10 



MASS. EXPERIMENT STATION BULLETIN 361 



Table 7. — Relation of Leaf-Mold Control to Yield and Value of 

 Tomatoes 



Fall Cropping Season 1928. 



With Poor With Good 



Management Management 



Average yield per plant: 



Firsts pounds 2.79 3.59 



Seconds pounds 1.56 1.55 



Total pounds 4.35 5.14 



Average value per plant: 



Firsts $0.66 $0.86 



Seconds .18 .18 



Total .84 1.04 



On j4-acre basis: 



Total yield pounds 9,004.5 10,639.8 



Total value $1,738.80 $2,152.80 



Forced Air Ventilation and Humidity Control 



By the simple method of introducing outside air over a heater. Stair et al. (48) 

 completely prevented tomato leaf mold from developing on plants in the hotbed. 

 Wilson (56) noted that with forced air no periods occurred up to May 14 when 

 the relative humidity was 86 percent for 10 hours or more, while naturally ven- 

 tilated houses showed at least 35 such periods. For the first monthly periods 

 in the spring, the average relative humidity in a naturally ventilated house was 

 77.7, 77.8, and 82.5 percent in contrast to 61.5, 67.5, and 72.1 percent in a forced- 

 air-ventilated house. The possibility of controlling the disease by combining 

 forced air with natural ventilation was suggested from these readings. The 

 admission of the inconsistency and unreliability of hand control of ventilation 

 and heating in commercial practice stimulated the commercialization of costly 

 automatic systems of greenhouse air conditioning. The Zephyrator (1, 2, 3, 27, 

 28, 29), reported to have been installed in several ranges in Ohio, relied upon an 

 electrically driven centrifugal blower to force air through underground pipes and 

 overhead ducts into the greenhouse, the purpose being to increase the evaporating 

 power of the air by stirring it. Evaporation was doubled without appreciable 

 draft (Newhall 29) when air was stirred at the rate of 44 feet per minute and in 

 a range of 13^ acres the air was changed theoretically in 22 to 30 minutes. Parker 

 (32) asserted that air must move at the rate of 2 feet per second to prevent atmos- 

 pheric stagnation and at this velocity there is a complete change of air without 

 drafts. Newhall (30) described an installation in which the blower is placed in 

 front of a series of radiators which may be fed with either steam or cold water so 

 that the greenhouse air may be warmed or cooled. 



Another system (4) first compressed the air to a fraction of its volume then 

 introduced it into the greenhouse through a series of perforated pipes, similar 

 to an overhead sprinkler system which could be adjusted for any desired angle 

 and volume. The compression of the air greatly increased its capacity to absorb 

 water. 



