https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-93-5-0440

Xanthomonas Wilt: A Threat to Banana Production in East and Central Africa

Xanthomonas is a well-studied genus of bacterial plant pathogens whose members cause a variety of diseases in economically important crops worldwide. Genomic and functional studies of these phytopathogens have provided significant understanding of microbial-host interactions, bacterial virulence and host adaptation mechanisms including microbial ecology and epidemiology. In addition, several strains of Xanthomonas are important as producers of the extracellular polysaccharide, xanthan, used in the food and pharmaceutical industries. This polymer has also been implicated in several phases of the bacterial disease cycle. In this review, we summarise the current knowledge on the infection strategies and regulatory networks controlling virulence and adaptation mechanisms from Xanthomonas species and discuss the novel opportunities that this body of work has provided for disease control and plant health.

Mechanistic insights into host adaptation, virulence and epidemiology of the phytopathogen Xanthomonas.

The Ralstonia solanacearum species complex includes R. solanacearum, R. syzygii, and the Blood Disease Bacterium (BDB). All colonize plant xylem vessels and cause wilt diseases, but with significant biological differences. R. solanacearum is a soilborne bacterium that infects the roots of a broad range of plants. R. syzygii causes Sumatra disease of clove trees and is actively transmitted by cercopoid insects. BDB is also pathogenic to a single host, banana, and is transmitted by pollinating insects. Sequencing and DNA-DNA hybridization studies indicated that despite their phenotypic differences, these three plant pathogens are actually very closely related, falling into the Phylotype IV subgroup of the R. solanacearum species complex. To better understand the relationships among these bacteria, we sequenced and annotated the genomes of R. syzygii strain R24 and BDB strain R229. These genomes were compared to strain PSI07, a closely related Phylotype IV tomato isolate of R. solanacearum, and to five additional R. solanacearum genomes. Whole-genome comparisons confirmed previous phylogenetic results: the three phylotype IV strains share more and larger syntenic regions with each other than with other R. solanacearum strains. Furthermore, the genetic distances between strains, assessed by an in-silico equivalent of DNA-DNA hybridization, unambiguously showed that phylotype IV strains of BDB, R. syzygii and R. solanacearum form one genomic species. Based on these comprehensive data we propose a revision of the taxonomy of the R. solanacearum species complex. The BDB and R. syzygii genomes encoded no obvious unique metabolic capacities and contained no evidence of horizontal gene transfer from bacteria occupying similar niches. Genes specific to R. syzygii and BDB were almost all of unknown function or extrachromosomal origin. Thus, the pathogenic life-styles of these organisms are more probably due to ecological adaptation and genomic convergence during vertical evolution than to the acquisition of DNA by horizontal transfer.

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Ralstonia syzygii, the Blood Disease Bacterium and some Asian R. solanacearum strains form a single genomic species despite divergent lifestyles.

Bacterial diseases of bananas and enset have not received, until recently, an equal amount of attention compared to other major threats to banana production such as the fungal diseases black leaf streak (Mycosphaerella fijiensis) and Fusarium wilt (Fusarium oxysporum f. sp. cubense). However, bacteria cause significant impacts on bananas globally and management practices are not always well known or adopted by farmers. Bacterial diseases in bananas and enset can be divided into three groups: 1) Ralstonia-associated diseases (Moko/Bugtok disease caused by Ralstonia solanacearum and banana blood disease caused by Ralstonia syzygii subsp. celebesensis); 2) Xanthomonas wilt of banana and enset, caused by Xanthomonas campestris pv. musacearum and (3) Erwinia-associated diseases (bacterial head rot or tip-over disease Erwinia carotovora ssp. carotovora and E. chrysanthemi), bacterial rhizome and pseudostem wet rot (Dickeya paradisiaca formerly E. chrysanthemi pv. paradisiaca). Other bacterial diseases of less widespread importance include: Javanese vascular wilt, bacterial wilt of abaca and bacterial fingertip rot (probably caused by Ralstonia spp., unconfirmed). This review describes global distribution, symptoms, pathogenic diversity, epidemiology and the state of the art for sustainable disease management of the major bacterial wilts currently affecting banana and enset.

