Phage-based biocontrol strategies can be an effective alternative to control Psa-induced bacterial canker of kiwifruit. The global production of kiwifruit has been seriously affected by Pseudomonas syringae pv. actinidiae (Psa) over the last decade. Psa damages both Actinidia chinensis var. deliciosa (green kiwifruit) but specially the susceptible Actinidia chinensis var. chinensis (gold kiwifruit), resulting in severe economic losses. Treatments for Psa infections currently available are scarce, involving frequent spraying of the kiwifruit plant orchards with copper products. However, copper products should be avoided since they are highly toxic and lead to the development of bacterial resistance to this metal. Antibiotics are also used in some countries, but bacterial resistance to antibiotics is a serious worldwide problem. Therefore, it is essential to develop new approaches for sustainable agriculture production, avoiding the emergence of resistant Psa bacterial strains. Attempts to develop and establish highly accurate approaches to combat and prevent the occurrence of bacterial canker in kiwifruit plants are currently under study, using specific viruses of bacteria (bacteriophages, or phages) to eliminate the Psa. This review discusses the characteristics of Psa-induced kiwifruit canker, Psa transmission pathways, prevention and control, phage-based biocontrol strategies as a new approach to control Psa in kiwifruit orchards and its advantages over other therapies, together with potential ways to bypass phage inactivation by abiotic factors.

Kiwifruit bacterial canker: an integrative view focused on biocontrol strategies.

Huanglongbing (HLB), formerly known as greening, is a bacterial disease restricted to some Asian and African regions until two decades ago. Nowadays, associated bacteria and their vectors have spread to almost all citrus-producing regions, and it is currently considered the most devastating citrus disease. HLB management can be approached in terms of prevention, limiting or avoiding pathogen and associated vectors to reach an area, or in terms of control, trying to reduce the impact of the disease by adopting different cultural strategies depending on infestation/infection levels. In both cases, control of psyllid populations is currently the best way to stop HLB spread. Best cultural actions (CHMAs, TPS system) to attain this goal and, thus, able to limit HLB spread, and ongoing research in this regard is summarized in this review. 

https://zenodo.org/record/7554445/files/cultural%20management%20of%20HLB.pdf

Cultural Management of Huanglongbing: Current Status and Ongoing Research

Fusarium circinatum is a harmful pathogenic fungus mostly attacking Pinus species and also Pseudotsuga menziesii, causing cankers in trees of all ages, damping-off in seedlings, and mortality in cuttings and mother plants for clonal production. This fungus is listed as a quarantine pest in several parts of the world and the trade of potentially contaminated pine material such as cuttings, seedlings or seeds is restricted in order to prevent its spread to disease-free areas. Inspection of plant material often relies on DNA testing and several conventional or real-time PCR based tests targeting F. circinatum are available in the literature. In this work, an international collaborative study joined 23 partners to assess the transferability and the performance of nine molecular protocols, using a wide panel of DNA from 71 representative strains of F. circinatum and related Fusarium species. Diagnostic sensitivity, specificity and accuracy of the nine protocols all reached values >80%, and the diagnostic specificity was the only parameter differing significantly between protocols. The rates of false positives and of false negatives were computed and only the false positive rates differed significantly, ranging from 3.0% to 17.3%. The difference between protocols for some of the performance values were mainly due to cross-reactions with DNA from non-target species, which were either not tested or documented in the original articles. Considering that participating laboratories were free to use their own reagents and equipment, this study demonstrated that the diagnostic protocols for F. circinatum were not easily transferable to end-users. More generally, our results suggest that the use of protocols using conventional or real-time PCR outside their initial development and validation conditions should require careful characterization of the performance data prior to use under modified conditions (i.e. reagents and equipment). Suggestions to improve the transfer are proposed.

/pdf/transferability-of-pcr-based-diagnostic-protocols-an-47j865319u.pdf

Transferability of PCR-based diagnostic protocols: An international collaborative case study assessing protocols targeting the quarantine pine pathogen Fusarium circinatum.

Introduction Huanglonbing (HLB) is the most serious disease of citrus in the world, associated with three non-cultivable phloem-restricted bacteria Candidatus Liberibacter asiaticus (CLas), Ca L. africanus (CLaf) and Ca L. americanus (CLam). CLas is transmitted by the Asian citrus psyllid Diaphorina citri, and has spread to several countries. The African psyllid Trioza erytreae, the vector of CLaf occurs in Africa and neighbouring islands. Only two major citrus-growing regions - Australia/New Zealand and the Mediterranean Basin - are still HLB-free in the world. However, T. erytreae has recently been introduced into continental Europe (Portugal and Spain) and has become a potential threat to citrus production. The transmission of CLas by T. erytreae had been postulated but never tested. To evaluate the risk of T. erytreae transmitting CLas, comparative transmissions of CLas by T. erytreae and D. citri were assessed. Methods Transmission tests were performed on excised leaves and seedlings of Citrus volkameriana with different inoculation access periods (in series) for both insect species. Quantifications of bacterial titers were made in excised leaves, seedlings three and six months after inoculation and on individual insects. Results Our results showed that T. erytreae was able to efficiently acquire CLas. Furthermore, T. erytreae carried significantly higher bacterial titers than D. citri, and was able to efficiently transmit the bacteria to seedlings at a similar rate that D. citri highlighting the high risk of spread of the most aggressive variant of HLB (CLas) by T. erytreae in Europe. Discussion Thus, extreme precautions to prevent any entry of CLas into Europe should be adopted.

