Citrus scab is a fungal disease that affects young leaves, branches, and fruits of citrus trees. The initial symptoms appear on new leaves, which become oily and develop a yellowish-brown color. As the disease progresses, these lesions grow into corky, raised bumps on the upper surface of the leaves. The affected areas often take on a cone-like shape, with a funnel-like depression on the leaf's front side. When the lesions merge, the leaves may twist and become rough in texture. Similarly, branches become shorter, twisted, and have a rough surface. On young fruits, the skin develops knob-like protrusions, becomes corky, and may thicken or deform, leading to premature fruit drop.
In wet weather, the lesions may appear grayish due to the presence of spores. Citrus scab and citrus canker (ulcer disease) both cause similar lesions, but they are caused by different pathogens—scab is a fungus, while canker is bacterial. The key difference is that scab causes deformed leaves, whereas canker does not alter the leaf shape significantly.
The pathogen responsible for citrus scab is a semi-mycotic fungus called *Sphaceloma fawcettii* (Jenk.). The fungus overwinters in infected plant parts, and under favorable conditions (temperatures above 15°C and adequate humidity), it produces conidia. These spores are spread by wind and rain, leading to direct infection of new shoots. Symptoms typically appear about 10 days after infection. The disease continues to spread through repeated infections, as conidia serve as secondary inoculum.
The optimal temperature range for the development of the pathogen is 16–23°C, with a maximum of 28°C. The disease thrives in rainy or foggy conditions, especially during spring and autumn flushes. Drought during the late autumn or winter shoot growth period generally inhibits the disease. Young fruits in May and June are particularly vulnerable. Among citrus species, oranges are most susceptible, followed by lemons and grapefruits, while mandarins show stronger resistance. Seedlings and young trees are more prone to infection than mature trees, and poor management of water and nutrients, along with poorly drained soils, increases the risk of disease.
To manage citrus scab, several control measures are recommended:
1. Plant disease-free seedlings in new orchards.
2. Prune and clear the orchard during winter and early spring. After pruning, apply protective sprays such as 30% copper oxychloride, 77% mancozeb, or "backing" suspension at 800–1000 times dilution, ensuring coverage of the tree canopy.
3. Apply timely sprays to protect new shoots and young fruits. For seedlings, the first spray should be applied when shoots reach 1–2 mm in length, followed by another spray every 10–15 days. For mature trees, the first spray should be applied when spring buds are 1–2 mm long, and the second spray when buds reach two-thirds of their full size. Other effective fungicides include 65% thiofanate-methyl diluted to 1000–1500 times, 43% dazomet suspension at 1000 times, 50% tebuconazole at 500 times, or 50% thiophanate-methyl wettable powder at 1000 times. Mixing these with Fuyouling Liquid Wax at 300–500 times dilution can enhance effectiveness.
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