DISEASES AND THEIR CONTROL 129 
parasitic fungi of greenhouse vegetable crops, though the 
opposite view is universally held by successful growers. 
Various diseases of greenhouse cucumbers and toma- 
toes are much more troublesome during the fall and win- 
ter months than during the spring and early summer. 
The success of the spring crops is attributed almost 
wholly to more light and more sunshine. The days are 
longer, and there is a larger proportion of bright, sunny 
weather to cloudy days than there is from November to 
March. This fact should be fully considered when mak- 
ing cropping plans. Some crops, such as lettuce, do bet- 
ter with the minimum amount of light than other crops 
like the tomato and cucumber. For this reason lettuce is 
most generally grown as a fall crop in preference to cu- 
cumbers and tomatoes. And this decision, too, is usually 
based on the fact that cucumbers and tomatoes are far 
more susceptible to disease during the fall and winter 
than from March 1 to August 1. Sunlight not only favors 
the most rapid growth of plants, but it also prevents the 
germination of certain disease spores. Shading, however, 
has a place in greenhouse management. The subject is 
discussed on page 36. 
The influence of moisture.—Excessive watering in the 
greenhouse invariably results in soft, tender plant tissues 
which permit the easy entrance and rapid development 
of parasitic diseases. 
The constant maintenance of high humidity in the 
greenhouses is just as dangerous as an excessive amount 
of moisture in the soil. It also encourages the growth of 
succulent tissues and is most favorable to the production 
and germination of spores. Growers who do not ventilate 
the houses freely and regularly, to reduce humidity as 
well as temperatures, are almost certain to experience 
heavy losses from the attacks of various diseases. 
While constant high humidity in the houses should be 
avoided, it is thought by some that moisture on the leaves 
is an advantage when bordeaux mixture is to be used. 
