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The generalised symptoms in vegetative parts of enset and banana are similar but the sequence of symptom development varies, probably according to the route of infection. Under cultivation, enset is not normally allowed to flower, being harvested for its starchy pseudostem before emergence of the floral raceme, and hence the floral symptoms do not appear to have been reported. In the leaves, the earliest symptoms are usually loss of turgor and wilting in the spear (youngest emerging leaf) or one or more of the young leaves, sometimes preceded by yellowing and distortion, especially in young plants. Older leaves often develop a pronounced yellowing, followed by wilting, necrosis and breakage of leaf bases owing to loss of turgor. Internally, vascular bundles show a cream, yellow or pinkish discoloration that may extend throughout the plant but is often most pronounced in the floral raceme. Droplets of cream to pale yellow bacterial ooze emerge slowly at first from cut tissues and cut pseudostems may yield copious quantities of ooze over a period of several days. Although the symptoms are characteristic of a vascular wilt, the bacterium does not remain confined to the vascular tissues; the large air spaces within the leaf bases may become filled with pockets of cream to pale yellow ooze and this characteristic feature appears to distinguish Xanthomonas wilt from other bacterial wilts of banana. In banana, infection of the corm may spread into daughter suckers, or in enset into plantlets induced by the removal of the vegetative meristem (enset does not normally produce suckers). Such seed pieces or plantlets from infected plants may show no visible signs of infection and can serve as sources of infection to spread the disease.
Symptoms in flowers and fruits are similar to those observed in other bacterial wilts of banana (Eden-Green, 2004a, b) and serve to distinguish these diseases from other conditions, such as Fusarium wilt (Panama disease). The sequence of symptom development probably depends on the mode of infection and the variety affected, but includes the following: blackening and shrivelling of part or whole of the flowers and developing fruit bunch, usually commencing with the male flowers and affecting the whole of the male bud; discoloration, ranging from pale yellow to reddish-brown, of vascular bundles of the flower stalk and the production of droplets of bacterial ooze; internal greyish-brown discoloration of the flesh of immature fruits; and premature ripening and sometimes distortion or abnormal development of individual fruits, although in many cases the whole bunch may appear outwardly normal. As in other bacterial wilts of banana, these symptoms, together with the rapid spread of disease, are consistent with infection of inflorescences and transmission of bacteria by insects visiting the flowers (Buddenhagen and Elsasser, 1962). Furthermore, the 'Bluggoe' (ABB) cooking banana variety that is known to be particularly susceptible to insect transmission of Moko, Bugtok and Blood diseases (see elsewhere in this Compendium) is also frequently the first variety to be affected as Xanthomonas wilt spreads into new areas.
Ideally, the movement of all parts of banana, enset and other Musaceous hosts (except as pathogen-free axenic tissue cultures) should be restricted from disease-affected areas, both within and between countries at risk from the disease. All fruits from disease-affected areas should be consumed or processed locally to minimise the risks of spread; fruit bunches that show premature ripening or other external symptoms are unlikely to enter trade but bunches that appear externally normal but are then found to be discoloured when cut open are a particular risk: these may be traded, discarded and serve as possible sources of infection. In practice it is rarely possible to enforce such controls and increased surveillance, backed by immediate eradication and phytosanitary measures, should be instigated in areas where introduction of the disease could cause economic loss.
Cultural Control and Sanitary Methods
In Uganda, efforts to eradicate the disease by poisoning, uprooting and burying affected and apparently healthy plants in the original outbreak areas failed to prevent subsequent outbreaks of disease in neighbouring villages, probably because spread had already occurred. There is evidence that these initial phytosanitary operations reduced the incidence and subsequent rate of spread within the original outbreak areas. Practices to prevent inflorescence infection is likely to be at least partially efficacious. Removal of the unopened male flower buds (debudding) is effective for Moko control and is routinely practised under plantation conditions where there is a risk of infection; however, the value of this practice has been questioned for Bugtok and Blood diseases where it is thought that infection may also occur via the female flowers (Thwaites et al., 2000). Bagging whole inflorescences has been advocated for Bugtok disease (Soguilon et al., 1995).
Cultivars (clones) of enset have been reported that appear to be tolerant following artificial inoculation with X. campestris pv. musacearum and to recover from natural infections in the field, and these have been recommended for planting in disease-affected areas (Thwaites et al., 2000). At present there is no information concerning the response of banana cultivars to experimental inoculation with X. campestris pv. musacearum; all of the cultivars that are commonly grown in Uganda are susceptible to natural infection.
Not known. Control of insect vectors may prove possible once these have been identified.
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:
X. campestris pv. musacearum is reported to cause sporadic but often considerable losses of enset in Ethiopia, sometimes causing farmers to abandon severely affected gardens for up to 5 years (Yirgou and Bradbury, 1968). Less susceptible varieties of enset have been identified to help manage the disease.
It is likely that the economic and social impact of X. campestris pv. musacearum in Uganda will be devastating, given the importance of banana both as a source of food and income. Preliminary evidence suggests that production from affected cultivations can be reduced by more than 90% in less than a year of the first appearance of the disease. Although many stools survive initial infection and may continue to produce suckers, these suckers often wilt before they flower and fruit bunches that are produced are usually affected and inedible. Further spread of the disease in East Africa and beyond would thus be of very considerable significance to both the cash economy as well as food security of the region.