Sweetpotato Genotypes 'CIP BRS Nuti' and 'Canadense' Are Resistant to Meloidogyne incognita, M. javanica, and M. enterolobii.

Sweetpotato is a staple crop in Brazil presenting a smaller number of pathogens and diseases in comparison with other root and tuberous crops. Root-knot nematodes are among the most serious sweetpotato root pathogens. The impact of these pathogens also extends to succeeding crops. Because in Brazil, it is common to cultivate more than one crop per season, this problem is rapidly disseminated. The aim of this study was to assess the resistance of two sweetpotato clones and four commercial sweetpotato genotypes to three different Meloidogyne species. Assays were performed under greenhouse and field conditions. 'CIP BRS Nuti' and 'Canadense' were resistant to Meloidogyne incognita, M. javanica, and M. enterolobii under greenhouse conditions. This finding was obtained based on the reproduction factor < 0.2, meaning that the initial population of 5,000 nematodes was reduced to <1,000 individuals after 90 days of inoculation. Tomato and sweetpotato cultivar Beauregard, known to be susceptible to the three nematode species, were highly damaged by the pathogens. Similar results were also observed under field conditions. To our knowledge, 'CIP BRS Nuti' and 'Canadense' are the first South American commercial cultivars with triple nematode resistance.

ARGONAUTE2 mediates RNA-silencing antiviral defenses against Potato virus X in Arabidopsis.

RNA-silencing mechanisms control many aspects of gene regulation including the detection and degradation of viral RNA through the action of, among others, Dicer-like and Argonaute (AGO) proteins. However, the extent to which RNA silencing restricts virus host range has been difficult to separate from other factors that can affect virus-plant compatibility. Here we show that Potato virus X (PVX) can infect Arabidopsis (Arabidopsis thaliana), which is normally a nonhost for PVX, if coinfected with a second virus, Pepper ringspot virus. Here we show that the pepper ringspot virus 12K protein functions as a suppressor of silencing that appears to enable PVX to infect Arabidopsis. We also show that PVX is able to infect Arabidopsis Dicer-like mutants, indicating that RNA silencing is responsible for Arabidopsis nonhost resistance to PVX. Furthermore, we find that restriction of PVX on Arabidopsis also depends on AGO2, suggesting that this AGO protein has evolved to specialize in antiviral defenses.

Citrus Stubborn Severity Is Associated with Spiroplasma citri Titer But Not with Bacterial Genotype.

The impact of citrus stubborn disease, caused by Spiroplasma citri, on citrus production is associated with the symptom severity of infected trees but its association with bacterial levels and virulence are unknown. Fifty-eight S. citri isolates were cultivated from severely and mildly symptomatic trees and randomly amplified polymorphic DNA and short-sequence repeat fingerprinting differentiated four major S. citri genotypes among these isolates. Each genotype was present in both mildly and severely symptomatic trees, suggesting that readily detectable genetic differences in the S. citri populations did not account for differences in disease severity. No variation in the size of amplicons of the pathogenicity-related fructose operon was observed in isolates from trees having varying degrees of symptom expression. Quantitative polymerase chain reaction demonstrated that spiroplasma titer is over 6,000 times higher in fruit from severely symptomatic than from mildly symptomatic trees. The genotypic similarities among S. citri isolates from severely and mildly symptomatic trees, and the consistently higher bacterial titer in the former than in the latter, suggests that titer but not genotype is, at least in part, responsible for the greater symptom severity in some of the S. citri-affected trees in the orchard evaluated.

Capsid protein of cowpea chlorotic mottle virus is a determinant for vector transmission by a beetle.

Cowpea chlorotic mottle virus (CCMV) is a bromovirus transmitted by species of chrysomelid beetles, including the spotted cucumber beetle, Diabrotica undecimpunctata howardii Barber. An experimental system was set up to identify the viral determinant(s) of the beetle transmission of CCMV. Nicotiana clevelandii was selected as an experimental plant host because it supports the replication and accumulation of both CCMV and a second member of the family Bromoviridae, cucumber mosaic virus (CMV). Using a reverse genetic system for CMV, a cDNA copy of the CCMV capsid protein (CP) gene was substituted for that of the CMV CP gene. The resulting 'CMV-hybrid' consisted of wild-type CMV RNA1, RNA2, and a chimeric CMV RNA3 expressing the CCMV structural protein. The CMV-hybrid replicated and formed virions in N. clevelandii; in electron micrographs the hybrid virus was indistinguishable from CCMV. In beetle feeding assays, both CCMV and the CMV-hybrid were transmitted by D. undecimpunctata, while beetle transmission of CMV was not observed. Conversely, only CMV was observed to be transmitted by the aphid Myzus persicae. Surprisingly, the CMV-hybrid was transmitted more efficiently than the parental CCMV, and a virus-induced alteration in beetle feeding behaviour is proposed to account for the difference. These results indicate that the CCMV CP is a viral determinant for beetle vector transmission.

Polymerase Chain Reaction-Based Detection of Spiroplasma citri Associated with Citrus Stubborn Disease.

Polymerase chain reaction (PCR)-based detection of citrus stubborn disease was improved using primers based on sequences of the P89 putative adhesin gene and the P58 putative adhesin multigene of Spiroplasma citri. Real-time PCR also was developed with detection limits estimated to be between 10-4 and 10-4 ng by serial dilution of a recombinant S. citri plasmid into DNA extracts from healthy Madam Vinous sweet orange. PCR for the detection of S. citri by these new primers was validated by comparing culturing of the pathogen, the traditional method of diagnosis, with PCR assays from samples taken from two citrus plots in Kern County, CA. Fruit columella was collected from 384 and 377 individual trees in each of two fields, respectively; one portion was used for culturing and the other for DNA extraction and PCR. PCR results matched those of culturing 85 to 100% of the time depending on the primers used. More importantly, PCR detected S. citri from culture-negative trees in 5 to 15% of the cases, suggesting that PCR performed as well or better than culturing for detection of S. citri in field samples. Real-time PCR proved to be the best method for detection. Differential reaction of the samples to the P58 primer pairs suggested that two populations of S. citri occur in historical and present-day field isolates. Citrus stubborn disease incidence was estimated to be 58.3 and 3.7% in the two orchards. The results presented here support the use of PCR for reliable detection of S. citri in field trees.