'Candidatus Liberibacter solanacearum' Titers in and Infection Effects on Potato Tuber Chemistry of Promising Germplasm Exhibiting Tolerance to Zebra Chip Disease.

Long-term sustainable management of zebra chip (ZC) disease of potato requires development of tolerant or resistant germplasm. To this end, 283 potato varieties and breeding clones were infected with the ZC putative causal agent 'Candidatus Liberibacter solanacearum' (Lso) by potato psyllid vector inoculations in 2010, 2011, 2012, and 2013. Potato germplasm was then examined for development of fresh and fried ZC symptoms. Over multiple years 29 breeding clones exhibited little to no symptoms in freshly cut tuber slices, and five exhibited little to no symptoms in fried slices. These five presumed tolerant breeding clones were chosen for further screening to determine whether the lack of physiological responses to Lso infection was the cause of observed tolerance. To this end, tuber amino acid, sugar, and phenolic levels were compared between noninfected and Lso-infected plants. The five putative tolerant clones had less dramatic shifts in host physiology following Lso infection than the susceptible Atlantic cultivar. This suggested lack of host responses to Lso infection that result in major changes in tuber biochemistry is a potential mechanism of ZC resistance. However, the susceptible Atlantic cultivar did have consistently greater Lso titers compared with two of the tolerant entries, so for these reductions in Lso pathogen progression also might be a factor. Regardless, lack of host responses could still remain one trait that could be used to aid in selection of ZC-resistant potato varieties, as other tolerant lines had infection levels consistent with susceptible Atlantic cultivar. These results also suggest that germplasm derived from relatives of cultivated potato plants are viable sources of ZC disease resistance.

Direct quantitative electrical measurement of many-body interactions in exciton complexes in InAs quantum dots.

We present capacitance-voltage spectra for the conduction band states of InAs quantum dots obtained under continuous illumination. The illumination leads to the appearance of additional charging peaks that we attribute to the charging of electrons into quantum dots containing a variable number of illumination-induced holes. By this we demonstrate an electrical measurement of excitonic states in quantum dots. Magnetocapacitance-voltage spectroscopy reveals that the electron always tunnels into the lowest electronic state. This allows us to directly extract, from the highly correlated many-body states, the correlation energy. The results are compared quantitatively to state of the art atomistic configuration interaction calculations, showing very good agreement for a lower level of excitations and also limitations of the approach for an increasing number of particles. Our experiments offer a rare benchmark to many-body theoretical calculations.

Seasonal population dynamics of the potato psyllid (Hemiptera: Triozidae) and its associated pathogen "Candidatus Liberibacter solanacearum" in potatoes in the southern great plains of North America.

The potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), and its associated pathogen "Candidatus Liberibacter solanacearum" (Ca. L. solanacearum), the putative causal agent of zebra chip (ZC) disease in potatoes (Solanum tuberosum L.), were sampled in commercial potato fields and untreated control plots for 3 yr in multiple locations in Texas, Kansas, Nebraska, and Colorado. Populations of the potato psyllid varied across years and across potato growing regions. However, the percentage of potato psyllids infected with Ca. L. solanacearum although variable across years, was consistently highest in the Lower Rio Grande Valley of Texas (LRGV), the reported overwintering location for this pest. The numbers of Ca. L. solanacearum-infected psyllids collected on field traps and large nymphs counted on leaf samples were both positively correlated with the final percentage of ZC in tubers. In the LRGV, where vector and disease pressure is the highest, population levels of immature life stages of the psyllid and percentage of ZC differed greatly between commercial and untreated fields. These results show that the pest management program that was used can be effective at controlling development of the psyllid and ultimately reducing the incidence of ZC.

Dependence of the redshifted and blueshifted photoluminescence spectra of single In(x)Ga(1-x)As/GaAs quantum dots on the applied uniaxial stress.

We apply external uniaxial stress to tailor the optical properties of In(x)Ga(1-x)As/GaAs quantum dots. Unexpectedly, the emission energy of single quantum dots controllably shifts to both higher and lower energies under tensile strain. Theoretical calculations using a million atom empirical pseudopotential many-body method indicate that the shifting direction and magnitude depend on the lateral extension and more interestingly on the gallium content of the quantum dots. Our experimental results are in good agreement with the underlying theory.

Tuning the exciton binding energies in single self-assembled InGaAs/GaAs quantum dots by piezoelectric-induced biaxial stress.

