3° 



isms are in no way responsible. These are the so-called physiologi- 

 cal disorders which an inherent predisposition may encourage, but 

 more commonly they are induced by abnormal environmental condi- 

 tions due to treatment. Sharp lines, however, cannot always be 

 drawn between the different t)'^pes of diseases, inasmuch as func- 

 tional disorders in themselves constitute favorable conditions for 

 the development of pathogenic organisms, and in many well known 

 instances these organisms would not be present, if the plants were 

 not weakened in some way or rendered more susceptible by injudi- 

 cious treatment. It is therefore quite essential that the conditions 

 underlying infection should be well understood in the case of plants 

 under glass,for here the environment is largely under control ; and, what 

 is equally important, the gardener should have a thorough knowledge 

 of the plant's normal requirements and limitations. Since a large 

 number of greenhouse diseases have their origin in unfavorable 

 environmental conditions, the logical manner of treatment, or, more 

 properly speaking, prevention, consists in applying the principles of 

 hygiene and rational cultural methods, rather than in resorting to 

 spraying. There are, indeed, few instances where spraying is neces- 

 sary for indoor crops, and the time utilized in this manner could 

 often be more profitably employed. Even in cases where a genuine 

 parasite appears, it is frequently possible to modify the method of 

 culture so as to reduce infection enormously. By modifying the 

 light and moisture conditions in a house in such a manner as to 

 cause the development of a firmer and more resistant texture in 

 certain tissues, liability to infection would be greatly reduced. In 

 order to bring this about, radical changes in cultural methods would 

 have to be adopted which would not in certain cases seem feasible 

 or practical at the present time. However, in many instances where 

 it is feasible it is not always accomplished, because the gardener 

 fails to understand the full significance of such matters. 



Diseases Induced by Abnormal Treatment. Pathogenic 

 Organisms Not Being Present. 



Leaf Curl. 



The curling of cucumber leaves into a spherical form is more or 

 less experienced by cucumber growers. This trouble manifests 



