BACTERIA WHICH CAUSE PLANT DISEASES 407 



grapes, raspberries, blackberries, the Paris daisy and many 

 other plants. 



Morphology and Culture. — The organism is a Gram- 

 negative rod, motile by 1 to 3 polar flagella. Growth 

 differs from most of the other members of the genus Pseudo- 

 monas in that it is white instead of yellow. Milk is made 

 alkaline. Gelatine is not liquefied. No ,gas is produced 

 from carbohydrates. 



Pathogenesis. — The organisms gain access to the plant 

 tissues through a wound. It is most common at the junc- 

 ture of graft and scion in grafted trees. If the meristematie 

 tissues giving rise to medullary rays or soft tissues are 

 invaded there is an enormous proliferation of cells, and a 

 soft gall is formed which usually ultimately disintegrates. 

 It the tissues invaded are those which give rise to the woody 

 tissues, the tumor developed is hard. Frequently the roots 

 are stimulated to abnormal development producing masses 

 which give rise to the name hairy root. Artificial inocula- 

 tion gives rise in suitable host plants to the development of 

 primary and secondary tumors. In many respects the 

 tumor in the plant simulates a cancer. The disease results 

 in severe loss to nursery men. Certain apples in particular 

 are very difficult to secure free from crown gall. It is prob- 

 able that infection occurs commonly at the time of grafting. 

 Trees grown from galled plants are frequently defective, 

 weakened or stunted in their development. 



PSEUDOMONAS STEWABTI 



Synonym. — Bacterium stewarti. 



This organism is the cause of a wilt of sweet corn. It 

 has been found in several sections of the United States, 

 though first reported from Long Island. 



The organism is a typical yellow pseudomonad in 

 morphology, culture and physiology. It invades the fibro- 

 vascular bundles, cutting off the supply of water, in eon- 



