FIELD DISEASES 143 



short peduncles as compared with the long 

 peduncles of healthy plants of the same va- 

 riety. Frequently, however, the affected 

 plants outgrow the disease entirely, and 

 thus a distinct line of demarcation is ob- 

 served between the previously diseased 

 part and the healthy part of the new 

 growth (fig. 28). In rare cases, infected 

 plants seem to thrive in spite of the dis- 

 ease. Such plants should be selected for 

 the purpose of breeding resistant strains. 

 Pathogenicity. Like peach yellows 

 and the mosaic disease of tobacco and 

 tomato, mosaic of the sweet pea can be 

 reproduced by puncturing with a sterile 

 needle from the diseased leaf into a healthy 

 one. As yet it has not been possible to 

 find an organism associated with the dis- 

 ease. Nevertheless the disease is conta- 

 gious. It takes from ten to fifteen days for 

 the disease to appear when artificial inocu- 



