Banana Xanthomonas wilt
|Xanthomonas campestris pv. musacearum|
|Xanthomonas campestris pv. musacearum
Banana Xanthomonas Wilt (BXW), or banana bacterial wilt (BBW) or enset wilt is a bacterial disease caused by Xanthomonas campestris pv. musacearum. After being originally identified on a close relative of banana, Ensete ventricosum, in Ethiopia in the 1960s, BXW emanated in Uganda in 2001 affecting all types of banana cultivars. Since then BXW has been diagnosed in Central and East Africa including banana growing regions of: Rwanda, Democratic Republic of the Congo, Tanzania, Kenya, Burundi, and Uganda.
Of the numerous diseases infecting bananas, BXW alongside banana bunchy top virus has been the most devastating in recent years. Global concern arose over the livelihoods of African banana farmers and the millions relying on bananas as a staple food when the disease was at its worst between the years 2001 and 2005. It was estimated that in Central Uganda from 2001 and 2004, there was a 30-52 % decease in banana yield due to BXW infection. Although extensive management of the disease outbreaks has helped reduce the impact of Banana Xanthomonas Wilt even today BXW continues to a pose a real problem to the banana farmer of Central and East Africa.
BXW symptoms can be sorted into two domains: symptoms on the inflorescence and symptoms on the fruit. Symptoms on the fruit are usually used to distinguish BXW from alternative banana diseases. A bacterial ooze is excreted from the plant organs and this is a mandatory sign that BXW may be present. Common symptoms on the fruit include internal discoloration and premature ripening of the fruit. A cross section of the BXW infected banana is characterized by the yellow- orange discoloration of the vascular bundles and dark brown tissue scaring. Symptoms on the inflorescence include a gradual wilting and yellowing of the leaves plus wilting of the bracts and shriveling of the male buds. Many factors may affect the combination of disease symptoms on show. These include the particular cultivar infected, how the disease has been transmitted and the current growing season.
Soil is one of the main sources for Xanthomonas campestris pv. musacearum inoculum. Xanthomonas campestris pv. musacearum may contaminate the soil for four months and more. BXW awareness campaigns have helped reduce the numbers of farmers growing bananas on contaminated plantains aiding in the control of the disease overall. Transmission of contaminated disease itself is thought to be low.
It widely thought that Xanthomonas campestris pv. musacearum bacteria is transmitted to airborne vectors through exposed male flowers (see plant reproductive morphology). Xanthomonas campestris pv. musacearum bacteria has been isolated from the ooze and nectar excreted from openings of fallen male flowers. Insects, namely stingless bees (Apidae), fruit flies (Drosophilidae) and grass flies (Chloropidae), transmit the disease from banana to banana after being drawn to the infected nectar. If the disease has been transmitted by insects the symptoms tend to first appear on the male buds of the banana plant.
The knife (panga) is used almost universally in African agriculture. Use of contaminated knives was a common method for disease spread when the disease first originated but increased knowledge of BXW transmission has led to increased numbers knives being disinfected after use. Herbicides are now advised as a more economical and effective way of destroying infected banana crop.
Infected plant material
BXW infects all parts of the plant. Disease spread has been primarily linked with the transport of plants shoots for replanting. Other parts of the plant such as the male buds (used in banana beer production) and mulch (banana waste material) can also expose novel regions to the disease.
Control of BXW is based upon a variety of methods to help prevent the spread of the disease. Vigilance and the quick removal of infected plants remain critical to minimising spread of the disease.
Infected plants can be removed using herbicides or more commonly by cutting the plant into small fragments and decomposing. The risk of infection can be lowered by removal of the male bud ('debudding') but many farmers believe this is essential to the quality of the banana fruit. The risk of infection decreases if the plants are not covered with topsoil. However the risk of disease should be balanced against the resulting decrease in yield of the banana plantain. A major part of disease control is the disinfecting of the tools used.
Much of the work in controlling BXW has been done through educational campaigns raising awareness of the disease to the banana farmers. For example: in Uganda and Tanzania where the government has actively worked alongside farmers to help limit spread of the disease, over 90% control of BXW has been reported. Moreover much of the information taught to the farmers can be used in the control of other banana infecting diseases.
BXW resistant banana
No banana cultivars in Central and Eastern Africa have shown any resistance to BXW despite some varieties, such as those in the 'Pisang Awak' region, showing increased susceptibility. Scientists have recently transferred two genes from sweet green pepper to bananas in order to confer resistance to BXW. This is a promising step forward in circumventing the time consuming and expensive practices of disease management such as 'debudding'.
Pflp and Hrap genes encoding the proteins plant ferredoxin-like amphipathic protein (pflp) and hypersensitive response-assisting protein (hrap) were isolated from sweet pepper and introduced to the genome of East African bananas using genetic engineering. The two proteins induced a hypersensitive response and systemic acquired resistance within the banana plant after being exposed to the bacterial pathogen. It was reported that over half of the transgenic bananas were resistant to BXW, resistance that was also found in field trials.
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