TOMATO LEAF MOLD 11 



Guba (18) described a system of relative humidity control and forced ventila- 

 tion relying on the variation of the greenhouse temperatures between a fixed 

 minimum and maximum and by the starting and stopping of exhaust fans. This 

 is brought about by a hygrostat which shifts from a low to a high thermostat and 

 which may be set at any desired temperature range. By increasing the tem- 

 perature when the relative humidity rises, the humidity will tend to be lowered 

 in a definite ratio for each degree rise in temperature. The starting of exhaust 

 fans simultaneously with the introduction of pipe heat stirs the air and causes 

 the infiltration of outside cooler air. This system insures considerable accuracy 

 since it relies upon the sensitivity of thermostats and hygrostats. There is no 

 draft of air or inside construction to make shadows or take up growing space 

 as in the Ohio installations. This system of humidity control and forced ventila- 

 tion was compared with a fixed thermostatic minimum temperature control and 

 hand ventilation in two sepaiates houses designated A and B. 



The minimum temperature in greenhouse B was adjusted for 60° F. (tem- 

 perature control) and Figure 5B shows the influence of such a prevailing minimum 

 temperature upon the course of the relative humidity of the greenhouse air in 

 the colder months of the year. In greenhouse A, air control instruments were 

 gauged for relative humidities below 85 percent (humidity control), which was 

 not always possible within a temperature range of 60° to 75° F., representing the 

 extreme adjustment for the low and high thermostats. This was especially true 

 during September, October, and critical periods in November. With the advance 

 of the season and the decline of the outside mean minimum temperature, the 

 maintenance of relative humidity below 80 percent within a fluctuating tempera- 

 ture range of 60° to 70° F. was more readily accomplished. In April to June, 

 conditions again are like those in the early autumn months. 



Less attention to ventilation w^as necessary in greenhouse A provided with air- 

 conditioning equipment than in greenhouse B with thermostatic minimum tem- 

 perature control only. In the former, the ventilators were opened when the 

 temperature reached 72° to 75° F.; in the latter, ventilation was much more 

 freely applied, but the conditions were not as accurately controlled. 



The results under the two conditions of air control in the fall of 1929 (Table 8) 

 showed a difference of 17.6 in percentage of infected leaflets, the greater amount 



Table 8. — Relation of Automatic Humidity Regulation and Automatic 

 Temperature Regulation to the Control of Tomato Leaf Mold 



Fall Cropping Season 1929. 



Percentage of Diseased Leaflets 



Date of 



Foliage Counts House A House B 



Humidity Temperature 



Control Control 



August 21, 1929 



September 24, 1929 



October 31, 1929 



November 27, 1929 



January 2, 1930 



January 15, 1930 



Total 33.2 50.8 



