10 



case they are called disinfectants. In respect to use, then, we distinguish 

 the two groups of fungicides, — protective sprays or dusts, and disin- 

 fectants. In some cases, however, the same substance may be used for 

 both purposes. 

 A good fungicide must have the following qualifications: — 



1. It must kill or inhibit 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 surface of the host. 



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

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

 atmospheric conditions, host or pathogen. 



5. It must be reaonably 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. 



1. Copper Fungicides. 

 (1) Bordeaux Mixture. 



The most popular and most extensively used of all copper fungicides 

 is Bordeaux mixture. For Massachusetts crops and conditions, the 

 4-4-50 formula (4 pounds copper sulfate, 4 pounds quicklime and 50 

 gallons of water) is preferred, except in the case of the late sprays for 

 potatoes and the spray for celery and grapes. In the latter cases 5-5-50 

 is recommended. 



Copper is the only active fungicidal agent in this mixture. The copper 

 sulfate as purchased is approximately of standard composition. There 

 is often great difficultly, however, in obtaining high quality lime for the 

 making of Bordeaux mixture, especially where spraying is done on a 

 small scale. It is partly for this reason that commercial Bordeaux prep- 

 arations have come on to the market. 



(2) Commercial Bordeaux Preparations. 

 The home preparation of Bordeaux mixture has a number of disad- 

 vantages : — 



1. It involves a number of distinct operations which require considerable time. 



2. The grower must keep in mind the proportions and various directions for 

 preparation, or always have available the printed directions for the same (con- 

 sidered a nuisance by the average grower). 



3. A number of suitable containers are required, and are frequently not at hand 

 when needed. 



4. Few growers keep on hand a supply of quicklime, and even at the store it 

 cannot always be obtained when wanted and of the quality wanted, especially in 

 small quantities. When a barrel of lime is opened it quickly carbonates, and the 

 merchant in the small place is reluctant to break a barrel for a few pounds, while 

 for the same reason the small grower does not wish to try to keep it at home. 



