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Disease symptoms caused by B. ribis may vary according to affected plant parts and plant species and their health. Describing symptoms for every susceptible host will be out of the scope of this data sheet. Therefore, an attempt has been made to describe infection courts and disease symptoms for some economically important woody plant species.
In general, B. ribis colonization of stem tissues of healthy as well as stressed plants are colonized through wounds, cracks (frost), sun-scorched bark, lenticels, leaf scars and branch stubs. Stems of healthy plants callus rapidly and the fungus may perpetuate underneath callus and may result in perennial canker when such plants are stressed. Cankers may be open and visible, partially covered by callus tissue or marked by a depressed area underneath the bark. Stems/branches of stressed plants are girdled quickly (due to inability to callus and/or compartmentalize fungus) by rapid colonization and discoloration of phloem, cambium and outer sapwood and the plants may manifest symptoms similar to vascular wilt (Wiehe, 1952; Witcher and Clayton, 1963; Schreiber, 1964; Punithalingam and Holliday, 1973; Rayachhetry, 1996b).
Progressive defoliation and dieback of twig and branch tips has been reported for mango trees in southern Florida, USA (Ramos et al., 1991). Affected tissues are discoloured brown to black. Canker or lesions on bark may cause defoliation and dieback of branches or the main stem, and lead to tree mortality. Other symptoms include resinosis on pine due to killing of branch tips (Punithalingam and Holliday, 1973), gummosis on peach trees (Weaver, 1974) and purple lesions on capsules or pedicels of Eucalyptus (Webb, 1983).
B. ribis has been reported to gain entry through stomata (leaf and green rachis) and fruit (lenticels) and cause panicle and shoot blight in female, and blossom and shoot blight in male, pistachio. Blight symptoms are marked by brown to black lesions on the current-season shoots (Michailides, 1991).
The fungus rapidly invades maturing apple fruits (entry points not known, speculated to be lenticels) and causes soft buttery rot compared with the relatively slow leathery rot caused by B. obtusa (Drake, 1971). Stem blight of blueberry starts with leaf browning or reddening due to rapid wilting which is often followed by mortality of entire plant as the fungus progresses through the vascular tissue (Cline and Milholland, 1992).
Although B. ribis is considered an opportunistic fungus (Smith et al., 1994), it can cause significant damage to many woody plant crops. Because of its widespread distribution and plurivorous nature, no established control measures are known for diseases caused by B. ribis.
The degree of pathogenicity of B. ribis varies from one host to another (Shear et. al., 1925). A Rhododendron isolate of B. ribis was able to cause more severe disease on Rhododendron than the apple and USA holly isolates of this fungus (Schreiber, 1964). Variation in pathogenicity for B. ribis isolates has been reported on 55 currant varieties in the northeastern USA (Hildebrand and Weber, 1944), almond cultivars in California (English et al., 1975), apple cultivars in Chile (Latorre and Toledo, 1984) and Malus quinquenervia in Florida (Rayachhetry et al., 1996c). Recent research in Florida has indicated the presence of field resistance of some mango cultivars towards the disease (Ramos et al., 1997).
Proper selection of cultivars, families or varieties for tolerance/resistance to B. ribis may provide a better chance of reducing crop loss than chemical control. Such a programme has been implemented on Eucalyptus in South Africa (Smith et al., 1994).
Cultural and Chemical Control
Formulation of environmentally sound cultural methods of disease control will be possible due to increasing knowledge of the biology and aetiology of B. ribis. The fungus is primarily a pathogen of stressed and injured plants (Ivory, 1967; Herbert and Grech, 1985; Pusey, 1989b; Old et al., 1990; Smith et al., 1994; Rayachhetry et al., 1996a). Major stresses that predispose plants to B. ribis invasion are drought, freezing and defoliation (Crist and Schoeneweiss, 1975; Wene and Schoeneweiss, 1980; Pusey, 1989a; Rayachhetry et al., 1996a). Families of sweetgum trees, inoculated in the autumn, developed more canker disease than those inoculated in the spring and summer (Filer and Randall, 1978). Almond trees are most susceptible to this fungus during the spring, summer and winter (English et al., 1975), while the canes of thornless blackberries are infected during spring to autumn (Mass and Uecker, 1994). Therefore, susceptibility of woody plants to B. ribis appears to be influenced by the season of the year and varies between species.
