Grassy Weeds and Corn as Potential Sources of Barley yellow dwarf virus Spread Into Winter Wheat.

Barley yellow dwarf virus (BYDV) is an important vector-borne pathogen of cereals. Although many species of grasses are known to host BYDV, knowledge of their role in virus spread in regional agroecosystems remains limited. Between 2012 and 2016, Idaho winter wheat production was affected by BYDV. BYDV-PAV and the bird cherry-oat aphid (BCOA) (Rhopalosiphum padi L.) vector were commonly present in the affected areas. A series of greenhouse bioassays were performed to examine whether two types of corn (Zea mays L.), dent and sweet, and three commonly found grassy weeds, downy brome (Bromus tectorum L.), green foxtail (Setaria viridis L.), and foxtail barley (Hordeum jubatum L.), can be inoculated with BYDV (species BYDV-PAV) by BCOA and also act as sources of the virus in winter wheat. BCOA successfully transmitted BYDV-PAV to both corn types and all weed species. Virus titers differed between the weed species (P = 0.032) and between corn types (P = 0.001). In transmission bioassays, aphids were able to survive on these host plants during the 5-day acquisition access period and later successfully transmitted BYDV-PAV to winter wheat (var. SY Ovation). Transmission success was positively correlated with the virus titer of the source plant (P < 0.001) and influenced by weed species (P = 0.028) but not corn type. Overall, the results of our inoculation and transmission assays showed that the examined weed species and corn types can be inoculated with BYDV-PAV by BCOA and subsequently act as sources of infections in winter wheat.

Effect of the level of "Candidatus Liberibacter solanacearum" infection on the development of zebra chip disease in different potato genotypes at harvest and post storage.

Potato psyllid (Bactericera cockerelli Sulc)-transmitted "Candidatus Liberibacter solanacearum" (Lso) has been negatively impacting the potato industry in the United States as well as other potato-producing countries. Lso has been linked to a condition known as zebra chip (ZC) that affects yield and quality of potato tubers. Efforts to find sources of resistance to ZC have primarily focused on greenhouse evaluations based on a single inoculation time prior to harvest. Plant response to infection, however, could be influenced by the developmental stage of the host plant, and ZC may continue to develop after harvest. The objectives of this study were to quantify Lso inoculation success, Lso titer, ZC severity and Lso development during storage in eight potato genotypes. These evaluations were conducted on plants infested with Lso-positive psyllids at 77, 12, and 4 days before vine removal (DBVR). The evaluated genotypes were categorized according to their relative resistance to Lso and tolerance to ZC symptoms. Lso inoculation success in the genotype family A07781, derived from Solanum chacoense, was lower than that of the susceptible control ('Russet Burbank'). A07781-4LB and A07781-3LB genotypes were characterized relatively resistant to the pathogen and highly tolerant to ZC symptoms, while A07781-10LB was categorized as susceptible to Lso but relatively tolerant to symptom expression. In stored potatoes, increase in Lso concentrations was observed for all infestation times. However, significantly higher Lso titer was detected in tubers infested 12 DBVR and the effect was similar across genotypes. Overall, the A07781 family can be considered as a promising source of resistance or tolerance to ZC.

Host plants and Wolbachia shape the population genetics of sympatric herbivore populations.

Changing climate and land-use practices have the potential to bring previously isolated populations of pest insects into new sympatry. This heightens the need to better understand how differing patterns of host-plant association, and unique endosymbionts, serve to promote genetic isolation or integration. We addressed these factors in populations of potato psyllid, Bactericera cockerelli (Sulc), a generalist herbivore that vectors a bacterial pathogen (Candidatus Liberibacter solanacearum, causal pathogen of zebra chip disease) of potato (Solanum tuberosum L.). Genome-wide SNP data revealed two major genetic clusters-psyllids collected from potato crops were genetically similar to psyllids found on a common weed, Lycium spp., but dissimilar from those found on another common non-crop host, Solanum dulcamara L. Most psyllids found on Lycium spp. and potato represented a single mitochondrial cytochrome oxidase I (COI) haplotype that has been suggested to not be native to the region, and whose arrival may have been concurrent with zebra chip disease first emerging. The putatively introduced COI haplotype usually co-occurred with endosymbiotic Wolbachia, while the putatively resident COI haplotype generally did not. Genetic intermediates between the two genetic populations of insects were rare, consistent with recent sympatry or reproductive isolation, although admixture patterns of apparent hybrids were consistent with introgression of genes from introduced into resident populations. Our results suggest that both host-plant associations and endosymbionts are shaping the population genetic structure of sympatric psyllid populations associated with different non-crop hosts. It is of future interest to explicitly examine vectorial capacity of the two populations and their potential hybrids, as population structure and hybridization might alter regional vector capacity and disease outbreaks.

Sequence characterization of a Peruvian isolate of Sweet potato chlorotic stunt virus: further variability and a model for p22 acquisition.

Sweet potato chlorotic stunt virus (SPCSV) is probably the most important virus infecting sweetpotato worldwide, causing severe synergistic disease complexes with several co-infecting viruses. To date only one isolate (Ug), corresponding to the EA strain has been completely sequenced. It was later shown to be unusual in that, in contrast to most isolates, it encoded an additional p22 protein at the 3' end of RNA1. We report the complete sequence and genome organization of a Peruvian isolate of SPCSV (m2-47) as determined by siRNA deep sequencing. We confirm that the ORF encoding p22 is lacking from m2-47 and all tested Peruvian and South American isolates, whereas additional isolates containing p22 were identified from Uganda. Other potentially important genomic differences such as two small ORFs encoding putative small hydrophobic proteins instead of one, upstream the hsp70h gene and a more divergent sequence at its RNA1 3'-UTR in contrast to SPCSV isolates that contain p22 are discussed and a model for recent acquisition of p22 in Uganda is proposed. A role for p22 as a pathogenicity enhancer of SPCSV is also provided by complementary expression of p22 in transgenic sweetpotato plants.