PLANT DISEASES 449 



becoming more and more intelligent and effective. There 

 are a few general principles that lie at the basis of any intel- 

 ligent and effective treatment, and these principles should 

 be known to all who cultivate plants. 



145. Infection. No intelligent control of disease is pos- 

 sible without exact knowledge of the sources of infection. 

 A good illustration of the truth of this statement may be 

 obtained from the case of peach curl. For a long time it was 

 supposed that the infection came from a parasite that lived 

 year after year in the peach tree, and therefore that no control 

 was possible. But when it was found that the infection came 

 from spores that lodged on the buds, thus getting a chance 

 at very young leaves, the control became obvious and easy. 



It is well to recall the various sources of infection known, 

 and to remember that they are not yet known in the case 

 of most diseases. There are soil infections, the parasites 

 living from season to season in the soil ; spore infections, in 

 which spores are carried by the wind, by insects, by rain- 

 drops, by seeds, etc. ; wound infections ; and infections by 

 parasites that live from season to season in the host. There 

 are also parasites on the surfaces of host plants, and para- 

 sites within the tissues of host plants; the former can be 

 treated easily, and the latter not. 



146. Fungicides. A fungicide is a substance that kills 

 fungi. It is applied usually either as a powder or as a 

 liquid, and it is obvious that its application depends upon 

 whether the fungus can be reached. The obvious conditions 

 for application are when the parasite is a superficial one, or 

 when its spores are lodged somewhere on the surface of the 

 plant. Such applications are clearly not appropriate in the 

 case of soil infections, or in the case of parasites living per- 

 manently within the tissues of the host. It should be remem- 

 bered, also, that some fungicides injure some plants, so that 

 their use upon them, no matter what may be the position of 

 the parasite, is impossible. 



