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

Abstract

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.

Keywords: Pseudomonas amygdali pv. lachrymans; bioaerosol; cucumber angular leaf spot disease; relative humidity; temperature.

Figure 1

Effect of temperature and relative humidity (RH) on the release of Pseudomonas amygdali pv. lachrymans (Pal) in aerosols from infected cucumber plants. Twenty cucumber plants at the early stage of the angular leaf spot (ALS) disease, were placed in nine exposure chambers, which maintained at temperatures of 18, 25 and 35°C with humidity levels of 35%, 65% and 95% RH per temperature, respectively. (A), Cucumber plants inoculated with sterile water. (B), Symptoms on cucumber plants at the early stage of the ALS disease, which inoculated with Pal::GFP suspension. (C), The release amount of Pal::GFP in aerosols, which detected at 24 h after incubation under different conditions of temperature and RH levels. The experiments were independently repeated three times. Columns and error bars represent the mean values and standard deviations (SDs) from three replicates. Columns labeled with different letters indicate statistically significant differences (p< 0.05).

Figure 2

Effect of temperature and relative humidity (RH) on the initial concentration of Pseudomonas amygdali pv. lachrymans (Pal) in aerosols after aerosolization. Pal::GFP suspension (10⁸ CFU/mL) was aerosolized to nine aerosol chambers, which maintained at 18, 25 and 35°C with three humidity levels of 35%, 65% and 95% RH per temperature, respectively, and the initial concentrations of Pal::GFP in aerosols were examined 0 min after nebulization. The experiments were independently repeated three times. Columns and error bars represent the mean values and standard deviations (SDs) of three replicates. Columns labeled with different letters indicate statistically significant differences (p< 0.05).

Figure 3

Effect of temperature and relative humidity (RH) on the survival rate of Pseudomonas amygdali pv. lachrymans (Pal) in the aerosol chambers. (A) 95% RH, (B) 65% RH and (C) 35% RH. The survival rates of Pal::GFP aerosol were determined at 0, 5, 15, 30, 45, 60 and 75 min after aerosolization, respectively. Error bars represent the standard deviations (SDs) of three replicates.

Figure 4

Effect of temperature and relative humidity (RH) on the size distribution of Pseudomonas amygdali pv. lachrymans (Pal) particles in the aerosol chambers. The size distribution of Pal::GFP aerosol were detected immediately after aerosolizing Pal::GFP suspension (10⁸ CFU/mL) for 30 min at the flowrate of 12 L/min by a six -jet Collision nebulizer. Error bars represent the standard deviations (SDs) of three replicates.

Figure 5

Effect of temperature and relative humidity (RH) on the pathogenicity and colonization of Pseudomonas amygdali pv. lachrymans (Pal) in aerosols on cucumber leaves. Healthy cucumber plant were exposed to Pal::GFP aerosol, generated by a six-jet Collison nebulizer for 30 min at the flow rate of 12 L/min, in chambers at different levels of temperature and RH. (A) The disease index and (B) colonization of Pal::GFP in cucumbers leaves were measured at 7 days after exposed to Pal::GFP aerosols at different levels of temperature and RH. The experiments were independently repeated three times. Columns and error bars represent the mean values and standard deviations (SDs) of three replicates. Columns labeled with different letters indicate statistically significant differences (p< 0.05).

Figure 6

The 3D surface plot of the co-influence of temperature and relative humidity (RH) on concentration levels of Pseudomonas amygdali pv. lachrymans (Pal) in aerosols in naturally infested cucumber greenhouses. Bioaerosol samples were collected from nine naturally infested greenhouses, which located in Tianjin City (n=2), Hebei (n=3) and Shandong (n=4) Provinces, from 2019 to 2021. The concentration of Pal in aerosols was detected by qPCR. Temperature and RH in greenhouses were obtained using a digital thermo-hygrometer. The co-influence of temperature and RH on Pal aerosol concentration presented as a six-parameter model: Z = Z₀ + ax + by + cxy + dx² + fy². In which, Z = Pal aerosols concentration, x = temperature, y = RH.