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Disease Note.

First Report of Downy Mildew Disease in Lettuce Caused by Bremia lactucae in Natal, Southern Africa. P. N. Achar, Department of Microbiology, University of Durban Westville, Private bag X54001, Durban, Southern Africa. Plant Dis. 80:464. Accepted for publication 19 February 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0464B.

In June 1994, typical downy mildew symptoms were observed in lettuce (Lactuca sativa L.) leaves in lettuce-cultivated areas in Natal, South Africa. Symptoms were characterized by an extensive growth of sporo-phores on the undersurface of the mature leaves. The sporophores appeared as white, discrete projections bearing spores. Discoloration was noticed in infected leaves at various growth stages of the hosts The lesions initially appeared as white, angular patches that turned black to brown as the leaves matured. Chlorotic lesions on older leaves turned necrotic; the leaves became brittle and eventually shriveled and died. The fungus was isolated from the leaf lesions and necrotic tissues at the base of the petioles and identified microscopically as Bremia lactucae Regel based on the following characteristics: asexual spores ranged from 17.0 to 18 µm long and 14.0 to 15 µm wide; sporophores ranged from 250 to 350 µm long and 10 to 12 µm wide; the sporophores were slender and dichotomously branched with disklike tips bordered by sterigmata; and the temperature range for asexual sporulation was 1 to 19C with an optimum at 15 to 17C (1). Furthermore, germ tubes entered the stomatal space through the stomata and ramified intercellularly. Germ tubes also penetrated directly through the cuticle into the underlying epidermal cells. Two types of haustoria, namely the knob and flask shapes (8.0 to 10.0 µm diameter), were observed. Artificially inoculated healthy lettuce leaves and petioles developed symptoms similar to those observed in the field and the fungus was reisolated from the above-infected tissues, completing Koch's postulates. Oospores were not found in any of the isolates collected at different locations in Natal. An average of 50% infection was recorded from 10 different sites in this region. Since lettuce is grown on a large scale as a fresh vegetable for South Africans, the appearance of the downy mildew disease in lettuce can lower its value in the market. This is the first report of the downy mildew of lettuce caused by Bremia lactucae in Natal, Southern Africa.

Reference: (1) H. Schultz. Phytopathol. Z. 10:490, 1937