https://biblio.ugent.be/publication/8594183/file/8594188.pdf

Bacterial Diseases of Bananas and Enset: Current State of Knowledge and Integrated Approaches Toward Sustainable Management

Banana Xanthomonas wilt (BXW), caused by the bacterium Xanthomonas campestris pv. musacearum (Xcm), is the most devastating disease of banana in east and central Africa. The spread of BXW threatens the livelihood of millions of African farmers who depend on banana for food security and income. There are no commercial chemicals, bio-control agents or resistant cultivars available to control BXW. Here we take advantage of the robust resistance conferred by the rice pattern recognition receptor (PRR), XA21, to the rice pathogen Xanthomonas oryzae pv. oryzae (Xoo). We identified a set of genes required for activation of Xa21 mediated immunity (rax) that were conserved in both Xoo and Xcm. Based on the conservation, we hypothesized that intergeneric transfer of Xa21 would confer resistance to Xcm. We evaluated 25 transgenic lines of the banana cultivar ‘Gonja manjaya’ (AAB) using a rapid bioassay and 12 transgenic plants in the glass house for resistance against Xcm. About fifty percent of the transgenic lines showed complete resistance to Xcm in both assays. In contrast, all of the non-transgenic control plants showed severe symptoms that progressed to complete wilting. These results indicate that the constitutive expression of the rice Xa21 gene in banana results in enhanced resistance against Xcm. Furthermore this work demonstrates the feasibility of PRR gene transfer between monocotyledonous species and provides a valuable new tool for controlling the BXW pandemic of banana, a staple food for 100 million people in east Africa.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/pbi.12170

Transgenic expression of the rice Xa21 pattern-recognition receptor in banana (Musa sp.) confers resistance to Xanthomonas campestris pv. musacearum

Banana production in Eastern Africa is threatened by the presence of a new devastating bacterial disease caused by Xanthomonas vasicola pv. musacearum (formerly Xanthomonas campestris pv. musacearum). The disease has been identified in Uganda, Eastern Democratic Republic of Congo, Rwanda and Tanzania. Disease symptoms include wilting and yellowing of leaves, excretion of a yellowish bacterial ooze, premature ripening of the bunch, rotting of fruit and internal yellow discoloration of the vascular bundles. Plants are infected either by insects through the inflorescence or by soil-borne bacterial inoculum through the lower parts of the plant. Short- and long-distance transmission of the disease mainly occurs via contaminated tools and insects, though other organisms such as birds may also be involved. Although no banana cultivar with resistance to the disease has been identified as yet, it appears that certain cultivars have mechanisms to ‘escape’ the disease. Management and control of the disease involve methods that reduce the inoculum’s density and spread of the pathogen. Removal of the male bud (de-budding) has proven to be very effective in preventing the disease incidence since the male bud appears to be the primary infection site. The economic impact of banana Xanthomonas wilt is not fully understood but its impact on food security in the region is very significant. While germplasm screening for the disease is ongoing, efforts to genetically engineer resistance in some banana cultivars are also making good progress. This paper presents a review of the disease and management strategies that have been successful in curtailing its spread.

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Banana Xanthomonas wilt: a review of the disease, management strategies and future research directions

