CERTAIN ELEMENTS IN NURSERY PRACTICE 195 



disease is of special importance on the fruit seedlings grown 

 in the western states. Often a large proportion of the seed- 

 lings show galls when dug. This is particularly true in case 

 of myrobalan plum seedlings. 



Description. The galls usually appear at the crown of 

 the plant (Fig. 211), but they are also often scattered over 

 the root system and may occur even 

 on the parts aboveground. Mature 

 galls measure from one-half to sev- 

 eral inches in diameter. They are 

 dark in color, with a roughened sur- 

 face. Young galls are small, greenish 

 and soft or even spongy. 



The disease may exhibit itself in 

 another form, known as hairy-root. 

 As the name suggests, there is an 

 excessive production of small fibrous 

 roots which cause the hairy appear- 

 ance. 



Cause. The disease is caused by 

 the bacterial parasite, Bacterium 

 tumefaciens. Presumably the bac- 

 teria hibernate in the soil and also 

 in the old galls that persist from year 

 to year. The organism is able to 

 live in dry soil for months. When 

 nursery stock is set in infected soil, the bacteria gain entrance 

 through wounds, and produce the galls. Often the disease 

 is spread by cuttings or grafts from diseased plants. 



Control. But little is known with respect to the control 

 of this disease in the nursery. Care should be exercised to 

 avoid the use of diseased cuttings or cions, especially since the 

 bacteria can be spread by means of pruning and grafting tools. 

 Newly planted grafts are often severely affected. The crown- 



FIG. 211. Crown-gall. 



