Pomelo Anthracnose Control Methods, a Must-Read for Growers
Symptom Characteristics
Harm to leaves, twigs and fruit. On leaves and twigs during continuous rainy and humid weather, acute-type symptoms appear: pale bluish patches with dark brown on leaf tips, like scalded by boiling water, with indistinct edges; tender shoots show acute wilting like scalded by boiling water. In briefly humid weather that quickly turns clear, chronic-type symptoms appear: leaf spots round or irregular, with dark brown edges, slightly raised, center grayish brown to grayish white, and concentric rings often appear on the spot surface; twig lesions mostly start at the leaf axils, developing from small brown spots into long spindle-shaped sunken lesions, and when a lesion extends around the stem for one full circle, it often causes the twig to turn yellowish brown to grayish white and die. Diseased young fruit, after rotting, shrinks and dries into mummified fruit, hanging on the tree or dropping off. Diseased mature fruit, under dry conditions, shows “dry scar type” spots, yellowish brown, slightly sunken, leathery, round to irregular, with distinct edges; under high humidity it shows “tear stain type” spots, with tear-like reddish brown patches appearing on the fruit surface; during storage and transport, it shows “fruit rot type” spots, mostly appearing from the stem end or its vicinity as tea-brown slightly sunken patches, eventually causing the cortex and inner pulp segments to turn brown and rot.
On the surface of each of the above affected parts, when moist, pinhead-sized vermilion dots appear; when dry, black dots appear (the fungus acervuli and spores).
Pathogen and Disease Characteristics
The pathogen is a fungus of the subphylum Deuteromycotina, called Colletotrichum gloeosporioides. The pathogen overwinters in the diseased parts as mycelium and acervuli, and uses conidia as primary and secondary inoculum, spread by wind, rain and insects, invading and causing disease through stomata or wounds. The pathogen has weak parasitic and latent infection characteristics, that is, after invading the host the pathogen can remain dormant without showing symptoms, and only shows symptoms when the vigor of the host tissue declines or deteriorates. Hot and humid weather favors disease development; plants frozen or drought-stressed in winter and spring; or citrus orchards waterlogged; or orchard soil heavy and clayey, with shallow soil layers and low organic matter content; or excessive or over-application of nitrogen fertilizer; or high groundwater level and poor drainage, all easily reduce plant stress resistance and make disease more likely. Usually varieties such as sweet orange, Ponkan, Jiaogan mandarin, Wenzhou mandarin, lemon, red tangerine and nian tangerine are more severely affected. For the same susceptible variety, the severity of disease is also closely related to tree vigor.
Control Methods
(1) Strengthen fertilizer and water management to improve plant vigor. Deep plow and improve the soil, increase application of organic fertilizer and phosphorus and potassium fertilizer, and avoid excessive or over-application of nitrogen fertilizer; renovate the irrigation and drainage system, and do a good job in waterlogging prevention, drought prevention, freezing prevention and pest prevention. (2) Thoroughly clean the orchard to reduce inoculum sources. Persist in combining winter and spring pruning and orchard cleaning with overall spraying (on the ground and on the trees) for prevention (80% copper oxychloride or “Kaoshan” suspension concentrate at 800-1000 times dilution, 1 time). (3) Spray pesticide in time to protect shoots and fruit. Master spraying once each at the tender leaf stage of spring, summer and autumn shoots; spray 2-3 times each during the young fruit stage in May-June and the fruit enlargement stage in August-September, with intervals of 10-20 days depending on weather and disease severity. Pesticides can be selected from 24% Yingde, or 25% Tanteling wettable powder at 400-600 times dilution, or 20% Shibaoling suspension concentrate at 800-1000 times dilution, or 43% Dashengfu, or "Kaoshan" suspension concentrate at 1000 times dilution, or 25% Shibaoke emulsifiable concentrate at 1000 times dilution, or 70% Thiophanate + 75% Chlorothalonil wettable powder (1:1) at 1000 times dilution, 40% Duoliu suspension concentrate at 600 times dilution. It is better to apply the above pesticides mixed with Fuyouling liquid wax agent at 500 times dilution.