Open field cultivation in warm regions often results stem blight (Phomopsis asparagi Sacc.) damage of asparagus (Asparagus officinalis L.). Therefore, the rain protected asparagus cultivation in a plastic house has been developed in Nagasaki Prefecture, Japan in 1987 for the purpose of high yield and disease-control. The occurrence of stem blight could be controlled by the rain protected cultivation of green asparagus. Annual yields are increased dramatically by mother fern culture after harvesting spring spears, but air temperature in plastic houses used for forced or protected cultivation often becomes too high during the summer season. The high temperature causes low spear quality and the deterioration of the working environment. In a previous report, it was shown that ventilation and shading were effective for suppression of temperature rise in a plastic house during summer (Inoue, 2008). In this report, we investigated the effect of ventilation and mist treatment on air temperature and spear yield in a plastic house. We also investigated an impact on the wet bulb globe temperature (WBGT) which is used as an index for evaluating heat stress for workers in a plastic house. Maximum and average temperatures in the plastic house were reduced by ventilation and mist treatment. The effect was highest by mist treatment, and side and roof ventilation, roof ventilation and side ventilation followed. By these treatments, yield and the average weight of spears during summer were increased and the percentage of abnormal spears was reduced. And unfavorable time for working (more than 31 in WBGT index) significantly decreased by the treatment. These results suggested that ventilation and mist treatment were able to suppress the temperature rise in the plastic house, and improve the spear yield and quality and reduce the high temperature stress of workers.
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