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Roots and basal regions are primary infection sites, but all plant parts may be infected. Symptoms vary with plant age and weather conditions. Seedlings develop damping-off symptoms in wet, mild weather, their stems becoming dark brown or black near the soil line, followed by rapid spread into the leaves. In the field, all leaves wilt (c.f. vascular wilts), begin to turn yellow and hang down around the stalk. Infection spreads into the stem and root system, lateral roots become blackened and fine lateral roots decay. The stem (shank) eventually turns black, 30 cm or more above ground, and the leaves turn brown and shrivel. The stem pith is dry, brown to black and is usually separated into plate-like discs. Lower leaves may become infected by rain splash and develop water-soaked, light green spots which rapidly expand, turning brown and necrotic. Infection may spread from leaf lesions to the stem (Shew, 1991).
Brown spots appear on ripe or green fruits. The spots enlarge and develop a concentric ring pattern of narrow, dark brown and wide, light brown bands; they remain firm and smooth, eventually covering half of the fruit. Lesion margins are smooth, but not sharply defined, and, under moist conditions, a white, cottony fungal growth appears on them. The foliage remains unaffected. Brown, water-soaked lesions appear on roots; they gradually dry and the discoloration may extend into the lower part of the stem. Severely affected roots may show extensive necrosis and decay (Stevenson, 1991).
Brown rot gummosis attacks the trunk near the soil level. The first symptom produced is a profusion of gums on the surface of the affected bark, followed by extension of the infection upwards and laterally. Lesions may occur below the soil level and are not readily seen on first inspection. Scraping the bark surface reveals that the affected tissues are brown and the bark is dead. Removal of infected bark reveals a dark brown staining of the cambial surface which is also killed (c.f. psorosis in which only the outer layers of the bark are destroyed). At a later stage, the dead bark dries, shrinks and cracks and patches may fall off, leaving an open canker. The leaves corresponding to the invaded part become chlorotic, first in the midrib and then on the lamina. The leaves then become chlorotic, fall off and twigs and branches begin to die back. Fewer and smaller leaves are produced on new shoots and infected fruits develop a soft brown rot.
A variety of root and fruit rots are produced on Capsicum spp., avocado, strawberry, pineapple, papaya, guava, aubergine, castor bean, Gossypium species, lucerne, rhubarb, sesame and Cinchona officinalis, all characterized by brown, water-soaked lesions on fruits or pods and a brown/black discoloration of roots.
Cultural Control and Sanitary Methods
For tobacco, overwatering seedlings should be avoided and the use of inoculum-free irrigation water is recommended. The removal of diseased plants, planting in well-drained sites and the avoidance of infested soil all help to reduce spread, but have limited use, as P. nicotianae is often imported in infected plants, soil or irrigation water. As P. nicotianae is plurivorous, normal crop rotation has little effect, but, where possible, flooding fields and growing swamp rice in a rice-tobacco cycle destroys P. nicotianae (van Schreven, 1948).
For tomato, heavy, poorly drained fields should be avoided and the implementation of practices which reduce soil compacting can improve control. Low areas are especially liable to heavy losses and it may help to grow plants in raised beds. Rotation with non-solanaceous hosts may also aid control.
The disease can be reduced in citrus by the removal of infected or dead trees, annual inspection down to the first lateral roots, planting trees high and avoiding wounding, especially near the trunk base. Ensuring that furrow irrigation water does not come into contact with the trunk by banking the soil up to a distance of 50 cm from and around the trunk base, and forming a watering basin around the banked area, also helps. Pathogen spread through irrigation channels can be reduced by introducing a sack of copper sulphate crystals which release copper ions slowly with the passage of water, but no more than twice during the irrigation period. The application of copper-based fungicides every 2-3 months during the wet season reduces disease incidence in areas where the disease is likely to be serious. Lesions may be painted with a copper fungicide paste following removal of the bark to expose the lesion.
Two pathogenic races of P. nicotianae, race 0 and 1, exist and may be characterized by a number of laboratory tests (Handelsman et al., 1991; Robin and Guest, 1994; Bonnet et al., 1994). Resistance to black shank was evaluated in 51 species and 13 hybrids of Nicotiana; results showed that N. longiflora and N. plumbaginifolia were highly resistant and N. undulata, N. nudicaulis and N. exigua were moderately resistant (Lautz, 1957). Sources of resistance, screening techniques and breeding methods were reviewed by Nielsen (1992a) and the results of performance testing of six tobacco genotypes against P. nicotianae in six countries was reported by Nielsen (1992b).
Resistance has been found in several tomato cultivars, especially Rossol and Roma Roma (Perez et al., 1986).
Successful control has been achieved in citrus using resistant rootstocks, budded at least 60 cm above ground, although rootstocks resistant to gummosis may be susceptible to other diseases or may not suitable to the climate or soil in a given area.
Inoculation of established trees of Citrus clementina with agar discs of Myrothecium roridum controlled previously existing gummosis infections (Tuset et al., 1990). Penicillium funiculosum and Pythium nunn controlled Phytophthora root rots of azalea and orange in vitro (Fang and Tsao, 1995a, b).
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:
Black shank occurs worldwide, but is most severe in warmer climates. Losses occur during all stages of development and can reach 100% in some fields (Shew, 1991). Buckeye rot is more common in tropical and subtropical regions. Gummosis occurs in all citrus-growing areas, but is more frequent in the humid tropics. Root and fruit rots of crops such as guava and aubergine are minor and vary greatly from year to year.