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INSECTS AND DISEASES '. >': \'^'j -lOl 



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Sprays or other topical applications are useless. The 

 most promising work for the future lies in the 

 selection of resistant or partially immune varieties 

 and strains. 



Slime molds cause but few diseases. The ones 

 best known are the club foot of cabbages and turnips 

 and the crown gall of peaches and other fruit trees. 

 Heavy applications of lime to the soil have been 

 shown to be an almost complete preventive for the 

 first of these diseases, but no remedy is known for 

 the crown gall. 



By far the larger number of vegetable plant para- 

 sites are to be found among the fungi. They cause all 

 such well-known diseases as smuts, rusts, cankers, 

 leaf spots, anthracnoses, scabs, mildews, molds, 

 and root rots. The nature of the treatment depends 

 largely on the part of the plant attacked. The super- 

 ficial powdery mildews can be killed with sulphur. 

 Most parasitic fungi, however, penetrate within the 

 tissues of the host plant. Those that reach the 

 plant from above ground can usually be prevented 

 by keeping the plant coated with fungicidal spra3^s 

 like Bordeaux mixture and thus preventing infection. 

 Once within the plant tissue the parasite is safe and 

 cannot be reached by applications. Sprays are thus 

 preventive, not curative, an important distinction 

 that should never be lost sight of. If the contagion 

 reaches the plant from the ground through the roots, it 

 cannot be prevented by spraying. In these cases soil 

 treatment is sometimes available, but reliance must 

 mainly be had on rotation of crops. Many smuts may 

 be prevented by treating the seed. Relief from fungous 



