BackgroundProduction of cassava (Manihot esculenta Crantz), a food security crop in sub-Saharan Africa, is threatened by the spread of cassava brown streak disease (CBSD) which manifests in part as a corky necrosis in the storage root. It is caused by either of two virus species, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), resulting in up to 100% yield loss in susceptible varieties.MethodsThis study characterized the response of 11 cassava varieties according to CBSD symptom expression and relative CBSV and UCBSV load in a field trial in Uganda. Relative viral load was measured using quantitative RT-PCR using COX as an internal housekeeping gene.ResultsA complex situation was revealed with indications of different resistance mechanisms that restrict virus accumulation and symptom expression. Four response categories were defined. Symptom expression was not always positively correlated with virus load. Substantially different levels of the virus species were found in many genotypes suggesting either resistance to one virus species or the other, or some form of interaction, antagonism or competition between virus species.ConclusionsA substantial amount of research still needs to be undertaken to fully understand the mechanism and genetic bases of resistance. This information will be useful in informing breeding strategies and restricting virus spread.Electronic supplementary materialThe online version of this article (doi:10.1186/s12985-014-0216-x) contains supplementary material, which is available to authorized users.

The banana Xanthomonas wilt disease (BXW) has threatened the livelihood of millions of farmers in East Africa. Use of resistant varieties is the most cost-effective method of managing this bacterial disease. A reliable and rapid screening method is needed to select resistant banana varieties. An in vitro screening method was developed for early evaluation of Xanthomonas wilt resistance using small tissue culture-grown plantlets. Eight cultivars of banana were screened with sixteen isolates of Xanthomonas campestris pv. musacearum using this method. There were significant differences (P<0.0001) in susceptibility among the various banana cultivars tested, whereas no significant difference (P=0.92) in pathogenicity was observed between the pathogen isolates. The cv. Pisang Awak (Kayinja) was found to be highly susceptible and Musa balbisiana resistant. Nakitembe was found to be moderately resistant while cvs Mpologoma, Mbwazirume, Sukali Ndiizi, FHIA-17 and FHIA-25 were susceptible. The susceptibility of these cultivars was further tested in vivo by artificial inoculation of potted plants with similar results. This study shows that an in vitro screening test can serve as a convenient, cheap and rapid screening technique to discriminate BXW-resistant from BXW-susceptible banana cultivars.

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A specific and rapid diagnostic tool has been developed to detect Xanthomonas campestris pv. musacearum, the causal agent of bacterial wilt of banana. PCR primers were developed from intergenic regions of X. campestris pv. musacearum following its partial sequence. A total of 48 primers were tested for specificity to X. campestris pv. musacearum strains collected from various regions in Uganda. These were also tested for specificity against related Xanthomonas species from the vasicola group, Xanthomonas species pathogenic to other crops, and against those existing saprophytically on banana plants. Seven primer sets (Xcm12, Xcm35, Xcm36, Xcm38, Xcm44, Xcm47 and Xcm48) were found to be very specific to X. campestris pv. musacearum. These primer sets directed the amplification of the expected product for all 52 strains of X. campestris pv. musacearum collected from different locations in Uganda. No amplification products were obtained with unrelated phytopathogenic bacteria or endophytic ⁄ epiphytic bacteria from banana. A detection limit of 10 3 CFU mL )1 corresponding to about four cells per PCR reaction was observed when X. campestris pv. musacearum cells were used for all the seven primer sets. The DNA samples from symptomless plant tissues also tested positive with primer set Xcm38. The specific PCR method described here is a valuable diagnostic tool which can be used to detect the pathogen at early stages of infection and monitor disease.

Polymerase chain reaction (PCR) primers (BXW-1 and BXW-3) for conventional PCR were developed from conserved sequences in the hrpB operon of the hrp gene cluster from Xanthomonas campestris pv. musacearum, the causative agent of banana Xanthomonas wilt (BXW). All 50 strains of X. campestris pv. musacearum, isolated from Uganda, Rwanda, and Tanzania, produced a 214-bp amplicon when whole cells, bacterial ooze from infected tissue, and genomic DNA purified from bacterial ooze or infected tissue were used as template. The BXW primers also detected strains of X. axonopodis pv. vasculorum isolated from sugarcane and maize and strains of X. vasicola pv. holcicola isolated from sorghum. All of the strains of X. campestris pv. musacearum were clonal when compared using enterobacterial repetitive intergenic consensus PCR.