Journal of Citrus Pathology

Surveys were conducted to assess Citrus tristeza virus (CTV) in California. Orchard surveys in central California during 2009 to 2013 estimated CTV incidence from 0.05% to 2.9%. Similar surveys in Ventura, Riverside, San Bernardino, and San Diego Counties in 2020-22 estimated CTV incidence from 6.3% to 34.9% while CTV was rarely found in the other five counties surveyed. T30 comprised over 95% of CTV detected alone or in mixtures with other strains in southern California and constituted 59% of 550 CTV accessions maintained in Tulare, California. VT, RB, and S1 genotypes were also found but T36 was rarely detected. No evidence of CTV-induced economic damage was noted except for occasional CTV quick decline on sour orange rootstock.

A stem-pitting isolate of citrus tristeza virus (CTV), spreading in Hunan province of  China (HU-PSTS), was sequenced and indexed on indicator plants. Biological assays showed that HU-PSTS is a highly aggressive stem pitting isolate belonging to biogroup 5. Viral small RNAs (18-26 nt) of the isolate were deep sequenced by Illumina technology to gain genomic information on the CTV strain infecting the source plant. The reads mapping with 17 CTV reference genomes enabled us to re-assemble the genomes of VT, T68, T30 and T3 strains. Among the VT, the highest number of mapped reads (47-41%) was with SG29, T318A, CT11A, Nuaga and AT-1 genomes, whereas T68, T30 and T3 genomes were less represented (28-20%). Alignments with genomes belonging to T36 and RB strains revealed percentage of mapped reads ranging from 10 to 12%. This is the first sequenced genome of a CTV isolate from Hunan province. According to the results, further sequencing and bioindexing need to be developed to explore the potential of a local cross protective strategy.

Currently six viroid species are recognised which infect the genera Citrus and Poncirus, with an additional tentative new species reported. Citrus bent leaf viroid (CBLVd) has been reported from various citrus growing regions world-wide, but has not been formally documented from South Africa. CBLVd was detected in field samples in various citrus growing regions in South Africa during routine diagnostic analyses conducted since 2011. The detection and sequence verification of CBLVd from field samples is reported in this study. Biological confirmation of CLBVd could be done for one sample that was shown to contain a single viroid infection. Bent-leaf symptom expression was observed after slash inoculation of sample RNA to the ‘Etrog’ citron indicator host. This study was a retrospective analysis, of previously identified CLBVd-positive samples, to document the long-standing presence of CBLVd in South Africa.

Florida citrus production from 1980-2021 was examined and modeled to determine the impacts associated with weather events and disease introductions.  Specifically, the study examined the effects of North Atlantic hurricanes, freezes events and two disease introductions -- Asiatic citrus canker (ACC), and Huanglongbing (HLB) -- on productions levels and on the structure of the Florida citrus industry.  The models estimated quantified the effects on production associated with the weather events and disease introductions.  Using the deterministic model generated, forecasts were generated to identify future implications of HLB on Florida citrus production.  Theses generated forecasts were compared to actual production levels and the USDA Crop forecast to test and validate the model.  Whereas testing indicated a significant structural change in the Florida citrus industry resulting from adverse weather events and disease introductions, published economic impact studies were examined and reviewed to gage the resulting reduction in total economic impact.

In the process of recording and studying citrus diseases in the 20^th century, citrus pathologists reported several suspected graft-transmissible disorders that were thought to be of virus or virus-like origin. While later work clarified and characterized most of these disorders, others were left unaddressed beyond their initial reports, and their status has remained unresolved for decades. For this reason, and for lack of a better term, such disorders are considered “phantoms”. In this work, our group performed an extensive literature review and communicated with renowned citrus pathologists and members of the International Organization of Citrus Virologists from around the world. Here we summarize and present in an organized manner the most up to date information for multiple phantom disorders, including disorders that have been subsequently characterized as a result of research efforts of the past 20 years. This review article could act as a reference point for citrus pathologists, regulatory agencies, and industry to clear up any confusion regarding citrus phantom disorders.