THE CROWN GALL: VARIABILITY 469 



plate 36). Further studies must be made. In this connection 

 read what Jensen says in his Danish paper (I.e.). 



Also two extremely virulent isolations, cultivated for several 

 years in my laboratory, gradually decreased in virulence and 

 finally lost all power to produce galls. Moreover, differences - 

 have been observed in the vigor of growth and harmfulness of 

 galls occurring naturally on various fruit trees. The subject, 

 therefore, is not only one of special interest to the pathologist 

 but also one of much complexity and considerable discourage- 

 ment to the nurseryman and tree inspector. 



Query: May a gall of little harm to one plant infect a soil 

 injuriously for another plant? 



Should galled apple trees be planted on land that might 

 later receive peaches, raspberries or grapes? Many such 

 queries must be left to the future. The subject is one which 

 invites careful and long-continued experimentation on the part 

 of various experiment stations and boards of inspection. 



Query : Can you produce the disease on olives ? On alligator 

 pears? On onions or on garlics? On daisy with the hop 

 organism? 



Transmission. Everything we know about crown gall 

 points to wounds as the usual, if not the only way of infection. 

 Nothing is known respecting insect carriers. In some cases it 

 would seem that the "heeling-in" of sound nursery stock in soil 

 containing the organism has served to infect the young trees 

 (O'Gara). How is the disease spread above ground on the 

 limbs of trees? 



Everything points to nurserymen as the world-wide dis- 

 tributors of this disease. Many of their soils are so badly in- 

 fected that good stock cannot be grown in them. Mr. Waite 

 has shown me young apple trees, in numbers, badly galled on 

 the graft and almost or quite free in the stock, so that we could 

 come to no other conclusion than that the disease was introduced 

 into the nursery on the grafts. I have seen badly diseased pear 

 stock that was shipped into the country from France, and badly 

 diseased peach trees that were shipped into California from the 

 eastern United States, and badly diseased gooseberries that were 

 shipped from Iowa to the Atlantic Coast, and badly diseased 

 roses that were shipped from Ohio to Florida. These are only 



