Guttation Fluids Containing Pseudomonas amygdali pv. lachrymans Are a Potential Source of Secondary Infection in Cucumber.

Guttation is a common feature of cucumber leaves under high relative humidity conditions; however, little is known about the role of guttation in the transmission of Pseudomonas amygdali pv. lachrymans, which is the pathogen of cucumber angular leaf spot disease. In this study, experimental evidence for the transmission of P. amygdali pv. lachrymans inside cucumber plants and through guttation was provided, and the results proved that P. amygdali pv. lachrymans can be transmitted from the bottom leaf to the upper leaves inside the plant and excreted from the upper leaves through guttation. After that, the third leaf of cucumber was inoculated with P. amygdali pv. lachrymans bacterial suspension, P. amygdali pv. lachrymans was detected on the fifth leaf, the petiole, and the stem and in guttation drops. Healthy cucumber seedlings were infected by P. amygdali pv. lachrymans in the guttation droplets, indicating that guttation fluids containing P. amygdali pv. lachrymans could become a potential source of secondary infection. The results from this study verified the hypothesis that guttation is a potential route for P. amygdali pv. lachrymans excretion from cucumber plants and may be a source of secondary transmission under high relative humidity.

Evaluation of a loop-mediated isothermal amplification assay based on hrpZ gene for rapid detection and identification of Pseudomonas syringae pv. lachrymans in cucumber leaves.

AIMS: Cucumber angular leaf spot caused by Pseudomonas syringae pv. lachrymans (Psl) is an important and destructive disease worldwide, and no effective technique has been developed for the control of the pathogen. Detection of infection or latent in cucumber plants is critical to evaluate disease progress and strengthening management to avoid a serious epidemic in the fields. In this paper, we developed a rapid and sensitive method for detection of Psl using an isothermal method known as loop-mediated amplification (LAMP). METHODS AND RESULTS: A set of six primers was designed to amplify the gene coding for the hrpZ, and conditions for detection were optimized to complete in 60 min at 67°C, and the amplification were confirmed through gel electrophoresis or visually inspected using calcein stain. The specificity of LAMP primers set was widely validated on Psl and nontarget strains. In sensitivity testing, LAMP allowed detection as low as 104  CFU per ml bacterial cells without DNA extraction. The novel method was also applied for detecting Psl in infected cucumber leaves, and even the early onset of disease can be detected by the assay. CONCLUSIONS: This study confirmed that the novel developed LAMP assay is an easy, rapid and sensitive method for the detection of Psl in infected leaves. SIGNIFICANCE AND IMPACT OF THE STUDY: The method is suitable for direct detection of Psl without strain enrichment and complex DNA extraction from samples in the field, and hence it has the capability to be used for on-site disease diagnosis and field surveys.

Effect of temperature and humidity on dynamics and transmission of Pseudomonas amygdali pv. lachrymans aerosols.

Cucumber angular leaf spot (ALS) disease, caused by Pseudomonas amygdali pv. lachrymans (Pal), is an emerging disease with a high incidence that causes severe damage to cucumber worldwide. Bacterial aerosols play a crucial role in the epidemiology of greenhouse ALS disease. However, little is known about the influence of temperature and relative humidity (RH) on the dynamics of Pal in aerosols. A study was conducted to investigate the relationships between the concentration of Pal aerosols and their dependence on temperature and RH in aerosol chambers and greenhouses. The results demonstrated that temperature and RH are both significant factors influencing the release amount, survival time and infectivity of Pal in aerosols, while RH has a greater influence on particle size than temperature across the range of conditions tested. The release amount and survival time of Pal in aerosols under high RH (95%) and low temperature (≤ 25°C) conditions were significantly higher than those under low RH (35%) and high temperature (35°C) conditions. The highest release amount of Pal aerosol (96 CFU/m3) and highest survival rate (98.41%) were found at 18°C and 95% RH, while the highest disease index (DI = 60.9) caused by Pal aerosol was found at 25°C and 95% RH. In addition, Pal aerosols presented a larger diameter (4.7->7.0 µm) under high RH (95% RH) than under dry conditions (≤ 65% RH). These findings will play a crucial role in elucidating the influence of environmental parameters on the dynamics and transmission of Pal in aerosols. Based on our findings, preliminary recommendations for controlling airborne Pal spread involve controlling air temperature and RH, which will contribute to the effective alleviation and control of cucumber ALS disease.

Aerosol transmission of Pseudomonas amygdali pv. lachrymans in greenhouses.

Most previous studies on aerosols have focused on the role of aerosols in the transmission of human and animal pathogens; however, little is known about the role of aerosols in the transmission of plant bacterial disease. In this study, experimental evidence for the emission and transmission of Pseunomonas. amygdali pv. lachrymans (Pal) aerosol was provided, and the results supported that diseased cucumber plants served as the main source of Pal aerosol. Bacterial aerosols released by infected plants played a significant role in the epidemiology of cucumber angular leaf spot (ALS) disease. Aerosol chambers were constructed to study the characteristics of Pal aerosols released by artificially infested cucumber plants. The particle size of Pal aerosol was predominately distributed from 1.1 to 4.7 µm, accounting for 72.16% of the total amount of Pal aerosol. The infection threshold of aerosolized Pal to cause ALS disease was 84-179 CFU/m3. In addition, the transmission dynamics of Pal aerosols from donor cucumber plants to recipient cucumber plants were also confirmed in exposure chambers and greenhouses. The results from the present study verified the hypothesis that aerosol dissemination is a potential route for the epidemiology of plant bacterial disease, and these data will contribute to the development of new strategies for the effective alleviation and control of plant bacterial diseases.

Genetic mapping of psl locus and quantitative trait loci for angular leaf spot resistance in cucumber (Cucumis sativus L.).

One of the most important cucumber diseases is bacterial angular leaf spot (ALS), whose increased occurrence in open-field production has been observed over the last years. To map ALS resistance genes, a recombinant inbred line (RIL) mapping population was developed from a narrow cross of cucumber line Gy14 carrying psl resistance gene and susceptible B10 line. Parental lines and RILs were tested under growth chamber conditions as well as in the field for angular leaf spot symptoms. Based on simple sequence repeat and DArTseq, genotyping a genetic map was constructed, which contained 717 loci in seven linkage groups, spanning 599.7 cM with 0.84 cM on average between markers. Monogenic inheritance of the lack of chlorotic halo around the lesions, which is typical for ALS resistance and related with the presence of recessive psl resistance gene, was confirmed. The psl locus was mapped on cucumber chromosome 5. Two major quantitative trait loci (QTL) psl5.1 and psl5.2 related to disease severity were found and located next to each other on chromosome 5; moreover, psl5.1 was co-located with psl locus. Identified QTL were validated in the field experiment. Constructed genetic map and markers linked to ALS resistance loci are novel resources that can contribute to cucumber breeding programs.