Bemisia tabaci (Gennadius) populations, collected from cassava and other plants in major cassava-cultivation areas of Sub-saharan Africa and from elsewhere around the world, were studied to determine their biotype status and genetic variation. Random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) markers were used to examine the genetic structure of the populations. The dendogram obtained using the neighbour joining method (NJ) split the cassava-associated populations from the non-cassava types with a 100% bootstrap probability. Analysis of molecular variance (AMOVA) of the RAPD fragments revealed that 63.2% of the total variation was attributable to differences among populations, while the differences among groups (host) and within populations accounted for 27.1 and 9.8% respectively. Analysis of the internally transcribed spacer region I (ITS 1) of the ribosomal DNA confirmed that the cassava populations of B. tabaci populations were distinct from non-cassava populations. Experiments to establish whitefly populations on various host plants revealed that cassava-associated populations were restricted to cassava only, whereas B. tabaci from other hosts were polyphagous but did not colonize cassava. Hence, populations of B. tabaci from cassava in Africa represent a distinct group.

Restriction fragment length polymorphism (RFLP) analysis of the ribosomal DNA internal transcribed spacer regions of Bemisia tabaci was used to distinguish cassava‐associated populations from other host‐associated populations. Endonuclease restriction profile analysis indicated that cassava‐associated populations from Africa represent a distinct group, with a significant level of separation into subgroups that were not linked to geographical origin. Analysis of molecular variance (amova) revealed that a high proportion of the total genetic variation (47%) was attributable to among‐population differences within the host‐associated groups. Principal coordinate analysis supported the differentiation between the cassava and the non‐cassava group, a result which was in agreement with the cluster analysis of the restriction fragment profile. Internal transcribed spacer RFLP markers, especially SmaI, identified in this study can be used to monitor the spread of B. tabaci biotypes, especially of the more virulent biotype B that has so far not been reported in the cassava‐growing belt of Africa.

Resting spores extracted from wart (Synchytrium endobioticum)-infected potato tubers were used for DNA extraction and amplification of 18S rDNA. Analysis of the cloned, sequenced fragment revealed high similarity to members of the Chytridiomycota. Using this information, specific oligonucleotide probes were designed and arrayed onto glass slides for detection of the pathogen. Viral sequence information available in the databank was retrieved, or new viral sequences were generated, and used to design probes for specific detection of important quarantine viruses of potato. To determine the sensitivity and specificity of the oligonucleotide probes, total RNA from infected plants was reverse transcribed, labelled with Cyanine 5, and hybridised with the microarray. A significant number of the oligonucleotide probes exhibited high specificity to S. endobioticum, Andean potato latent virus, Andean potato mottle virus, Potato black ringspot virus, and Potato spindle tuber viroid. Hybridisation signals of sub-arrays within slides were reproducible (r = 0.79) with a high correlation coefficient of hybridisation repetitions (0.73). Our results demonstrate the potential of microarray-based hybridisation for identification of multiple pathogen targets, which will find application in quarantine laboratories, where parallel testing for diverse pathogens is essential.

