CHAPTER XXXIV 



BACTERIA WHICH CAUSE PLANT DISEASES THE GENERA 

 ERWINIA, PSEUDOMONAS AND LACTOBACILLUS 



IN recent years it has come to be recognized that a com- 

 siderable number of plant diseases are due to the activity of 

 pathogenic bacteria. Practically all of the forms producing 

 disease in plants belong to two genera, Erwinia and Pseudo- 

 monas. 



The genus Pseudomonas is characterized by being made 

 up of rod-shaped, short organisms, usually motile by means 

 of polar flagella, rarely nonmotile. They are aerobic and 

 facultative anaerobic. Frequently they are capable of 

 liquefying gelatine ; they do not produce spores. The Gram 

 stain is variable, usually negative; usually there is com- 

 paratively slight fermentation of carbohydrates. Many of 

 the species produce a water soluble pigment, green, blue, 

 purple, brown, etc. Among the plant pathogens, a nondif- 

 fusible yellow pigment is frequently produced. Some 

 species are white. 



The genus Erwinia is very closely related to the genus 

 Bacterium, that is, to the organisms of the colon typhoid 

 series of bacteria which have already been discussed. All 

 of them are plant pathogens. The growth upon culture 

 media is usually whitish and often slimy. Acid is usually 

 formed in certain carbohydrate media and occasionally gas. 

 The cells are rod-shaped, without endospores, Gram-neg- 

 ative and either motile by means of peritrichic flagella or 

 occasionally nonmotile. 



ERWINIA AMYLOVORA 



Synonyms. Bacillus amylovorus. 



This organism is the cause of a specific disease, pear 



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