In May 2004, following reports from local farmers of a devastating new banana disease, the first three authors visited Masisi District, 72 km northwest from Goma in North Kivu Province, Democratic Republic of Congo (DRC) (altitude 1700 m above sea level) and diagnosed banana bacterial wilt caused by Xanthomonas campestris pv. musacearum (Xcm). Symptoms were similar to those seen in Uganda (Tushemereirwe et al., 2003) and included: progressive yellowing, wilting and blackening of leaves; yellow or brown vascular streaks throughout the plant; pockets of pale yellow bacterial ooze in airspaces within leaf bases; premature ripening and internal discoloration of fruits; and shrivelling of male inflorescence buds. Inflorescence symptoms probably result from transmission of bacteria by insects, and were uncommon. This may explain the limited spread in DRC since 2001, only c. 10 km from the original focus at Bashali Mokoto village, compared with 400 km in Uganda over the same period (see www.banana.go.ug). As in Uganda, ABB banana genotypes (especially 'Pisang Awak') appear to be the first to be infected, and matooke clones (Musa AAA-EA group) the last. Affected stools do not always die; new suckers emerge and these initially appear healthy but usually become infected from the mother plant, rarely surviving to flowering stage. The epicentre of the outbreak in Masisi was devastated, with total loss of yield and an alarming impact on food security. Using methods described by Tushemereirwe et al. (2004), yellow pigmented bacteria were isolated as almost pure cultures from samples of diseased inflorescence stalks sent to the Global Plant Clinic at CABI Bioscience, UK. Biochemical and molecular characteristics of two isolates were indistinguishable from Xcm from Uganda. Both caused rapid wilting within 7-10 days of inoculation into young banana plants from which the same organism was reisolated. It may be feasible to eradicate this outbreak by destroying affected plants and cleaning up affected fields, combined with removing male flower buds in surrounding healthy plants to prevent insect transmission. However, the first author has recently observed a new disease focus c. 20 km from the first, so continued vigilance and control action will be needed. The origin of these outbreaks is unknown. Until 2001, Xcm was known only from Ethiopia, where it causes disease in enset (Ensete ventricosum) and cultivated banana (Yirgou and Bradbury, 1968, 1974). It is possible that the disease has spread from wild or semicultivated enset plants, which can be found throughout the Masisi region. (Author's abstract).

Presence of banana xanthomonas wilt ( Xanthomonas campestris pv. musacearum ) in the Democratic Republic of Congo (DRC)

Banana xanthomonas wilt (XW) caused by Xanthomonas campestris pv. musacearum (Xcm) attacks all banana cultivars. Xcm in inflorescence-infected Pisang Awak plants with wilting male bud bracts is restricted to the upper parts of the true stem; therefore, cutting these plants at the pseudostem base has been recommended to prevent further Xcm spread. In order to fine-tune existing control strategies, this study examined the movement of Xcm into plants and mats, in relation to disease incubation period. Mature Pisang Awak and East African highland (AAA-EA) plants were inoculated with Xcm through abscission wounds of female bracts, male bud bracts, male flowers, a combination of male bud bracts and flowers, and by cutting male buds with a contaminated machete. Thirty plants per genotype and treatment were monitored for 24 months for disease symptoms. An additional 68 AAA-EA and 33 Pisang Awak plants were sampled weekly to assess the rate of Xcm spread within the plants. All floral entry points resulted in disease, with the highest incidence in combined male bract and male flower abscission wound inoculations. The study confirmed the systemicity of Xcm, with the pathogen able to live within the mat for long periods (5-16 months) without causing disease. Reliance on disease symptom expression to manage XW is therefore not sufficient. The long incubation period in lateral shoots may explain the current resurgence of the disease in locations where the disease was thought to have been successfully eradicated.

Systemicity of Xanthomonas campestris pv. musacearum and time to disease expression after inflorescence infection in East African highland and Pisang Awak bananas in Uganda