High Plains wheat mosaic virus (HPWMoV) is a monocistronic octapartite single-stranded negative-sense RNA virus in the genus Emaravirus, family Fimoviridae (ICTV 2018). It was first reported in 1993 in several High Plains states of the USA (Jensen et al. 1996). It infects a number of cereal crops including wheat (Triticum aestivum L.), maize (Zea mays L.), barley (Hordeum vulgare L.), and some weeds (Seifers et al. 1998). Symptoms induced by HPWMoV are similar to those caused by wheat streak mosaic virus (WSMV) with leaf veins showing yellow flecks and streaks, and severity varying from mild to high. HPWMoV is transmitted by the wheat curl mite Aceria tosichella Keifer (Seifers et al. 2009). It can co-infect wheat with some cereal viruses including wheat streak mosaic virus (WSMV), triticum mosaic virus, barley yellow dwarf virus PAV, and cereal yellow dwarf virus RPV (Burrows et al. 2009). In aA field survey conducted in Southern southern Alberta, Canada, in late June 2017, we sampledevaluated 34 and 37 plants of wheat and grassy weed species, respectively, for the presence of cereal viruses. The sampled wheat plants showed yellow flecks and streaks, with some heavily chlorotic leaves showing necrotic patches along necrosis at leaf margins. Some leaves evenLeaves with extreme symptoms were became completely scorched and/or desiccated. Grassy weed species were collected nearby symptomatic wheat, but did not show any typical symptoms. The incidence of symptoms varied among the sampled wheat fields, with some fields showing more than 75% crop impactedranging from trace levels to 75% infected. The samples were subjected to RT-PCR test for HPWMoVwas performed using primers WMoV-F (Byamukama et al. 2016) and WMoV-RR targeting the RNA 3B region of HPWMoV, after total nucleic acid extraction using in-house procedure (SOP L036). All primer sequences, their location in the genome, and amplicon sizes are given in supplementary table S1. Four wheat plants (1 plant of cv. AAC Elevate, and 3 plants of unknown cultivar), and one plant of foxtail barley plant (Hordeum jubatum L.), from Lethbridge County tested positive for HPWMoV. One wheat plant (cv. Glenn) from the Medicine Hat area also tested positive for HPWMoVthe virus. The assay wasResults were confirmed by two independent RT-PCRs with primers targeting RNA-6 segment (WmoV-F1 and WmoV-R1) and RNA-2 segment (WmoV-F2 and WmoV-R2), respectively. BLASTn analysis of a representative of each of the two fragments, which have been submitted to NCBI under accession numbers MT124696 and MT124697, showed that they share more than 99 % sequence identity with respective RNA species of somethe HPWMoV isolates from the USANebraska and Kansas (KT988861, KJ939624, KJ939629, and KT988866). All but one of the HPWMoV infected wheat plants were also positive for WSMV. WSMV infection was confirmed using DAS-ELISA (Agdia, IN, USA) and RT-PCR with primers targeting the WSMV coat protein (WSMV-CP2 and WSMV-CP4, Dwyer et al. 2007). NAs a second confirmatory method, next-generation sequencing analysis of one RT-PCR positive sample yielded 10,012,849 clean reads. The dsRNA extraction method, library preparation, sequencing and bioinformatics analyses were done according to Rott et al. (2017). The obtained clean reads were searched for viral sequences using Virtool software (https://www.virtool.ca). This analysis revealinged 31 HPWMoV specific contigs. Sixteen contigs, ranging in length from 252 to 1728 bp, covered 72.92% of HPWMoV KS7 isolate’s genome (KT988860 – KT988868) and shared 95-100% identity with it. This virus could be present in, or spread into, other parts of the Canadian prairies where complex co-infections with the other wheat curl mite-transmissible viruses could occur. Co-infections of WSMV and HPWMoV have been reported to correlate with increased symptom severity (Burrows et al. 2009). Primer sequences, their location in genome and amplicon sizes are given in supplementary table S1. WSMV infections have led to wheat yield losses of 7-18% in the Texas panhandle, and the North American prairies, including Alberta, Canada (Hadi et al. 2011). The appearance of HPWMoV in Canada is concerning because co-infections with WSMV could increase severity and yield loss.

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A survey was carried out in the 1996/97 and 1997/98 growing seasons on a field planted in three replicates with five clones of cassava at the International Institute of Tropical Agriculture, Ibadan, located in a transition forest, to determine the effects of cassava genotype and climate on the development of African cassava mosaic geminivirus (ACMV) and changes in the Bemisia tabaci population. Cassava genotype, climate and their interactions have significant (P<0.01) effects on the population of B. tabaci and the development of ACMV. The incidence of ACMV was significantly (P<0.01) higher in clones 81/01635 and 92/0520 than in TMS 30572 and 94/0239, while 91/02327 showed the greatest resistance. A positive correlation between the incidence and severity of ACMV was observed, but this did not correlate with the whitefly population density.

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