TOMATO LEAF MOLD 15 



tion, it is obvious that at this season ideal temperature and moisture conditions 

 for the fungus prevail. 



A clearer understanding of temperature and humidity conditions prevailing 

 under glass in the culture of tomatoes in Massachusetts may be obtained from a 

 study of some of the weekly record charts. Under summer weather conditions 

 day temperatures often reach 95° F. and higher, in spite of ventilation, and the 

 minimum temperature is hardly ever below 60° F. A peak relative humidity of 

 100% prevails almost every night (Fig. 5-A). In bright weather and with ventila- 

 tion, the relative humidity values in the greenhouse recede with rising tempera- 

 tures, and the lowest relative humidity values are associated with the highest 

 air temperatures. These are the conditions of the air some distance from the leaf 

 itself. At the leaf surface higher humidity prevails. The records of temperature 

 and relative humidity outside the greenhouse for the week ending September 

 30, 1929, show the same wide fluctuations in night and day conditions (Fig. 5-B). 

 Moisture saturation with the lowest temperatures prevails at night, and the lowest 

 relative humidities (below 50%) with the highest temperatures occur at midday. 

 It is evident, as Rippel (22) noted in Germany, that the relative humidity in the 

 greenhouse is above 80% for much of the day, and only for a short time under 

 60% in the ofT or mild heating season. 



With the beginning of the boiler heating season in September and October, 

 some influence of heat on the humidity of the greenhouse air (Monday and 

 Tuesday, Fig. 6-A) is shown; while lacking heat (Wednesday to Sunday) a relative 

 humidity of 100% prevails regularly in spite of ventilation. At this season, the 

 temperature difference between the inside and outside air is so small, or zero, 

 that there is usually little if any effect from boiler heat on the humidity of the 

 greenhouse air, even when supplemented with ventilation, unless the temperature 

 is raised unreasonably high. The influence of heat on the humidity of the air 

 is thus clearly demonstrated in Fig. 6-A, 6-B, and 7-B. 



During cold winter weather and with the greenhouse temperatures maintained 

 at and near 60° F., the wide fluctuations of temperature and humidity prevailing 

 during the warmer months are smoothed off to rather narrow limits (Fig. 7-A 

 and 7-B) except when periods of warm weather interrupt to cause the outside 

 temperature to converge closely upon the minimum inside growing temperature. 

 These periods introduce high relative humidities which may be controlled by pro- 

 portionately higher temperatures. The need for ventilation supplementary to 

 heat at these times is significant. 



To obtain more intimate knowledge of the problem, frequent readings of the 

 Mason hygrometer and psychrometer were made in different locations in the 

 greenhouse at, and away from, the foliage and with different outside weather 

 conditions and amounts of ventilation. A set of readings was taken on a rainy 

 day in April when the outside temperatures were 42° to 52° F. and the relative 

 humidity 100%. A rotary fan was operated 6^/^ feet from the ground and in the 

 center of a greenhouse of 10,000 cubic feet (Table 6). Without ventilation and 

 with the particular outside conditions prevailing when these readings were taken, 

 the relative humidity at the foliage was 100%; at the ground between the rows 

 of plants 83 to 88%; and at the fan 83 to 88%. Inside temperatures were in the 

 -range of 60° to 70° F. In these instances, the air re-circulated by fanning lacked 

 sufficient evaporating power to influence the relative humidity at the foliage. 

 Obviously, the greenhouse air must be changed with much drier outside air, or 

 diffused with much colder outside air and heated, to obtain an appreciable reduc- 

 tion of the relative humidity. 



