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The first indication of blister blight disease is a small, pale-green, pale-yellow, or pinkish, translucent spot on the tea leaf which is readily seen against the darker green colour when the leaf is held against the light. Reitsma and van Emden (1949) refer to these tiny infection spots as stages in the disease. The circular spots enlarge until they reach a diameter ranging from 3 to 12.5 mm. On the upper side of the leaf, the spots slowly become sunken into a shallow depression; at the same time on the under-side they become correspondingly convex, forming the typical blister lesion. The upper concave surface of the lesion is smooth and shiny, whereas the lower convex surface is at first dull, then grey and finally pure white, due to a dense, velvety growth on which the spores are produced. Affected tea leaves can often be distorted, folded or irregularly rolled, owing to the development of the blister lesions on the mid-rib and the margin.
The disease also affects the tender and young green stems. On the stem a pale yellow spot appears; this gradually elongates and encircles the whole stem which becomes slightly swollen at this point. When the spores are ripe the patch on the stem turns grey. The fungus penetrates the tender stem and consequently the leaves and buds above the diseased part wither and necrotize. Finally the stem bends over and breaks off at the affected spot (Petch, 1923).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
E. vexans is capable of inflicting serious crop losses. Because the fungus infects the tender foliage that forms the harvest, direct crop losses can occur. The tea industry in southern India suffered enormous crop losses in the early years of blight (Subba Rao, 1946). Between 1948 and 1952 the annual loss amounted to 18 kilotonnes of tea, before control measures were fully implemented on an extensive scale (Venkata Ram, 1964). During a 3-month period a crop gain ranging from 57 to 139%, depending on the intensity of protection, was recorded (Venkata Ram, 1968), and in tea recovering from pruning, regular fungicide treatment resulted in a crop increase of 127% in a 4-month period (Chandra Mouli, 1985).
Blister blight inflicted severe crop losses in Sri Lanka; in a study over 42 months unprotected tea suffered a loss of 33% (Loos, 1952). On estates in Indonesia situated at high elevations, crop losses ranged from 20 to 25% because of inadequate protection (Schweizer, 1950), and in the early 1950s Indonesia lost about 10 kilotonnes of tea because of blister blight epiphytotics (de Weille, 1959b). It has been established that failure to control blister blight would lead to an annual loss of 43% of the tea crop (Ordish, 1952).
Diseased leaves also affect the quality of finished tea if included in processing as several biochemical characteristics are changed resulting in reduced quality (Baby et al., 1998). However, others suggest that some infection can actually lead to an increase in quality (Rajalakshmi and Ramarethinam, 2000).