap of leaves, causing chlorotic spots on the leaves; in severe cases the entire leaf loses its green color and turns yellow.
After the scale insect eggs hatch, the nymphs can move around to find a place where they can feed on leaves, and then they secrete a protective waxy layer and no longer move. When the eggs have just hatched and before the insects have settled down, since the scale covering has not yet thickened, spraying pesticides gives the best control effect. General organophosphorus insecticides such as dichlorvos, malathion, and fenitrothion can be used for control. After the scale insects have settled down, because they are protected by a waxy covering, only systemic pesticides can be used for control, commonly including dimethoate, omethoate, acephate, etc. Special-effect pesticides such as sujieke can also be used.
For a small number of scale insects, manual removal can also be used. For example, gently scrape off the insect bodies with a small bamboo strip, or carefully brush them off with a soft brush, or gently wipe off the insect bodies with a cotton ball dipped in vinegar or medical alcohol.
3. Thrips Thrips are very small and can harm flowers and leaves. They use their mouthparts to scrape open the epidermis and suck the sap, causing chlorotic spots or markings at the damaged areas; in severe cases the petals turn brown and become deformed. Suitable insecticides include qixingbao, dimethoate, omethoate, malathion, acephate, dichlorvos, carbaryl, etc.
4. Aphids Aphids are also small and can harm leaves and flowers; when numerous, they can cover the surface in a full layer. Aphids suck sap through their needle-like mouthparts, causing deformity or curling of the damaged areas. There are many suitable insecticides, such as dimethoate, omethoate, acephate, fenitrothion, phoxim, pirimicarb, derris extract, pyrethrin, and fenvalerate, etc.
When cultivating African violets, the potting medium should be clean and sterile, drain well and be airy, water should not be applied too much, and attention should be paid to ventilation under high temperature and high humidity; this can greatly reduce the occurrence of pests and diseases. Because some pesticides can cause yellow spots on the leaf surface, thereby reducing the ornamental value of the plant, try not to use pesticides to control pests and diseases.