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A Disease Similar to Greasy Spot but of Unknown Etiology on Citrus Leaves in Argentina. G. M. Marcó, Estación Experimental, Concordia, Entre Rios, Argentina. J. O. Whiteside, Citrus Research and Education Center, Lake Alfred, FL 33850. Plant Dis. 70:1074. Accepted for publication 3 June 1986. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-1074a.

In northeast Argentina, a serious leaf spot disease of sweet orange (Citrus sinensis (L.) Osbeck) and other Citrus spp. is so similar in appearance to greasy spot caused by Mycosphaerella citri Whiteside that it was previously assumed to be greasy spot. With both diseases, hypertrophy and eventual gummosis in the spongy mesophyll occur, and pressure on the lower leaf surface is sufficient to raise, but not rupture, the epidermis. The first indication of a difference in etiology was a lack of M. citri ascocarps on fallen, decomposing citrus leaves. Also, the Argentine material did not show persistent appressoria in the outer stomatal chambers or filamentous hyphae in the diseased spongy mesophyll, characteristic of M. citri. Another difference from greasy spot was the presence of numerous, as yet unidentified spherical structures 2–12 μm in diameter in and near the necrotic mesophyll tissue, mostly just within the lower epidermis. Although the disease in Argentina is not caused by M. citri, a fungal cause is still considered probable because foliar applications of copper fungicides provide good control.