Greasy spot is fungal disease that normally is found on citrus trees. Tell-tale signs include yellowish-brownish blister spots on leaves, often on the underside. As the disease develops, the spots develop into oily looking blisters. Greasy spot can cause significant leaf loss, particularly during winter and can also infect the fruit rind (particularly on grapefruit).
To control Greasy Spot, regularly collect and remove any fallen leaves, thus reducing the source of new spores and re-infection. Spay the tree with Liquid Copper Fungicide in June or July. A second spray application may need to be applied in August or September to protect late-summer growth.