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. 



