The citrus industry is facing an unprecedented challenge from Huanglongbing (HLB). All cultivars can be affected by the HLB-associated bacterium ‘Candidatus Liberibacter asiaticus’ (CLas) and there is no known resistance. Insight into HLB pathogenesis is urgently needed in order to develop effective management strategies. Here, we use Sec-delivered effector 1 (SDE1), which is conserved in all CLas isolates, as a molecular probe to understand CLas virulence. We show that SDE1 directly interacts with citrus papain-like cysteine proteases (PLCPs) and inhibits protease activity. PLCPs are defense-inducible and exhibit increased protein accumulation in CLas-infected trees, suggesting a role in citrus defense responses. We analyzed PLCP activity in field samples, revealing specific members that increase in abundance but remain unchanged in activity during infection. SDE1-expressing transgenic citrus also exhibit reduced PLCP activity. These data demonstrate that SDE1 inhibits citrus PLCPs, which are immune-related proteases that enhance defense responses in plants.

The citrus industry is facing an unprecedented crisis due to Huanglongbing (HLB, aka citrus greening disease), a bacterial disease associated with the pathogen Candidatus Liberibacter asiaticus (CLas) that affects all commercial varieties. Transmitted by the Asian citrus psyllid (ACP), CLas colonizes citrus phloem, leading to reduced yield and fruit quality, and eventually tree decline and death. Since adequate curative measures are not available, a key step in HLB management is to restrict the spread of the disease by identifying infected trees and removing them in a timely manner. However, uneven distribution of CLas cells in infected trees and the long latency for disease symptom development makes sampling of trees for CLas detection challenging. Here, we report that a CLas secreted protein can be used as a biomarker for detecting HLB infected citrus. Proteins secreted from CLas cells can presumably move along the phloem, beyond the site of ACP inoculation and CLas colonized plant cells, thereby increasing the chance of detecting infected trees. We generated a polyclonal antibody that effectively binds to the secreted protein and developed serological assays that can successfully detect CLas infection. This work demonstrates that antibody-based diagnosis using a CLas secreted protein as the detection marker for infected trees offers a high-throughput and economic approach that complements the approved quantitative polymerase chain reaction-based methods to enhance HLB management programs.

Spiroplasma citri is a bacterium that causes stubborn disease of citrus and infects other crops, ornamentals, and weeds. It is transmitted by leafhoppers in a circulative manner. Due to limited sequence data on S. citri, the bacterium was isolated from naturally infected Chinese cabbage grown on a farm in Fresno County, CA. DNA from S. citri CC-2 was extracted from a pure culture in LD8 and subjected to PacBio sequencing. Four contigs were obtained with a single circular chromosome of 1,709,192 bp and three plasmids of 40,210, 39,313, and 2,921 bp in size. The genome developed herein extends the sequence database of S. citri and is the first whole-genome sequence record of S. citri from California.

Objectives: Spiroplasma citri is a bacterium with a wide host range and is the causal agent of citrus stubborn and brittle root diseases of citrus and horseradish, respectively. S. citri is transmitted in a circulative, persistent manner by the beet leafhopper, Neoaliturus (Circulifer) tenellus (Baker), in North America. Five strains of S. citri were cultured from citrus, horseradish, and N. tenellus from different habitats and times. DNA from cultures were sequenced and genome assembled to expand the database to improve detection assays and better understand its genetics and evolution. Data description: The whole genome sequence of five strains of S. citri are described herein. The S. citri chromosome was circularized for all five strains and ranged from 1,576,550 to 1,742,208 bp with a G + C content of 25.4-25.6%.

The term 'transmissible small nuclear ribonucleic acids' (TsnRNAs) describes well-characterised viroid RNA species that do not induce any pronounced disease syndromes in specific citrus hosts, but rather act as regulatory genetic elements modifying tree performance. The canopy volume (CV) of 13-yearold navel orange trees (Citrus sinensis) on Poncirus trifoliata rootstock treated with TsnRNA-IIIb (syn. Citrus dwarfing viroid) was reduced by 45% and 53.5% in standard-density (6 m × 6.7 m) or high-density (3 m × 6.7 m) plantings, respectively. The total yield of eight consecutive harvests was not affected significantly by the TsnRNA-IIIb treatments or the two planting densities. However, the yield per land surface unit (Y/LSU) was almost doubled (increased by 97.5%) for the high-density plantings over the standard-density plantings of the untreated controls. The Y/LSU of the TsnRNA-IIIb treated navel orange trees in the standard-density planting was reduced by 32.7%. The TsnRNA-IIIb treatment in both planting densities concentrated significantly more fruit production (approximately 60%) in the economically advantageous middle canopy height zone (0.6-2.4 m) in comparison with the untreated controls (35%). Fruit grade, size, appearance, organoleptic characteristics or time of maturation of the TsnRNA-IIIb dwarfed navel trees were not significantly different between the two planting densities and the controls. Fruit with higher commercial value was produced in the TsnRNA-IIIb dwarfed navel trees in the high-density planting by 3.9% and 4.6% over the TsnRNA-IIIb or controls in standard-density planting respectively. The increase in Y/LSU and fruit value for the TsnRNA-IIIb treated navel trees in the high-density plantings in combination with the reduced management cost of dwarfed trees could result in substantial higher profits for a commercial grove despite the higher establishment cost of high-density plantings.

Phytophthora root rot (PRR) of avocado (Persea americana), caused by Phytophthora cinnamomi, is the most serious disease of avocado worldwide. Previous studies have determined that this pathogen exhibits a primarily clonal reproductive mode but no population level studies have been conducted in the avocado-growing regions of California. Therefore, we used amplified fragment length polymorphism based on 22 polymorphic loci and mating type to investigate pathogen diversity from 138 isolates collected in 2009 to 2010 from 15 groves from the Northern and Southern avocado-growing regions. Additional isolates collected from avocado from 1966 to 2007 as well as isolates from other countries and hosts were also used for comparative purposes. Two distinct clades of A2 mating-type isolates from avocado were found based on neighbor joining analysis; one clade contained both newer and older collections from Northern and Southern California, whereas the other clade only contained isolates collected in 2009 and 2010 from Southern California. A third clade was also found that only contained A1 isolates from various hosts. Within the California population, a total of 16 genotypes were found with only one to four genotypes identified from any one location. The results indicate significant population structure in the California avocado P. cinnamomi population, low genotypic diversity consistent with asexual reproduction, potential evidence for the movement of clonal genotypes between the two growing regions, and a potential introduction of a new clonal lineage into Southern California.

Hylocereus (Berger) Britton and Rose and Selenicereus (Berger) Britton and Rose are two genera of vine cacti, commonly named pitahaya, that produce fruit that are gaining in popularity as an exotic fruit in many countries. There has also been an increasing interest in these fruits because they can be grown in areas that are prone to drought and heat where other fruit production is not possible. However, there is significant taxonomic confusion regarding species within these genera as well as some uncertainty among named varieties. Amplified fragment length polymorphism (AFLP) was used to genotype a large pitahaya collection to determine if there was redundancy in some of the commonly named varieties as well as to assess the overall diversity of the collection. A total of 51 markers were scored from 230 accessions. Seven distinct clades were found with 126 putative clones but only three clades had high bootstrap (greater than 80%) support and one had moderate support (60%). Some of the differently named varieties were identical based on our analysis, but there was also genotypic diversity within putatively named varieties. The results of this study will help growers and researchers to choose genetically distinct accessions from the germplasm collection to investigate how different accessions perform in their growing regions.

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