INSECTICIDES AND FUNGICIDES. 21 



to man. The material rapidlj^ loses its effectiveness unless carefully stored 

 in tight receptacles. 



Pj'rethrum is quite effective on soft-bodied insects and larvte not pro- 

 tected by hairs, and is especially useful against young cabbage worms on 

 cabbage and cauliflower plants which are soon to be harvested. It acts 

 purely as a contact insecticide; the application must be made very thor- 

 oughl}^, therefore, to bring the material in actual contact with the insects 

 to be treated. Its action is of short duration, the active principles being so 

 volatile, and if used too sparingly some of the insects are merely numbed 

 and eventually recover. Its usefulness is consequently very limited. It 

 is rather costly, and is apt to vary as to purity. 



PjTethrum may be applied as a dry powder, pure or diluted with two 

 to three times its bulk of flour, air-slaked lime, etc., which increases its 

 adhesiveness. When diluted with any carrier it is well to keep the mixture 

 in some tightly closed receptacle for twenty-four hours before using. It 

 can also be applied as a spray at the rate of 1 ounce to 2 gallons of water, 

 which should stand for twenty-four hours before using. For immediate use 

 a decoction can be made by extracting in a quart of boiling water for from 

 five to ten minutes, then adding the rest of the water. 



B. FUNGICIDES. 



Fungicides, as the term is applied in this bulletin, are substances used 

 to kill or prevent the growth of fungi. They are applied to the host as 

 spray, dust or fumes. For the most part, they are used as preventives and 

 not cures, and therefore should l^e applied before the fungus is present on 

 the surface of the host plant. As such, they protect by forming a poison 

 barrier through which the threatening fungus cannot penetrate. Some- 

 times, however, they are used to destroy a pathogen (parasitic organism 

 which causes the disease) which is already present; e.g., powdery mildews 

 and potato tuber organisms, in which case they are called disinfectants. 

 In respect to use, then, we distinguish the two groups of fungicides: (I) 

 Protective sprays or dusts, and (II) Disinfectants. In some cases, how- 

 ever, the same substance may be used for both purposes. 



A good fungicide must have the following qualifications : — 



1. It must kill or inliibit the growth of the pathogen at the concentration used. 



2. It must not seriously injure the host plant at this same concentration. 



3. If used as a spray it must adhere tenaciously to the suiface of the host. 



4. If used as a protective spray it must be practically insoluble in water after it 

 diies on the host, but still go very gradually into solution under the influence of 

 atmospheric conditions, host or pathogen. 



5. It must be reasonably low in cost, both of material and of labor of application. 



Most of the fungicides which are in general use owe their effectiveness 

 to the presence in some form of one of three elements, — copper, sulfur 

 or mercury. Formaldehyde, effective on account of its reducing qualities, 

 is an exception. On this basis we shall divide them for convenience of dis- 

 cussion, as follows: — 



