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Pink rot of potato
Tubers used as seed tubers do not emerge when they are infected or planted in heavily infested soil. The sprouts die, the development of roots is poor and generally the mother tuber is rotted. If affected sprouts emerge, the plants are weak, and later show symptoms of wilt and die during the first stage of development. When the soil is heavily infested, affected plants show symptoms of wilt at all stages of development. The disease affects the roots, stems, stolons and tubers (Fernandez-Northcote et al., 1972; Rowe and Nielsen, 1981). Aerial tubers may form in leaf axils (Pethybridge, 1914; Goss, 1949).
Infected stems show internal vascular necrosis and, in some cases, the lower part of affected stems show the same external symptoms of black leg, caused by Erwinia spp. (Fernandez-Northcote et al., 1972; Rowe and Nielsen, 1981).
Affected tubers have superficial necrotic or blemished areas with watery secretions from the eyes. When affected tubers are cut transversally, the cutting surface is spongy in texture and changes colour after 15-20 minutes of exposure to the air from cream to pinkish, then brown and finally black (Pethybridge, 1913; Torres et al., 1970; Fernandez-Northcote et al., 1972; Vargas and Nielsen, 1972; Carroll and Sasser, 1974; Rowe and Nielsen, 1981). When the soil is heavily infested, tubers are completely destroyed and only the residual tissues of the potato tubers are observed at harvest time (Torres et al., 1986).
Seed-piece root rot of sugarcane
Stem pieces of sugarcane, which are used to start new plantings, fail to develop roots and the buds do not germinate when planted in infested soil. Infected tissues become water soaked. As the disease progresses, salmon-pink to orange-red streaks appear throughout the interior of the stalk. In yellow-stalked cultivars, the streaking is usually observed throughout the intact cane. In the advanced stages, the whole cane becomes water-soaked and reddish-brown and has a distinct ether-like odour (Steib and Chilton, 1951; Sivanasen and Waller, 1986).
Crown and root rot of cultivated wild rice
The leaves of affected wild rice (Zizania palustris) plants in California, USA, progressively desiccate and become brittle, turning from green to grey-green and finally to tan. The crown showed the most severe necrosis. Plants of all stages, from the early tillering to the grain-filling stage, are affected and die (Gunnell and Webster, 1988).
Cultural control methods that can reduce the severity of disease caused by P. erythroseptica var. erythroseptica include the use of healthy potato seed tubers; incorporation of organic matter to improve poor soil drainage; the use of greening potato seed tubers; the avoidance of excessive irrigation late in the growing season; crop rotation every 4 years, and avoiding planting in heavily infested soil.
Some native potato clones are reported to be resistant to P. erythroseptica var. erythroseptica (Fernandez-Northcote et al., 1972), but resistance is conditioned by greening and the age of tubers (Chattopadhyay, 1955; Fernandez-Northcote et al., 1972). Some varieties of sugarcane are also resistant.
The application of fumigants to heavily infested soils before planting, complemented with metalaxyl treatment at planting time, controls pink rot of potatoes (Torres et al., 1986), but some insensitivity to metalaxyl has been reported (Lambert and Salas, 1994; Goodwin and Mc Grath, 1995).