Control of anthracnose of grapefruit

Symptom characteristics

Harm to leaves, shoots and fruits. Leaves and branches show acute symptoms in continuous rainy and humid weather: leaf tips show light cyan with dark brown patches, such as boiling water hot, the edge is not obvious; young shoots show boiling water scalding acute withering. In the short, moist and sunny weather, it shows chronic symptoms: leaf spots are round or amorphous, the edges are dark brown, slightly raised, the middle part is gray-brown to gray-white, and the spots often show wheel lines; the disease spots on branches often start from the axils of leaves, and develop from brown spots to long fusiform sunken spots. When the spot expands around the stem for a week, it often causes the branches to turn yellowish brown to gray and die. The young fruit is sick, dried and shrunk into a stiff fruit after decay, hanging from the tree or falling off. Mature fruits show "dry scar" spots under dry conditions, yellowish brown, slightly sunken, leathery, round to amorphous, with obvious edges; "tear marks" when humidity is high, and tears-like reddish-brown patches appear on the fruit surface; during storage and transportation, there are "fruit rot" plaques, and tea-brown slightly sunken patches appear from or near the pedicle, and eventually turn brown and rot in the cortex and internal vesicles.

On the surface of the above-mentioned affected parts, large scarlet dots appear when the needle is wet, and black dots appear when dry (germ conidia and spores).

Pathogen and characteristics of disease

The pathogen is a subphylum fungus, which is called Colletotrichum gloeosporioides, which overwinters with mycelium and conidia in the diseased part, and uses conidia as the initial invasion and re-invasion seed body, which is transmitted by wind, rain and insects to invade from stomata or wounds to cause disease. The pathogen has the characteristics of weak parasitism and latent infection, that is, the pathogen can be dormant after invading the host and show symptoms only when the activity of the host tissue decreases or declines. Hot and humid days are favorable for disease; winter and spring plants are frozen or drought; or citrus orchards are waterlogged; or heavy clay, shallow soil layer, low content of organic matter; or excessive application of nitrogen fertilizer; or high groundwater level and poor drainage are easy to reduce plant resistance and disease. The disease is usually more serious in sweet orange, mandarin, banana orange, Wenzhou mandarin, lemon, red orange, tangerine and so on. The severity of disease of the same susceptible variety is closely related to the strength of the tree.

Prevention and cure method

The main results are as follows: (1) strengthen fertilizer and water management and improve plant vitality. Deeply turn over and improve the soil, increase the application of organic fertilizer and phosphorus and potassium fertilizer, avoid excessive application of nitrogen fertilizer, improve the drainage and irrigation system, and do a good job in preventing waterlogging, drought, freezing and insect pests. (2) clear the garden thoroughly and reduce the source of bacteria. Adhere to the combination of winter and spring pruning and garden cleaning, comprehensive spraying (ground, tree) prevention (80% copper oxychloride or "backing" suspension 800x 1000 times once). (3) timely spraying to protect shoots and fruits. Spray once in spring, summer and autumn shoots and tender leaves, 2-3 times in young fruit stage from May to June and fruit expansion period from August to September, once every 10-20 days depending on weather and disease. The medicament can choose 24% due, or 25% carbon tetralin wettable powder 400 × 600 times, or 20% Shibao Ling 800 × 1000 times, or 43% Dasengfu, or "backing" suspension 1000 times, or 25% Spock EC 1000 times, or 70% topzine + 75% chlorothalonil wettable powder (1:1) 1000 times, 40% polysulfide suspension 1000 times. It is better to mix the above pesticides with Fuyou Ling liquid wax 500 times.