1898.] PUBLIC DOCUMENT — No. 33. 51 



it from disease, when the soil was soaked with water and 

 the plants in some instances practically submerged for days 

 at a time. Every plant is surrounded by a host of para- 

 sitic organisms, which, given the proper conditions, will 

 manifest their distinctive properties. The healthy, vigor- 

 ous plant is always less susceptible to the attacks of fungi 

 than the weakly, abnormally developed one, — a fact which 

 every practical gardener readily understands. We have 

 seen this illustrated so many times in our work in the 

 greenhouse that it may be well to give an example of it 

 here. Certain species of non-parasitic nematode worms, 

 which are always present in greenhouse soil, although appar- 

 ently doing no harm as long as the plants are vigorous, 

 will, as soon as the plant becomes weakened or abnormal 

 from any cause, penetrate the tissues and cause rapid decom- 

 position of the same. What is true in regard to nematodes 

 applies also to fungi and bacteria, and, indeed, these various 

 forms of organisms are most frequently to be found together 

 in the decayed tissues of the plant. 



Before any attempt is made to spray diseased plants, it is 

 well worth while to find out something about the nature of 

 the disease with which the plants are affected. It is, for 

 example, iinwise to spray roses for the black spot or mil- 

 dew when the roots are half decayed by the action of para- 

 sitic gall-forming nematode worms ; and for the same reason 

 it would be unwise to treat the spot disease of the English 

 violet, when the roots are covered with hundreds of minute 

 galls, and when the supply of nutriment from the root is 

 greatly interfered with. 



On the other hand, spraying the apple, grape, potato and 

 plum is at the present time justifiable and necessary ; and 

 there are many diseases common to greenhouse cucumbers 

 and tomatoes which can be largely controlled by spraying, 

 although it must be said here that by judicious management 

 of the various conditions surrounding the plants these dis- 

 eases can be checked. 