The banana bacterial wilt caused by the Xanthomonas campestris pv. musacearum (Xcm) is one of the major constraints to banana production in Uganda. Field observations suggest that the primary means of disease spread is by insect transmission through the male flowers. This study carried out an inventory of insects found on banana inflorescence, investigated possible sources of inoculum in banana plants and determined insect species that carried the bacterium on their bodies and thus possible vectors of the disease. The most abundant insects visiting banana flowers are stainless bee, Plebeina denoiti (Vachal) (Apidae), fruit flies (Drosophilidae) and grass flies (Chloropidae). Female flowers had twice as many insects as male flowers. The bacterial cells have been isolated from the stainless bee P. denoiti, honey bees (Apis melifera), fruit flies and grass flies that had been collected from male flowers of both asymptomatic and symptomatic plants. The bacterial cells isolated from P. denoiti were more than two times as many as other insect groups. Further studies to confirm the mode of transmission by insects, to investigate transmission epidemiology and biology of banana Xanthomonas wilt have been initiated. (Author's abstract).[Le fletrissement bacterien du bananier cause par le Xa est l'une des contraintes majeures de la production de bananes en Ouganda. Les observations de terrain suggerent que les premiers moyens de la dispersion de la maladie sont les fleurs males transportees par les insectes. Au cours de cette etude, on a inventorie les insectes trouves sur le bananier pendant la floraison, les sources possibles d'inoculum sur les bananiers et pour determiner les especes d'insectes qui transportent les bacteries sur leurs corps et ainsi deviennent les vecteurs de la maladie. Les insectes qui visitent le plus souvent les fleurs du bananier sont les abeilles Plebeina denoiti (Vachal) (Apidae), les mouches (Drosophilidae) et les Chloropidae. Les fleurs femelles avaient deux fois plus d'insectes que les fleurs males. Les cellules bacteriennes ont ete isolees du P. denoiti, les abeilles (Apis melifera), les Drosophilidae et les Chloropidae qui ont ete collectees sur fleurs males et les plantes asymptomatiques et symptomatiques. Les cellules des bacteries isolees a partir de P. denoiti etaient deux fois plus importantes que les autres insectes. D'autres etudes sont necessaires pour confirmer le mode de transmission par les insectes et les aspects epidemiologique et la biologie de Xanthomonas sur le bananier. (Resume d'auteur).]

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Role of insects in the transmission of banana bacterial wilt

A semiselective medium was developed for isolating Xanthomonas campestris pv. musacearum (Xcm) from infected banana plants, soil and insect vectors. The new medium was named cellobiose-cephalexin agar (CCA) and it contained (L−1): 1 g yeast extract, 1 g glucose, 1 g peptone, 1 g NH4Cl, 1 g MgSO4·7H2O, 3 g K2HPO4, 1 g beef extract, 10 g cellobiose, 14 g agar, 40 mg cephalexin, 10 mg 5-fluorouracil and 120 mg cycloheximide. The medium was evaluated for selectivity using 21 bacterial isolates and for plating efficiency using Xcm. The bacterial isolates included a soilborne Xanthomonas species and three pathogenic Xanthomonas strains that infect cassava, cabbage and beans. Although the plating efficiency of Xcm on CCA was lower (59%) than on non-selective yeast extract peptone glucose agar (YPGA), its selectivity was significantly higher, averaging 60 and 82%, when isolating from banana fruits and soil, respectively. CCA was also superior when isolating Xcm from insect vectors, with selectivity of 48–75%, compared with 8–17% on YPGA. Xanthomonas campestris pv. phaseoli did not grow on CCA, while X. campestris pv. campestris and X. axonopodis pv. manihotis grew, but their colonies were smaller than those of Xcm. Twenty-nine out of 33 suspected Xcm strains isolated from plants, soil and insects using CCA were pathogenic when inoculated onto banana plants, indicating that CCA can be a reliable tool in isolating Xcm populations. The medium should prove useful in studies on ecology, epidemiology and management of the banana bacterial wilt pathogen that is currently ravaging bananas in East and Central Africa.

S. Eden-Green

Papers

Banana Xanthomonas wilt: a review of the disease, management strategies and future research directions

Presence of banana xanthomonas wilt ( Xanthomonas campestris pv. musacearum ) in the Democratic Republic of Congo (DRC)

Systemicity of Xanthomonas campestris pv. musacearum and time to disease expression after inflorescence infection in East African highland and Pisang Awak bananas in Uganda

Role of insects in the transmission of banana bacterial wilt

Development of a semiselective medium for isolating Xanthomonas campestris pv. musacearum from insect vectors, infected plant material and soil