Effect of Different Ventilation Methods in Plastic Sheds on the Spread of Cucumber Bacterial Angular Leaf Spot

Cucumber bacterial angular leaf spot, a global bacterial disease, has caused significant economic losses to the cucumber industry. Ventilation mode, temperature, humidity and other environmental factors in the greenhouse are important factors that affect the diffusion and transmission of pathogens. In this study, the impact of different ventilation methods in plastic sheds on the distribution and variation of temperature and humidity inside the greenhouse, as well as the diffusion of pathogens and disease transmission were evaluated.

Three ventilation methods (20.0 m×7.0 m×3.0 m) were set up in this experiment, including bottom and top ventilation, top ventilation, and bottom ventilation. After atomizing the bacterial suspension of Pal pathogens manually at the central position of the plastic shed for 0.5, 1.5, and 2.5 h, nutrient agar plates were used to collect Pal pathogens at distances of 0, 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0 m from the center of the inoculum source. Based on the results of colony counts, a dynamic diagram of the deposition and diffusion of Pal pathogens was drawn to analyze the impact of ventilation modes on pathogen diffusion. An inoculation area for Pal pathogens was set up in the central location of the plastic shed. The pathogen sampler for agricultural cultivation space was used to collect Pal aerosols at distances of 0, 0.5, and 1.5 m from the inoculation area. Next, qPCR technology was adopted to detect the concentration of Pal pathogens, while the disease index of cucumbers was also investigated. Temperature and humidity recorders were used to monitor temperature and humidity data at different locations inside the cucumber greenhouse. Then, temperature and humidity distribution maps under different ventilation modes in the longitudinal and cross sections of the greenhouse were drawn and the impact of ventilation methods on the distribution and changes of temperature and humidity inside the greenhouse was analyzed.

The different ventilation methods in plastic sheds had a significant impact on the spread of Pal pathogens. With bottom and top ventilation, the concentration of Pal pathogens at different locations within the plastic shed was lowest, and the colony count was 3-469 CFU/m²; whereas with only bottom ventilation or top ventilation, there was local accumulation and more deposition of the pathogens, with the colony counts being 2-847 and 5-800 CFU/m², respectively. In field experiments on cucumber cultivation in plastic sheds, ventilation methods significantly affected the regulation of temperature and humidity inside the greenhouse as well as the transmission efficiency of cucumber bacterial angular leaf spot. Among them, the bottom and top ventilation treatment resulted in the best overall cooling and dehumidification effect inside the plastic sheds, with the lowest relative humidity at night. The humidity of the longitudinal and cross sections inside the greenhouse at 24: 00 were 70.6%RH-83.7%RH and 80.3%RH-93.0%RH, respectively, which were lower than those under the bottom ventilation treatment (80.2%RH-87.3%RH and 91.1%RH-96.1%RH) and top ventilation treatment (78.2%RH-92.2%RH and 91.9%RH-96.4%RH). Additionally, the concentration of Pal pathogens was the lowest at 8 128 CFU/m³, leading to the lightest disease incidence of cucumbers with a disease index of 23.33. The top ventilation treatment was the second best, with a concentration of Pal pathogens at 13 542 CFU/m³ and a cucumber disease index of 27.39. For the bottom ventilation treatment, the concentration of Pal pathogens was the highest, reaching 27 954 CFU/m³, resulting in the most severe disease incidence of cucumbers, with a disease index of 32.96.

The bottom and top ventilation strategy in plastic sheds has a good effect on cooling and dehumidifying the greenhouse. The spread of Pal pathogens is slow, resulting in mild disease incidence in cucumbers. The results of this study can provide a basis for guiding ventilation regulation in cucumber cultivation.