XV. 



BACTERIA OF PLANT DISEASES. 



I SHALL not attempt to give a full account of the bacteria which 

 have been described as bearing an etiological relation to various in- 

 fectious diseases of plants, but a " text- book of bacteriology " would 

 be incomplete without some reference to the best known of these 

 bacteria. In the following descriptions of species I have preferred 

 to quote largely from the published papers of Dr. Erwin F. Smith, 

 of the Department of Agriculture, United States, a recognized au- 

 thority in the investigation of plant diseases, rather than to rewrite 

 his descriptions. 



BACILLUS SOLANACEARUM (Smith). 



Causes a bacterial disease of the tomato, egg plant, and Irish 

 potato. 



"Morphology. A medium-sized bacillus, with rounded ends; often in 

 pairs, with a plain constriction ; elliptical, but of variable size, depending- on 

 age of culture or the length of time the tissues of the plant have been occu- 

 pied ; usually one and one-half to three times as long as broad. On cover- 

 glass preparations made from peptone beef bouillon cultures forty-eight 

 hours old and stained with a watery solution of methyl violet, many are 1.5 

 by 0.5 //, but these measurements must not be taken too literally, since the size 

 depends not only on the age of the culture but also on the kind of stain em- 

 ployed, i.e., on whether or not the cell wall stains. Organism motile, often 

 only sluggishly so, especially when taken from the plant, but sometimes very 

 actively motile, especially in young cultures. Flagella much longer than 

 the rod ; several exact number and place of attachment not made out 

 clearly, owing to imperfect preparations (Van Ermengem's method), but ap- 

 parently arising from any part of the rod. An attempt to stain them by 

 L6 filer's method was unsuccessful. No spores observed either in the plant 

 or in culture media, but the search has not been continued long enough to 

 warrant any opinion as to their existence. Zooglcea are formed almost from 

 the start in fluid culture media. 



"Symptoms Produced in the Plant. The first indication of this disease, 

 or at least the first one to attract the farmer's attention, is the sudden wilting 

 of the foliage. This may occur first on a single shoot, but finally it affects 

 the whole plant. Subsequently, and especially if the plant is young or not 

 very woody, the stem shrivels, first changing to a yellowish-green or to a 

 muddy green, and finally to brown or black. The vascular bundles become 

 brown long before the shrivelling takes place, and in the potato often show 

 through the outer green parts of the stem as long, dark streaks, or the bac- 

 teria run out on the petioles, after the manner of pear blight, forming nar- 

 row, black lines. The vessels of such bundles are filled with the bacilli, 



