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Initial symptoms generally include a yellowing or bronzing of the leaves and an increase in leaf temperature. Leaves appear flaccid, develop visible wilt and usually within 3 days of these initial symptoms, become permanently wilted and die (Ezekiel and Taubenhaus, 1932; Streets and Bloss, 1973; Watkins, 1981). Leaves desiccate, but generally remain attached to the plant.
During the initial wilting, root decay is usually confined to the lower taproot, with laterals appearing healthy. In field crops and vegetables, the entire root system rapidly decays within a few days of wilting. Discoloration of xylem elements in the roots and lower stems can often be found with infected cotton plants (Olsen et al., 1983; Rush et al., 1985). Fruit trees may die suddenly or show signs of slight wilting over two growing seasons before succumbing to the pathogen. Some root decay will be evident during the development of above-ground symptoms in tree crops, but extensive rotting will usually require two growing seasons.
For most crops and soil types, there are no control measures that are both effective and economically justified. Soil fumigants such as 1,3-dichloropropene have been shown to provide control of the pathogen. These treatments are cost prohibitive for annual row crops, but may be justified as pre-plant treatments for orchards or vineyards established in soils that allow for adequate gas penetration. The fungus is highly sensitive to the triazole fungicides, but these compounds have herbicidal activity in cotton if applied to the seed (Whitson and Hine, 1986; Riggs and Lyda, 1988). Propiconazole and triadimenol applied in subsurface drip irrigation have been demonstrated significantly to reduce plant mortality in cotton (Olsen and George, 1987). The application of a systemic triazole fungicide deep in the soil near the root appears to offer the potential for disease reduction.
Currently, other control measures (e.g. resistant varieties, biological, cultural) have not yielded adequate or sufficiently consistent reduction in plant mortality to be useful (Kenerley and Jeger, 1992).
The most extensive disease loss surveys have been conducted on cotton in Texas and Arizona where average loss of raw fibre has been estimated to be 3.5 and 2.2%, respectively (Streets and Bloss, 1973). An intensive aerial survey conducted in several regions in Texas illustrated that 25% of the cotton-producing areas (6429 ha and 3339 ha in 1979 and 1981, respectively) were infested with the pathogen (SD Lyda, Department of Plant Pathology and Microbiology, Texas A&M University, Texas, USA, personal communication, 1991). Field surveys combined with aerial photography and image analysis have been used to estimate losses on the basis of loss per hectare, hectares affected and fibre quality and marketability in Upland and Pima cotton in Arizona (Mulrean et al., 1984). Yield loss was 10 and 13% for infested fields of Upland and Pima cottons, respectively, compared to healthy plants. In the case of orchard crops (e.g. apples, almonds), mortality exceeding 50% of the established trees has been found on some sites.