We study the effect of an external biaxial stress on the light emission of single InGaAs/GaAs(001) quantum dots placed onto piezoelectric actuators. With increasing compression, the emission blueshifts and the binding energies of the positive trion (X+) and biexciton (XX) relative to the neutral exciton (X) show a monotonic increase. This phenomenon is mainly ascribed to changes in electron and hole localization and it provides a robust method to achieve color coincidence in the emission of X and XX, which is a prerequisite for the possible generation of entangled photon pairs via the recently proposed "time reordering" scheme.

First Report of 'Candidatus Liberibacter psyllaurous' in Zebra Chip Symptomatic Potatoes from California.

A disease that severely affects processing potatoes (Solanum tuberosum L.), termed zebra chip (ZC), has been identified in several locations in the United States (Texas, Nebraska, Colorado, Kansas, New Mexico, Arizona, and Nevada), Mexico, and Central America (4). The disease name comes from the rapid oxidative darkening of freshly cut tubers and the dark stripes and blotches that occur in chips processed from infected tubers. Recently, the disorder has been associated with a new 'Candidatus Liberibacter' species in New Zealand (3). Also, a bacterium designated 'Candidatus Liberibacter psyllaurous' has been identified recently in potato plants with "psyllid yellows" symptoms that resemble foliar symptoms of ZC (2). In the fall of 2008, 10 tubers were received at the Prosser laboratory from a commercial potato grower and five had symptoms characteristic of ZC. The tubers, cv. Dakota Pearl, were grown near Lancaster in southern California. The tubers showed rapid oxidation upon slicing and the sunken stolon attachment characteristic of ZC (4). Nucleic acid was extracted from symptomatic tubers (1) and tested by PCR for 'Ca. Liberibacter' species with primer pairs OA2/OI2c (5'-GCGCTTATTTTTAATAGGAGCGGCA-3' and 5'-GCCTCGCGACTTCGCAACCCAT-3') and CL514F/R (5'-CTCTAAGATTTCGGTTGGTT-3' and 5'-TATATCTATCGTTGCACCAG-3'), which amplify from the 16S rDNA and rplJ and rplL ribosomal protein genes, respectively (3). Four of the five tubers with distinct ZC symptoms yielded the expected amplicons with both primer pairs. Two tubers with mild internal discoloration yielded correctly sized amplicons but in lesser amounts than from the severely affected tubers. Nucleic acid from healthy potato tubers yielded no product with these primers. One clone of the 1,168-bp OA2/OI2c amplicon and two clones of the 669-bp CL514F/R amplicon from a strongly positive sample were sequenced in both directions (ACGT, Inc., Wheeling, IL). BLAST alignments of the consensus sequences of the OA2/OI2c and CL514F/R amplicons (GenBank Accessions Nos. FJ498802 and FJ498803, respectively) revealed 100% identity with analogous 'Ca. Liberibacter' sequences reported from ZC-symptomatic potatoes in New Zealand (GenBank Accession Nos. EU849020 and EU919514). The OA2/OI2c amplicon was also identical to a sequence of 'Ca. Liberibacter psyllaurous' (GenBank Accession No. EU812559) from psyllid yellows-affected potatoes in the United States (2) and also showed a 99% identity with sequences from a 'Ca. Liberibacter' species reported in ZC tubers from Kansas (GenBank Accession No. EU921626). Potato crops with symptoms of ZC have been observed previously in California (4), but this is the first identification of 'Ca. Liberibacter psyllaurous' from diseased potatoes grown in California. Since ZC was first reported in the mid- to late-1990s, information from potato growers and processors suggests that ZC is becoming more important. The disease has caused millions of dollars in losses, particularly in south Texas (4). The identification of 'Ca. Liberibacter psyllaurous' in California provides additional evidence that the disease is increasing in importance in other potato-growing regions. References: (1) J. M. Crosslin et al. Plant Dis. 90:663, 2006. (2) A. K. Hansen et al. Appl. Environ. Microbiol. 74:5862, 2008. (3) L. W. Liefting et al. Plant Dis. 92:1474, 2008. (4) J. E. Munyaneza et al. Subtrop. Plant Sci. 59:30, 2007.

Fine structure of negatively and positively charged excitons in semiconductor quantum dots: electron-hole asymmetry.

We present new understanding of excitonic fine structure in close-to-symmetric InAs/GaAs and InGaAs/GaAs quantum dots. We demonstrate excellent agreement between spectroscopy and many-body pseudopotential theory in the energy splittings, selection rules and polarizations of the optical emissions from doubly charged excitons. We discover a marked difference between the fine structure of the doubly negatively and doubly positively charged excitons. The features in the doubly charged emission spectra are shown to arise mainly from the lack of inversion symmetry in the underlying crystal lattice.