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In Fiji, symptoms have been described as diamond-shaped, greyish-white spots on the upper leaf surface with dimensions of 2-4 mm x 3-5.5 mm, the longer axis being parallel to the leaf veins. The spots, which sometimes have brown centres, were surrounded by a black border about 0.5 mm wide. On the undersurface of the leaf, the lesion could be covered with a dense, velvety brown mass (Knowles, 1916). Water-soaked areas, often several times the size of the spot, have been observed surrounding lesions (Firman, 1971). Diamond-shaped lesions have also been reported in Samoa (Gerlach, 1988).
Symptoms seen on specimens collected in the highlands of Peninsular Malaysia have been reported to be well-defined, white, grey or pale brown, elliptical and round, spots. The dimensions of the elliptical spots were 2-4 mm x 3-12 mm, and the round spots were 2-5 mm diameter. The lesions were often pale on the upper leaf surface and darker on the lower surface, with dark purple borders (Ellis, 1957).
In the Southern Highland Province of Papua New Guinea, round and ellipsoidal lesions of various sizes with well-defined, dark brown borders and grey centres have been seen on the upperside of the leaf lamina and midrib. Few were reported as having a true diamond shape as described in Fiji, and resembled more those described from specimens collected in Malaysia. In places where infection density was high, surrounding leaf tissue yellowed slightly and large necrotic areas developed. The dark brown borders and grey centres of lesions were discernible in patches of dead tissue. This diploid banana clone was extremely susceptible to the disease and first symptoms could be seen on the second and third fully expanded leaf (Jones, 1999).
Shade, high humidity and cool tropical environments encourage the disease. Greater spacing between plants in plantations to reduce shade and humidity levels should be beneficial. Leaves sprayed with oil in Fiji seemed more prone to Malayan leaf spot and it was suggested that this was because oil inhibits photosynthesis and transpiration similar to when plants are shaded. Maneb applied in water delayed the appearance of the disease, and symptoms were never severe (Firman, 1971).
Malayan leaf spot has little economic significance, although it has been recognized as potentially serious in Fiji where it has been reported as the worst disease in the cool season, especially in the upper Waidina Valley (Firman, 1971). In Fiji, Malayan leaf spot is not usually a significant problem on unsprayed banana plants except those in the shade. However, on plants receiving regular applications of maneb-oil-water emulsion or oil-water emulsion, the disease built-up and replaced black leaf streak (Mycosphaerella fijiensis) as the dominant leaf spot between August and October, 1970, with above 20,000 lesions per leaf. The pathogen caused rapid leaf loss at this time, but disease incidence declined rapidly after October (Firman, 1971). In 1970, Malayan leaf spot was reported as being epidemic in Fiji with 30% of the crop being rejected due to immaturity (Ellis and Holliday, 1976).
In Papua New Guinea, Malayan leaf spot has been severe on the cultivar Mala (AA) growing in gardens at altitude in the Southern Highland Province (Jones, 1999). The cultivars Porapora (AA), Hogolo (AA), Gebi (AS), Meko (AAA), Horul (AAB), Kimbem (AAB) and Wissen (AAB) growing in the same area near Mendi were less seriously affected (DR Jones, PNG, 1989, personal observation).
The disease is not thought to be seedborne. Seeds are not usually formed in cultivated banana fruit and wild diploid Musa species that produce seed have not been seen with symptoms.
The disease could be disseminated over long distances with sword sucker propagating material. International movement of all banana germplasm should be in tissue culture (Diekmann and Putter, 1996).
