14 FUXGT AND FUXGICIDES 



Hand Picking. — With diseases like the hrown rot 

 of peaclies and pkims, plum pockets, etc., much ma}^ be 

 done in reducing injury by persistent hand-picking of 

 the affected parts, which, of course, shoukl be burned 

 or buried deeply in the soiJ. 



Use of Non-infected Seed. — Xumerous funsrous 

 diseases start from the seed. For instance, the potato 

 rot exists in seed potatoes, and so infects the new crop. 

 In such cases care should be taken to obtain seed not so 

 infected, or, when possible, so to treat the seed that the 

 spores are destroyed. 



Destruction of Alternating Forms. — As shown 

 on other pages, some of the rusts exist in two or more 

 forms on different host-plants. For instance, the apple 

 rust has one form that produces '^ cedar balls" on cedar 

 trees. The destruction of the latter will prevent the 

 development of the former. 



Use of Fungicides. — The most practicable way of 

 preventing the great majority of the fungous diseases of 

 cultivated crops is by the use of fungicides (a word 

 meaning, literally, killers of fungi). These are discussed 

 more fully under the following heading. The majority 

 of them are aj)plied externally to plants by spraying or 

 dusting. In such cases they may act iu either or both 

 of two ways : (1) By directly destroying any fungus 

 spores present at the time of application ; and (2) hy 

 remaining on the surface in a condition to destroy, either 

 before germination or during that process, any sj^ores 

 that may light upon the plant thereafter. 



THE IMPORTANT FUKGICIDES 



The followino^ list includes the funo-icides that have 

 proved of practical value up to the present time. It is a 

 list, however, that will probably be extended during the 

 next decade, as experiments are constantly progressing 

 to learn the fungicidal value of many other substances. 



