Effects of mancozeb on citrus rhizosphere bacterial community.

Multiple and consecutive application of fungicide might damage the rhizosphere bacterial community of citrus. In order to evaluated effect of mancozeb on the chemical properties of citrus-cultivated soil and the richness and diversity of rhizosphere bacterial community. The abundance response of rhizosphere bacterial groups without application or with application of 1.333 g mg-1 mancozeb for 2, 4, 6 and 8 times were investigated, and further studied the relationship between the rhizosphere bacterial community and chemical properties of citrus-cultivated soil. We found the rhizosphere bacterial composition and diversity were distinct between soil planted with citrus and without citrus, in addition, the abundance of rhizosphere-associated bacterial species in the soil planted with citrus increased significantly. Meanwhile, the chemical properties and the richness and diversity of rhizosphere bacterial community of the soil planted with citrus did not significantly change among different application frequence of mancozeb. Moreover, with the increased applying times of mancozeb, the relative abundance of Candidatus, Saccharibacteria, Parcubacteria, and Proteobacteria increased but the abundance of Nitrospirae decreased. In our one-year trial, there were less adverse effects of mancozeb on the citrus-cultivated rhizosphere by the repeated application of mancozeb. Therefore, mancozeb, as a fungicide, could be used multiple times to control citrus disease.

Morphological, molecular and virulence characterization of three Lencanicillium species infecting Asian citrus psyllids in Huangyan citrus groves.

Citrus greening or Huanglongbing (HLB) is caused by the infection of Candidatus Liberibacter spp. in citrus plants. Since Asian citrus psyllid is the primary vector of this bacterial pathogen, the spread of HLB can be mitigated by suppressing Asian citrus psyllid populations in citrus groves using entomopathogens. To expand the current data on entomopathogens infecting Asian citrus psyllids, we isolated and characterized three different entomopathogens. Strains ZJLSP07, ZJLA08, and ZJLP09 infected the Asian citrus psyllid, Diaphorina citri Kuwayama, in Huangyan citrus groves. Based on molecular and morphological analyses, two were identified as Lecanicillium attenuatum and Lecanicillium psalliotae, and the third was recognized as an unidentified species of the genus, Lecanicillium. The corrected mortalities caused by strains ZJLSP07, ZJLA08 were 100% at 7days post-inoculation, while by ZJLP09 complete mortality occurred at 6days after inoculation, with 1.0×10(8)conidia/ml at 25°C and a relative humidity of 90% in the laboratory. Under the same condition, the corrected mortalities caused by strains ZJLSP07, ZJLA08 and ZJLP09 were 100%, 92.55% and 100%, respectively at 9days post-inoculation in the greenhouse. Our findings also revealed that these fungal strains infected D. citri using hyphae that penetrated deep into the insect tissues. Further, all three strains secreted the enzymes proteinases, chitinases and lipases with a potential to destroy insect tissues. Interestingly, strain ZJLP09 had an earlier invasion time and the highest levels of enzyme activities when compared to the other two strains. These findings have expanded the existing pool of entomopathogenic fungi that infect D. citri and can be potentially used for the management of D. citri populations.

A New Diagnostic System for Detection of 'Candidatus Liberibacter asiaticus' Infection in Citrus.

In this study, two polyclonal antibodies were produced against the Omp protein of 'Candidatus Liberibacter asiaticus'. First, omp genes were sequenced to exhibit 99.9% identity among 137 isolates collected from different geographical origins. Then, two peptides containing the hydrophobic polypeptide-transport-associated (POTRA) domain and ß-barrel domain, respectively, were identified on Omp protein. After that, these two peptides were overexpressed in Escherichia coli and purified by affinity chromatography to immunize the white rabbits. Finally, the antiserum was purified by affinity chromatography. The two Omp antibodies gave positive results (0.454 to 0.633, 1:1,600 dilution) in enzyme-linked immunosorbent assay against 'Ca. L. asiaticus'-infected samples collected from different geographical origins but revealed negative results against other pathogen-infected, nutrient-deficient and healthy samples. The antibody against the POTRA domain of Omp protein could detect 'Ca. L. asiaticus' in 45.7% of the symptomatic samples compared with a 56.2% detection rate with a polymerase chain reaction assay. These new antibodies will provide a very useful supplement to the current approaches to 'Ca. L. asiaticus' detection and also provide powerful research tools for tracking distribution of this pathogen in vivo.

Genetic diversity and population structure of rice stripe virus in China.

Rice stripe virus (RSV) is one of the most economically important pathogens of rice and is repeatedly epidemic in China, Japan and Korea. The most recent outbreak of RSV in eastern China in 2000 caused significant losses and raised serious concerns. In this paper, we provide a genotyping profile of RSV field isolates and describe the population structure of RSV in China, based on the nucleotide sequences of isolates collected from different geographical regions during 1997-2004. RSV isolates could be divided into two or three subtypes, depending on which gene was analysed. The genetic distances between subtypes range from 0.050 to 0.067. The population from eastern China is composed only of subtype I/IB isolates. In contrast, the population from Yunnan province (southwest China) is composed mainly of subtype II isolates, but also contains a small proportion of subtype I/IB isolates and subtype IA isolates. However, subpopulations collected from different districts in eastern China or Yunnan province are not genetically differentiated and show frequent gene flow. RSV genes were found to be under strong negative selection. Our data suggest that the most recent outbreak of RSV in eastern China was not due to the invasion of new RSV subtype(s). The evolutionary processes contributing to the observed genetic diversity and population structure are discussed.

Pc4, a putative movement protein of Rice stripe virus, interacts with a type I DnaJ protein and a small Hsp of rice.

Rice stripe virus (RSV) infects rice and causes great yield reduction in some Asian countries. In this study, rice cDNA library was screened by a Gal4-based yeast two-hybrid system using pc4, a putative movement protein of RSV, as the bait. A number of positive colonies were identified and sequence analysis revealed that they might correspond to ten independent proteins. Two of them were selected and further characterized. The two proteins were a J protein and a small Hsp, respectively. Interactions between Pc4 and the two proteins were confirmed using coimmunoprecipitation. Implications of the findings that pc4 interacted with two chaperone proteins were discussed.