The specificity and sensitivity of polymerase chain reaction (PCR) primers developed for ‘Candidatus Liberibacter solanacearum’ and ‘Candidatus Liberibacter psyllaurous’ were evaluated in conventional and real-time PCR assays. All PCR primers were specific for ‘Ca. L. psyllaurous’ and ‘Ca. L. solanacearum’ insomuch as they did not detect other prokaryotic plant pathogens that affect potato except for the putative pathogens associated with psyllid-yellows and haywire. Conventional PCR assays were capable of detecting 0.19 to 1.56 ng of total DNA per reaction, and real-time PCR was found capable of detecting 1.56 to 6.25 ng of total DNA per reaction, depending on the specific PCR primer set used. ‘Ca. Liberibacter’ species associated with zebra complex disease (ZC) was confirmed in plants affected by this disease throughout Texas from 2005 to 2008, in seed tubers produced in Wyoming in 2007, and in Colorado, Kansas, Nebraska, and Mexico in 2008. A multiplex PCR assay using ‘Ca. L. solanacearum’–specific primers and primers specific for the β-tubulin DNA regions from potato was developed, providing possible utility of the multiplex assay for ‘Ca. Liberibacter’ detection in different solanaceous plant species. Preliminary studies suggest silverleaf nightshade (Solanum elaeagnifolium), wolfberry (Lycium barbarum), black nightshade (S. ptychanthum), and jalapeno pepper (Capsicum annuum) as additional solanaceous hosts for the ZC-associated bacterium. The ‘Ca. Liberibacter’ species detected in all samples divided into two clusters sharing similarity of 99.8% in their partial 16S rRNA gene sequences and 99.3% in their partial intergenic spacer region (ISR)-23S rRNA gene sequences. Genetic variation in the 16S rDNA region consistently matched that of the ISR-23S rDNA region. In this partial 16S-ISR-23S rDNA region, there was a total of eight single nucleotide polymorphisms among ‘Ca. L. psyllaurous’ and ‘Ca. L. solanacearum’ “strains” investigated in this study. ‘Ca. L. solanacearum’ and ‘Ca. L. psyllaurous’ were shown to be very closely related bacteria, if not the same, by successful amplification using a combination of forward primer of ‘Ca. L. solanacearum’ and reverse primer of ‘Ca. L. psyllaurous’ in ZC-affected potato samples. This finding clarifies the current taxonomic status of ‘Ca. L. solanacearum’ and ‘Ca. L. psyllaurous’. The detection of ‘Ca. L. solanacearum’ from haywire-symptomatic potato samples demonstrates that this bacterium might also be associated with this disease.

Field surveys in 2008 determined the prevalence and diversity of viruses present in the Great Plains wheat crops. Symptomatic plants (n = 754) in nine states were tested for Wheat streak mosaic virus (WSMV), Wheat mosaic virus (WMoV, formerly known as High Plains virus), Triticum mosaic virus (TriMV), Barley yellow dwarf virus-PAV (BYDV-PAV), and Cereal yellow dwarf virus-RPV (CYDV-RPV), using indirect ELISA. Virus prevalence varied greatly, with average frequency of detection highest for WSMV (47%), followed by WMoV (19%), TriMV (17%), BYDV-PAV (7%), and lowest for CYDV-RPV (2%). Most positive plant samples (37%) had one virus present, with decreasing frequencies for co-infection by two (19%), three (5%), or four viruses (1%). TriMV was detected for the first time in Colorado, Nebraska, Oklahoma, South Dakota, Texas, and Wyoming. WMoV was identified for the first time in Montana and Wyoming. Chlorotic streaks were more frequently associated with WSMV, WMoV, and TriMV (R = 0.166 to 0.342; P < 0.05), and stunting was more frequently associated with WMoV (R = 0.142; P = 0.004) or TriMV (R = 0.107; P = 0.033) than WSMV. Symptom severity did not increase with co-infection as compared to single virus infections, with the exception of plants co-infected with mite transmitted viruses in Texas. Accepted for publication 1 May 2009. Published 6 July 2009.

Wheat streak mosaic (WSM), caused by Wheat streak mosaic virus (WSMV), is a devastating disease in wheat (Triticum aestivum L.) in the Great Plains of North America. Use of resistance is an effective and environmentally sound method to control the disease. In this study, six wheat genotypes were compared for their responses to WSMV infection under growth chamber conditions. The three resistant genotypes, KS96HW10‐3 (Wsm1), Mace (Wsm1), and CO960293‐2, had disease scores significantly lower than the remaining three genotypes without major resistance. Disease in TAM 111 and TAM 112 was consistently less severe than Karl 92. A population consisting of 188 F2:3 families derived from the cross CO960293‐2 × TAM 111 was used for determining inheritance of the WSMV resistance and for molecular mapping of the resistance in CO960293‐2. Data on segregation of resistance indicated that the resistance in CO960293‐2 is conditioned by a single dominant gene, which was named Wsm2 Transgressive segregation toward susceptibility occurred in the population suggesting a minor gene in the moderately susceptible parent TAM 111, which was not allelic to Wsm2. Wsm2 was mapped to the short arm of chromosome 3B with two flanking simple sequence repeat markers. The single dominant gene inheritance for WSMV resistance in CO960293‐2 has been consistent with the observations that the resistance can be readily transferred to adapted cultivars.

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.

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