6 MASS. EXPERIMENT STATION BULLETIN 361 



Spore germination occurs in relative humidities above 95 percent and is best 

 at 100 percent or air saturation and in water; and these conditions often occur 

 on the leaf surface and in the immediate atmosphere of the foliage when lower 

 relative humidities exist in the open greenhouse atmosphere. The more humid 

 conditions on the leaf are the result of transpiration and guttation from the 

 foliage and moisture condensation (dew) from the air. A temperature range of 

 75° to 80° F. is optimum for spore germination and infection, and it is also about 

 the ideal day temperature range for growing tomatoes under glass. 



Air circulation is very important in the correction of excessive temperature 

 and moisture conditions. If the ventilators admit insufficient air, opening of 

 doors and construction of additional ventilators in stagnant corners and in the 

 ends of the greenhouse are desirable. Ventilation must be increased with in- 

 creased heat from the sun. Late in the spring the maximum amount of ventila- 

 tion may be provided day and night. At this season and in the early fall, when 

 the outside temperatures on occasional nights drop to 50° F., a minimum heating 

 temperature of 60° to 65° F. and ventilation seems best and some ventilation 

 may be necessary from both the sides and the spans of the greenhouse. The 

 ventilators should be lowered but not closed at sundown after the heat is in the 

 pipes. This practice will prevent a sudden rise in the relative humidity of the air. 



The appearance of dew or "sweat" on the plants in the morning is rather 

 common in the off-heating season. This happens when the greenhouse is closed 

 at night and the warm confined air is gradually cooled, and can be avoided by 

 ventilation or, if there is a considerable temperature decline, by heat and ventila- 

 tion. In the fall an inside minimum temperature of 60° to 65° F. with ventilation 

 is often definitely necessary to prevent high relative humidities. This can be 

 appreciated by a study of hygrothermographic records for August, September, 

 and other off-heating periods, of which Figure 5 is typical. The records show 

 that moisture saturation and minimum temperatures of 55° to 60° F. occur at 

 night and high temperatures and low relative humidity in the daytime. The 

 effect of heating on the relative humidity of the air at night is quite insignificant 

 at this season because of the very narrow difference existing at night between the 

 inside and outside temperatures unless the temperatures are raised above 60° F. 

 When the outside night temperatures drop to 50° F. and below, as they do in 

 October and later months, the effect of heat or an inside minimum temperature 

 of 60° F. on lowering the humidity of the air may be considerable, as is indicated 

 by Figure 5B. 



In the morning a gradual warming of the greenhouse air above the prevailing 

 night temperature is desirable. This is accomplished by gradually increasing 

 the ventilation as the sun increases in power. In constrast, a sudden cooling of the 

 air at this time may cause a heavy deposit of dew which may remain for some 

 time even after the temperature rises. 



Wilson and Alexander (55) and Alexander (5) reported less leaf mold in Ohio 

 greenhouses when heating with ventilation was practiced after June 25. Ale.xander 

 implied the need of heat as late in the summer as tomatoes are picked and as early 

 as September 1 in the fall season. Walker and Sumner (53) reported less leaf 

 mold in Wisconsin greenhouses when heating began in August. Heating up to 

 late June and as early as the first of September is uncommon in Massachusetts, 

 but where it is practiced, development of the disease is delayed. With coal at 

 nearly $7 per ton delivered, and the average price of tomatoes much less than in 

 prc-depression years, late and early heating obviously absorbs a good share of 

 the profits; and therefore, it has not become a generally accepted practice in 

 Massachusetts. 



