PLANT DISEASES 



253 



very carefully indeed, since this disease usually makes its 

 appearance in the first place in the nursery. If nursery stock 

 is secured that is absolutely free from crown-gall, the chances 

 of development later in the orchard are very much lessened. 

 Every precaution must be taken to see that roots are not in- 

 jured in the process of cultivation or in other ways. As before 

 pointed out, the disease is transmissable and it finds ready 

 access through wounds, especially if these are close to the 

 surface of the ground . There 

 is a possibility that a root 

 may be found which will be 

 immune to this disease. 



364. Potato scab 

 (Fig. 129). This common 

 disease of the potato, occur- 

 ring in all parts of the 

 country, causes an immense 

 amount of damage. It is 

 readily recognized, for it 

 covers the tuber with rough brown blotches as shown in 

 Fig. 129. The outer tissue is destroyed and the tuber be- 

 comes unsightly in appearance. 



The disease is produced by a fungus which gains access 

 from the seed or from the soil. Since it lives in the ground 

 from year to year, fields in which it has appeared should not 

 be planted to potatoes or root-crops for four or five seasons 

 after it was last detected. There is no other way known at 

 present whereby it can be eradicated from the soil after it has 

 once gained access. 



The common treatment to prevent infection from seed 

 potatoes is by soaking in formaldehyde solution. Of course, 

 potatoes which are in the least scabby or which are known to 

 have come from a field where scab has been present, should 

 not be used for seed. It is not possible, however, in the 

 larger number of cases, to ascertain the facts with regard to 



FIG. 129. Potato scab. 



