Symptoms vary slightly in strength and aspect depending on the tree variety but all commercial grooves are affected to some extent. They may be visible first on the upper side of mature leaves as yellow to dark brown spots surrounded by a chlorotic zone. Slightly raised, pale orange to yellowish brown blisters can be observed underneath these spots on the lower leaf surface. Later the symptoms on both sides become darker brown or black and take a more 'greasy' appearance. Affected trees may gradually loose their leaves, reducing tree vigor and fruit yield. On fruits, greasy spot is characterized by the appearance of small, necrotic black specks surrounded by a green area, a symptom called greasy spot rind blotch. These can cover a large portion of the surface of the fruit. In areas with high temperature and high rainfall the infection may occur at any time.
The symptoms are caused by the fungus Mycosphaerella citri, which survives in crop debris on the soil surface when no suitable crop is available. In the spring, when conditions are favorable, the fungus produces spores that are released by rain splashes, overhead irrigation or heavy dew. Wind can also transport them to other citrus groves. Once they land onto the lower side of a leaf, they germinate and the fungi slowly penetrate the tissues through the natural pores on the blade. This process is also favored by elevated temperatures, high humidity and prolonged leaf moisture. Several months can pass between the primary infection during the summer and the appearance of the first symptoms in winter. By contrast, cool temperatures and dry weather result in lower spore number and less infection. Leaves remain susceptible to infection throughout the growth stages of the tree if environmental conditions are favorable. The presence of rust mites on trees has also been associated with the disease.
Sorry, we don't know of any alternative treatment against Mycosphaerella citri . Please get in touch with us in case you know of something that might help to fight this disease. Looking forward hearing from you.
Always consider an integrated approach with preventive measures together with biological treatments if available. Greasy spot is usually controlled with a one or two well-timed applications of petroleum oil during the summer months. This reduces the penetration of the spores into the leaves and fruits and thus delays the appearance of symptoms, even when the pathogen has already colonized the leaf. Products containing copper or copper sulfate are normally added to the oil to achieve a successful control of leaf and fruit symptoms. Other fungicides have also been used previously (for example strobilurins) but have lead in some cases to the development of resistance.