Journal of Citrus Pathology

Citrus has been promoted in the Lao People’s Democratic Republic (Lao PDR) as a poverty reduction strategy for at least two decades. However, citrus trees have been in widespread decline for no less than ten years. Since 2010, the authors have observed symptoms on citrus trees consistent with the bacterial disease huanglongbing (HLB). These symptoms included asymmetric leaf mottle, small lopsided fruit, poor fruit production and tree decline. The authors then initiated a long-term study on the occurrence of HLB in southern Lao PDR. Samples of leaf mid-ribs were collected from citrus trees in orchards, nurseries, and backyards across four provinces: Champasak, Sekong, Salavan, and Savannakhet. The presence of ‘Candidatus Liberibacter asiaticus’, the putative causal agent of the Asiatic form of HLB, was confirmed in 59 of 109 samples collected in all four provinces. The Asian citrus psyllid, the vector of ‘Ca. L. asiaticus’, was also observed on citrus trees and tested positive for the pathogen. The implications of these findings for citrus production in Lao PDR are discussed.

Citrus virus A (CiVA) was detected for the first time in Australia in a living pathogen collection. Buds were originally collected from a Washington navel field tree prior to 1970 and graft-inoculated onto a Symons sweet orange indicator plant. The virus was detected using conventional and quantitative reverse transcription polymerase chain reactions and high-throughput sequencing. This variant shares 96.3% (RNA1) and 96.7% (RNA2) nucleotide identity with isolates of CiVA from South Africa and China, respectively. Foliar symptoms of leaf flecking and oak leaf patterns, consistent with detections of CiVA in other regions, were observed on the foliage of the original accession and inoculated indicator plants. Subsequent surveys of an Australian citrus variety collection detected a different CiVA sequence variant in two accessions of Pera sweet orange; this variant had 97% similarity to the other Australian variant.

Samples collected from citrus trees with viroid-like symptoms in citrus orchards in Tucumán, Salta, and Jujuy provinces(northwestern Argentina) were initially indexed on citron (Citrus medica) and then analysed by s-PAGE. These samples werefound to be infected with different viroid species, among them, CEVd and HSVd have been already identified. In order todetermine the presence of other viroids, we performed a RT-PCR assay using specific primers for CBLVd, CDVd, Citrus barkcracking viroid (CBCVd) and Citrus viroid V (CVd-V).

Forty-two samples including 15 lemons, 15 oranges, 8 grapefruits, 2 citrumelos and 2 Cleopatra mandarins were analysed.On the basis of amplification of the appropriately sized DNA, CDVd was detected in thirty-eight samples and CBLVd in allgrapefruit samples. CBCVd and CVd-V were not found in any samples to date. Analysis of the amplicon sequences revealed96% and 97% identity with CBLVd GenBank reference sequences, and 96% to 98% with CDVd GenBank referencesequences.

This is the first report of CBLVd and CDVd in citrus trees in Argentina. The results indicate that CDVd is widely spreadthroughout the surveyed areas and it is even more prevalent than CEVd and HSVd. Moreover, a considerably high percentageof citrus species are affected by mixed viroid infection. It would be important to take it into consideration when developingcitrus disease management strategies.

The dynamics and intensity of new shoot flushes of ‘Pera’ sweet orange scions [Citrus × sinensis (L.) Osbeck] grafted onto ‘Rough’ lemon (Citrus × limonia var. jambhiri Lush.), ‘Swingle’ citrumelo [Citrus × aurantium var. paradisi x Poncirus trifoliata (L.) Raf.] and ‘Sunki’ mandarin (Citrus reticulata ‘Sunki’) rootstocks were evaluated in the field at a citrus farm located in a northern region of the state of Sao Paulo, Brazil. Every 20 days for 16 months, new shoots were counted within a square frame of 0.25 m² set on the central portion of the canopy and classified based on their phenological stages. Trees on ‘Swingle’ rootstock produced a lower area under the flush shoot dynamics curve (AUFSD) and mean number of new shoots than trees on ‘Rough’ lemon or ‘Sunki’ mandarin. For trees on all three rootstocks, new shoot intensities varied significantly over time with the greatest number of new shoots developing during late spring and early summer. Increases in minimum air temperature and available soil water were important indicators of overall emergence of new shoots.

Huanglongbing (HLB), also known as citrus greening, is the most serious disease affecting citrus production in Asia, Africa, the Americas, and the Arabian Peninsula. HLB is associated with the α-Proteobacteria “CandidatusLiberibacter asiaticus” (CLas), “Ca. L. africanus” (CLaf), and “Ca. L. americanus” (CLam). The Bangladesh citrus industry comprises mandarins, sweet oranges, pummelos, limes, and lemons. In 2017-2018, a survey was conducted for two consecutive years in 18 sweet orange growing areas of Bangladesh, and the presence of CLas in these areas was confirmed by polymerase chain reaction. HLB incidence and severity were assessed based on leaf symptoms. The results unveiled a widespread prevalence of HLB with incidence ranging between 0.08 and 56% and severity between 1.80 and 28.33. Information on the genetic diversity of CLas Bangladeshi isolates was obtained based on the presence or absence of Type 1 (SC1, NC_019549.1) and Type 2 (SC2, NC_019550.1) prophages. In silico phylogenetic analyses based on Type 1 and Type 2 prophage sequences showed the presence of four and three clusters of CLas isolates, respectively. Combined phylogenetic analyses of Type 1 and Type 2 prophages indicated the existence of four clusters of CLas isolates. Bangladeshi CLas isolates were found to harbor multiple copies of prophages. The diversity analysis revealed different CLas isolates distributed to different citrus growing areas, indicating spread through propagated materials.

The value of California citrus production in the 2020-21 marketing year was $3.63 billion. The total economic impact of the industry on California’s economy in 2020-21 was $7.6 billion. The California citrus industry added $1.9 billion to California’s state GDP in 2020. Estimated full time equivalent jobs in the California citrus industry in 2020-21 totaled 24,247. Estimated wages paid by the California citrus industry income in 2020-21 totaled $759 million. A 20% reduction in California citrus acreage would cause a loss of 8,213 jobs, $214 million in employee income, and reduce state GDP by $569 million.