4 TIMELY HINT 118 



disease of the apple tree is caused by this same bacterium, or at 

 least by a very similar one. 



How crown gall is spread. It has already been observed that a 

 few diseased trees in an orchard are sufficient to infect the entire 

 lot. The germs are easily carried from one tree to another by ro- 

 dents, including field mice, gophers, and rabbits, by insects (par- 

 ticularly borers) , the careless use of pruning tools, and also through 

 irrigation and cultivation. It should be a regular practice among 

 nurserymen and orchardists to sterilize pruning tools frequently 

 while using them. This may be done conveniently by dipping the 

 blades of pruning shears, knives, and similar tools in a 4 or 5 per 

 cent solution of formaldehyde and coating over with a small brush 

 the blades of saws, pruning hooks, and such tools as can not easily be 

 dipped. This can be done with small tools by immersing the blades 

 in the formalin solution contained in a small vessel while passing 

 from one tree to another. 



Fig. 2. Crown gall on nursery tree sh pped into Arizona by mail. 

 A. W. Moirill, Fifth Annual Report, Arizona Commission of Agr. and Hort., p 20 



Trees, the roots of which have been damaged in carriage or in 

 transplanting, are more easily infected with crown gall than similar 

 trees the roots of which are in good condition, since the germs can 

 easily gain access to the tissues of the plant through injuries. It 

 would be a good practice to paint over the wounds (when these are 

 of any considerable size) of the roots of young trees, with a thick 

 lead paint. Orchard trees that have died from crown gall are a 

 source of infection and should be destroyed. Also trees that appear 

 to be unhealthy should be examined, and, if found affected with crown 

 gall, should be destroyed. Root-grafted trees are more liable to 



