What specific effective fungicide should be used for citrus scab?

Citrus scab is a common fungal disease in citrus cultivation. To control it effectively, you must first understand it. What are citrus scab and its specific fungicides?

I. Citrus scab

1. The pathogen of this disease is Elsinoe fawcettii, a fungus of the genus Elsinoe in the class Deuteromycetes. The scab pathogen overwinters as mycelium in diseased tissue. When the spring temperature rises above 15°C and there is abundant rain and high humidity, the old lesions produce conidia, which are spread by wind, rain or insects to the young leaves, flowers and young fruit of the spring shoots. After invading the epidermis, new lesions appear after an incubation period of 3-10 days, completing the primary infection.

2. Afterwards, conidia are produced again and cause damage to the young leaves, young shoots and fruit of the summer and autumn shoots, and the mycelium overwinters in the diseased parts. The most suitable temperature for disease development is 20-21°C, and disease development stops when the temperature exceeds 24°C. If continuous rainy weather, or heavy morning fog and dew, occurs when new shoots are sprouting and expanding their leaves, the disease is likely to become epidemic. Therefore, temperature and humidity play a decisive role in the occurrence and epidemic of scab.

3. The main characteristics of citrus scab: it mainly damages the young tissues of leaves, new shoots and fruit, and also damages the calyx and petals. When leaves are damaged, oily yellowish-brown round small spots initially appear, gradually expand, and the color changes to wax yellow. Later, the lesions become suberized and raised, mostly protruding on the underside of the leaf, with the upper surface sunken, forming conical grayish-brown scab spots. The lesion diameter is 0.3-2mm. The leaves of new shoots severely damaged early often wither and fall off. When the weather is humid, a grayish-brown powdery substance grows on the surface of the lesions (the pathogen's acervuli and conidia). Sometimes many lesions gather together, causing the leaves to become distorted and deformed.

II. Specific fungicides for citrus scab

1. Focusing on the control of scab on young fruit, spray when 2/3 of the flowers have fallen, and if conditions are especially favorable for disease development, spray again half a month later. For the first application, 0.8% equal-strength Bordeaux mixture can be used. Add 0.6 jin of table salt to the prepared Bordeaux mixture; this has a synergistic effect and resists rain washing, and is also specifically effective against citrus white plaster disease.

2. Effective fungicide varieties include Bordeaux mixture, 55% procymidone-thiophanate-methyl wettable powder at 1000-1200 times dilution, 50% duomeiqing wettable powder at 800-1000 times dilution, 77% Kocide 2000 at 800 times dilution, 80% Biben wettable powder at 400-600 times dilution, 80% Sande Sheng (Dacotech M-45) wettable powder at 600 times dilution, 30% copper wettable powder at 400-500 times dilution, 20% Longkejun (copper fungicide) suspension concentrate at 500 times dilution, and 12% Lujunling emulsifiable concentrate at 500 times dilution.

Control temperature and humidity well, and at the same time carry out fungicide protection to effectively control the occurrence of the disease.