Effect of condensation dehumidification on nocturnal humidity regulation and gray mold prevention in tomato solar greenhouses

Tomatoes are commonly cultivated in solar greenhouses worldwide to extend growing seasons and secure stable yields. In winter, the low nocturnal temperatures of cold seasons combined with the high airtightness of solar greenhouse structures severely restrict air circulation. This poor airflow leads to the accumulation of water vapor released by tomato plant transpiration and soil evaporation, thereby forming persistent high air relative humidity that lasts for extended nightly periods. Such prolonged high-humidity environments significantly increase the susceptibility to Botrytis cinerea infection—a destructive fungal disease that impairs tomato quality and reduces marketable yields. To address this critical production issue, the present study introduced refrigerated dehumidifiers into the nocturnal environment of solar greenhouses, with one unit placed at the northeast corner and another at the southwest corner of the experimental greenhouse plot to ensure uniform dehumidification coverage. Air temperature and relative humidity were closely monitored using high-precision sensors of air temperature and humidity, which were deployed at representative sampling points in both the test area (with dehumidifiers) and the control area (without dehumidifiers). By systematically comparing the dynamic differences in temperature and humidity parameters between the two areas, the study further explored the regulatory effect of condensation dehumidification on the nocturnal humidity environment of solar greenhouses and quantitatively evaluated its preventive efficacy against Botrytis cinerea. The results demonstrated that refrigerated dehumidifiers could operate effectively under the typical "low-temperature and high-humidity" nocturnal conditions of solar greenhouses, with a stable operation temperature threshold of 15 ℃ for the ambient air. Under suitable temperature conditions (≥15 ℃), the equipment ran steadily: the average temperature difference (ΔT) between the inlet and outlet of the dehumidifier reached 13.5 ℃, the average relative humidity difference (ΔR_RH) was 44.8%, the effective dehumidification duration approached 100%, the dehumidification capacity (D) was 1.31 kg/h, and the dehumidification energy consumption index (R) was 0.77 kW·h/kg, showing economical energy efficiency. In contrast, under unsuitable temperature conditions (<15 ℃), the refrigerated dehumidifiers operated intermittently with periodic fluctuations due to necessary defrosting processes. These defrosting cycles interrupted continuous dehumidification, thereby weakening the equipment's temperature-raising and humidity-reducing effects. Under such conditions, the average ΔT and ΔR_RH between the inlet and outlet were reduced to 7.6 ℃ and 29.5%, respectively, while D decreased to 0.99 kg/h and R increased to 1.01 kW·h/kg. These findings clearly indicated that the dehumidification performance of the equipment was significantly influenced by the ambient air temperature during the nighttime in solar greenhouses. Condensation dehumidification effectively regulated the nocturnal humidity environment of solar greenhouses: it maintained the nocturnal relative humidity at approximately 80%, which was 14.2% lower than that of the control plot, and reduced the nocturnal air relative humidity by 6.7% ～ 10.6%. Additionally, this method increased the nocturnal saturation vapor pressure deficit (ΔP) to 0.4 kPa—a parameter closely related to inhibiting fungal spore germination—exerted negligible impacts on the air temperature, and lowered the dew point temperature (T_d) by 1.4 ℃. Notably, the dehumidification process did not introduce low-temperature air into the greenhouse, thus preventing any adverse reduction in the indoor air temperature and ensuring favorable tomato growth conditions. Condensation dehumidification also exhibited a remarkable ecological preventive effect on tomato leaf Botrytis cinerea. By shortening the duration and reducing the frequency of high relative humidity (>85%) periods during nighttime, this method effectively inhibited the initial infection and subsequent spread of Botrytis cinerea. Statistical analysis showed that the dehumidification treatment achieved a preventive rate (P_r) of 85.04% for the disease index (D_i) and 73.74% for the disease incidence rate (D_r) of tomato leaf Botrytis cinerea, outperforming many traditional physical control measures. Furthermore, future studies should investigate the optimal intermittent operation mode of dehumidifiers to reduce operational costs while maintaining stable dehumidification efficacy, such as adjusting operation intervals based on real-time humidity monitoring data. In conclusion, condensation dehumidification can effectively improve the high-humidity nocturnal environment of solar greenhouses and prevent the occurrence of tomato leaf Botrytis cinerea. This study provides a feasible technical solution for the precise regulation of nocturnal humidity environments and the green prevention and control of diseases in solar greenhouse tomato production, holding practical significance for promoting sustainable protected horticulture development.