It is called sweet orange scab, coming from the term elsinoe australis, which is a plant fungal disease that damages your sweet oranges/citrus trees. This disease can be spread from one tree to another and originates from water specially during wet and rainy conditions. When damage have already spread, certain quarantine procedures are made to prevent further damage. Here are some natural and chemical ways to prevent sweet orange scab disease from destroying your citrus trees.

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Source: Gardening Know How