1908.] PUBLIC DOCUMENT — No. 33. 147 



trouble to crops. In the northern latitudes many greenhouse 

 crops do not obtain sufficient light daring the winter months, 

 and when cloudiness prevails it is with some difficulty that 

 crops are matured without becoming diseased. All expert 

 greenhouse men mature their crops when the weather condi- 

 tions will permit, and not according to the calendar ; in other 

 words, it requires a certain definite amount of light, or so many 

 light units, as it were, to mature a crop. The light in May, 

 for example, is equal in intensity and amount to about twice 

 that of corresponding periods of a day in November ; con- 

 sequently, it requires about twice as much time to bring a crop 

 to the same degree of maturity in November as it would in 

 May. 



Lack of light is responsible for various mildews and leaf 

 spots, top-burn or tip-burn, wilts, etc. Many of these leaf 

 spots are seldom if ever found on plants to which sunlight has 

 access. The Sclerotinia diseases of lettuce, water cress and 

 parsley are likewise induced by crowding and shading, and 

 light in such cases will prevent infection by the formation of 

 resistant tissues. It is well known that absence of light causes 

 the so-called " layering " of wheat and " damping off" of cut- 

 tings, and the mildews of various plants grown in the shade are 

 too well known to need consideration, 



The improper regulation of atmospheric moisture and venti- 

 lation is responsible for many fungous diseases, and the control 

 of these factors is important in preventing the troubles. 

 Among the mildews, Cladosporium can be entirely controlled 

 by holding the moisture in the greenhouse in check, and by 

 paying strict attention to proper ventilation and to normal 

 light conditions. Many gardeners have succeeded in control- 

 ling the chrysanthemum rust by using proper precautions in 

 regard to moisture. 



A series of the most troublesome diseases common to cucum- 

 bers and melons out of doors — Plasmopara, Alternaria and 

 Anthracnose — can be absolutely controlled in the greenhouse 

 by paying attention to moisture, light and ventilation. The 

 circulation of air, as well as light, has a marked effect upon the 

 development of resistant tissues in greenhouse crops, and the 

 control of moisture is necessary to prevent the germination of 