/pdf/the-african-citrus-psyllid-trioza-erytreae-an-efficient-fejuhxyo.pdf

The African citrus psyllid Trioza erytreae: An efficient vector of Candidatus Liberibacter asiaticus

Grapevine bacterial canker caused by Xanthomonas citri pv. viticola (X. campestris pv. viticola) (Xcv), was detected in Brazil in 1998 and is currently regarded as a quarantine disease with limited distribution in the country. To improve sensitivity and speed in the detection of Xcv in asymptomatic grapevines, two pairs of primers were designed, targeting sequences of a pathogenicity gene (hrpB) and the xanthomonadin coding cluster. Both pairs were tested in conventional PCR (cPCR) and real-time PCR (qPCR) formats. Primers targeting the hrpB gene showed cross reactions with other Xanthomonas spp. but were effective for use in both cPCR and qPCR, whereas primers for the xanthomonadin gene were highly specific for Xcv but showed low efficiency in qPCR. Enrichment of plant extracts in semi-selective medium before qPCR allowed a significant increase in sensitivity when compared to total DNA extraction, making it possible to detect as low as 101 CFU ml-1. Under natural infection conditions, symptomatic and asymptomatic grapevines were tested by qPCR with hrpB primers and cPCR with xanthomonadin primers. In both cases, plant extracts were enriched for 36-72h. Xcv was detected in all symptomatic samples by qPCR and the result was confirmed by cPCR. For the asymptomatic samples, Xcv was detected in 93.4% with qPCR and in 89.5% with cPCR. These two methods offer advantages in terms of sensitivity and specificity, and they could be useful in quarantine programs, certification of grapevine propagating material and detection of inoculum sources in alternative hosts, contributing to the prevention of pathogen spread to disease-free areas.

Detection of Xanthomonas citri pv. viticola on grapevine by real-time PCR and BIO-PCR using primers designed from pathogenicity and xanthomonadin gene sequences

The aim of this study was to characterise the performance of new molecular methods for the detection and identification of Pseudomonas syringae pv. actinidiae (Psa) and to provide validation data in comparison to the assays mentioned in official diagnostic protocols and being currently used. Eleven molecular tests for the Psa detection were compared in an inter-laboratory comparison where each laboratory had to analyse the same panel of samples consisting of thirteen Psa-spiked kiwifruit wood extracts. Laboratories had to perform also isolation from the wood extracts. Data from this interlaboratory test performance study (TPS) was statistically analysed to assess the performance of each method. In order to provide complete validation data, both for detection and identification, this TPS was supplemented by a further study of identification from pure culture of phylogenetically closely related Pseudomonas spp., Psa, and bacterial strains associated with kiwifruit. The results of both these studies showed that simplex-PCRs gave good results, whereas duplex-PCR and real-time PCR were the most reliable tools for detection and identification of Psa. Nested and multiplex-PCR gave false-positive results. The use of the most reliable detection test is suggested for routine analyses, but when Psa-free status needs to be accurately assessed, it is recommended that at least two detection tests are used. This work provides a wide comparison of the available diagnostic methods, giving new information for a possible revision of the official diagnostic protocols (e.g. European and Mediterranean Plant Protection Organization (EPPO) protocol PM7/120 for the detection of Psa).

/pdf/performance-of-diagnostic-tests-for-the-detection-and-35gyisq5ie.pdf

Performance of diagnostic tests for the detection and identification of Pseudomonas syringae pv. actinidiae (Psa) from woody samples.

Huanglongbing disease affects the Rutaceae family and is associated with three phloem-limited bacterium species: Candidatus Liberibacter asiaticus, africanus and americanus. These species are considered quarantine pathogens in the world, and pose major risks for citrus production and industry. Due to the low titer and the uneven distribution of the bacteria within its host plant, conventional PCR detection protocols can lead to false negative results, especially for early detection. Herein, three real-time PCR diagnostic methods recommended by the EPPO and FAO for asiaticus and africanus species detection were evaluated for their performance and compared with a conventional duplex PCR. Assessments were done as part of an international cooperative project under the EUPHRESCO guidance. Intra-laboratory assessment of the analytical specificity and analytical sensitivity was performed on 33 target or non-target DNA samples and seven target DNA samples were used to determine the sensitivity. Thereafter, repeatability, reproducibility, and concordance odds ratio were assessed on 20 target or non-target DNA samples through a collaborative test performance study organized among eight international laboratories. Results showed that the Li protocol proved to be the best method for asiaticus and africanus species detection, along with the conventional duplex PCR; whereas the Morgan protocol showed high performance only for asiaticus species. Interlaboratory reproducibility was high, suggesting that these real-time PCR methods can be readily transferred to diagnostic laboratories.

Comparison of the performance of the main real-time and conventional PCR detection tests for ‘Candidatus Liberibacter’ spp., plant pathogenic bacteria causing the Huanglongbing disease in Citrus spp.

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Performance of diagnostic tests for the detection and identification of Pseudomonas syringae pv. actinidiae (Psa) from woody samples.

Comparison of the performance of the main real-time and conventional PCR detection tests for ‘Candidatus Liberibacter’ spp., plant pathogenic bacteria causing the Huanglongbing disease in Citrus spp.