20 



MASS. EXPERIMENT STATION BULLETIN 350 



In contrast to the previous tests, readings were taken under prevailing bright 

 weather conditions in May, October, and November (Table 8) when ventilation 

 was provided from the top and sides of the greenhouse. These readings show 

 higher relative humidities and lower temperatures at the foliage than in the open 

 atmosphere between the rows of plants. The introduction of drier outside air by 

 ventilation reduces the relative humidity at the foliage, and the reduction is 

 greatest when both the top and side ventilators are opened. Previously it was 

 shown that saturated moisture conditions prevail at night during the warmer 

 months. It is also evident that relative humidities above 95%, accompanied by 

 optimum temperatures for the fungus at the leaf surfaces in consequence of high 

 rates of transpiration, are of common occurrence during the day throughout the 

 warmer months in spite of ventilation. 



Table 8. Temperature and Relative Humidity in Greenhouse Planted 



TO Tomatoes 



(Readings in bright weather in May, October and November. Ventilators Open.) 



Newhall and Wilson (16) and New hall (17) asserted that with rapid air changes 

 a relative humidity as high as 90% in the greenhouse may exist without infection. 

 With infrequent air changes a relative humidity as low as 60%, would be required 

 to prevent infection. They noted that in a stagnant atmosphere infection proceeds 

 when the greenhouse air is as low as 80% relative humidity. The writer's readings 

 reveal that the recirculation of air of high relative humidity and of nearly similar 

 temperature does not influence humidity conditions at the leaf surface and has 

 little efTect on the humidity of the air itself. The infiltration of cold outside air and 

 warming it to maintain a minimum growing temperature of about 60° F. produces 

 a drying greenhouse atmosphere and this condition is most pronounced in the 

 coldest months of the year. In the warmest months of the year the introduction 