Sanitation activities (removal and destruction of dead and dying trees/branches) have been reported to be effective in controlling cane blight in currant (Punithalingam and Holliday, 1973). Excision of cankerous bark and cambium and application of a mixture of glycerol, anhydrous lanolin and phenylmercuric nitrate did not stop the expansion of B. ribis stem canker on almond due to incomplete removal of actively growing mycelium from sapwood underneath the lesion (English et al., 1975). A coat of benomyl-kaolin clay slurry around blueberry roots is known to provide systemic protection against B. ribis stem blight for 3-6 months, but high concentrations may result in phytotoxicity (Cline and Milholland, 1992). Use of indole butyric acid with benomyl or thiabendazole has been reported to enhance rooting and reduce disease impact on Rhododendron (Punithalingam and Holliday, 1973).
Preventive methods should focus on: maintaining tree health through proper irrigation, fertilization and the use of B. ribis-tolerant families, cultivars and strains (wherever possible); avoiding or minimizing unwarranted injury on valuable woody plants and using proper pruning techniques on horticultural and forestry crops; pruning trees when the level of airborne spores is low in the area; removing and disposing dead and dying branches properly to reduce the source of inocula; and using protective chemicals, such as benomyl at an appropriate concentration when and where a given crop is most susceptible to infection. None of these actions can ensure complete disease prevention but may reduce the disease intensity and crop losses.
Curative methods may include complete excision of discoloured bark, cambium and sapwood followed by immediate dressing with Bordeaux mixture (Punithalingam and Holliday, 1973) and the removal of infected side branches to avoid disease progress into the main stems.
Originally, B. ribis was described as a serious disease of cultivated currants (Ribes spp.) and was referred to as the cane blight fungus (Grossenbacher and Duggar, 1911, Stevens, 1926). Since then this species has been reported to cause disease on many cultivated and wild woody species. It is known to gain entry into the stem mainly through wounds (frost crack, mechanical wounds caused by inappropriate pruning or other means, sun scorch), branch stubs, lenticels and peduncles (Hildebrand and Weber, 1944; Wiehe, 1952; Taylor, 1958; Rayachhetry et al., 1996a, b).
B. ribis causes cankers and dieback on Aleurites montana and A. fordii (tung tree), C. limon (lemon), Cryptomeria japonica (Japanese cedar), L. styraciflua (sweet gum), M. domestica (apple), Pinus spp. (pine), Rhododendron spp., Salix spp. (willow), Tilia spp. and U. americana (American elm) (Punithalingam and Holliday, 1973; Lattore and Toledo, 1984). It also causes canker and fruit rot of P. persica (peach) (Stevens, 1926), canker and gummosis on lemon (Grasso, 1983), trunk, branch, and twig canker on Eucalyptus marginata (Davison and Tay, 1983), canker and mortality on Eucalyptus radiata (Shearer et al., 1987), seed capsule abortion and twig dieback on Eucalyptus camaldulensis (Webb, 1983), tip dieback of Mangifera indica (Ramos et al., 1991) and stem canker on Melaleuca quinquenervia (Rayachhetry et al., 1996b). Pathologists that consider B. ribis and B. dothidea to be synonymous reported that the incidence of stem blight disease on blueberry (Vaccinium corymbosum and V. ashei) in North Carolina has increased from 9% in 1959 (Witcher and Clayton, 1963) to 23% in 1985 (Cline and Milholland, 1992).
The fungus can reduce the fruit yield of thornless blackberry to an uneconomic level (Mass and Uecker, 1984). A 40-80% crop loss has also been reported for pistachio in commercial orchards in California (Michailides, 1991). If plant health is not properly maintained, B. ribis may cause significant economic damage to any susceptible plant listed under the host range (and others yet to be discovered).