PREVENTION AND CONTROL 271 



Clean Seeds. - - The progressive agriculturist will 

 always take the precaution to use clean seed, if possible 

 selecting the seed from the healthiest, most desirable 

 plants. Seed may carry the organisms of disease in 

 two ways : the spores may cling to the seeds and be 

 planted with them, germinate in the soil and penetrate 

 the young plant ; or the organisms may have penetrated 

 the seeds, sometime during their development, and lie 

 dormant, ready to start into activity and grow with 

 the growth of the young plant. 



Sterilization of Seed. Where the spores of the 

 organism cling to the seeds, it is frequently possible to 

 destroy the spores without injuring the germinating 

 qualities of the seeds by dipping the seeds in certain 

 solutions (see page 280). This method may also be used 

 with certain diseases of tubers (potato scab), but can- 

 not be used successfully where the disease organisms 

 penetrate the seeds during their development. 



Clean Plants. Where the young plants are grown 

 in the seed beds and then transplanted to the fields, 

 care should be taken that none but healthy plants are 

 used. These plants can also be sprayed to advantage 

 with fungicides before the plants are pulled. 



The soil of the nursery beds can also be treated to 

 destroy the organisms of disease (see page 149). 



Nursery Stock. Nursery stock should be carefully 

 guarded by the nurserymen and by the buyer. It is 

 much easier to care for healthy than for diseased trees, 

 and no man can be justified in knowingly selling or in 

 planting diseased trees or shrubs. The fact that the 

 disease is already prevalent in the community is no 

 excuse for furthering its distribution. 



Destruction of Insects. Insects are frequently the 

 carriers of organisms of disease, and also make wounds 

 through which these organisms penetrate. While it is 

 impractical to take preventative measures against some 

 of these insects, others can be held in check very 

 advantageously. Flea beetles not only cause direct 

 injury to the plants, but also make favourable conditions 



