A BLACK KNOT DISEASE OF DIANTHERA 

 AMERICANA L. 



I. M. Lewis 

 (With Plates 58-61, Containing 10 Figures) 



Introduction 



During the past two years the water willow, Dianthera ameri- 

 cana, growing along a small creek near the campus of the Uni- 

 versity of Texas has been affected with a disease which does 

 not appear to have been heretofore reported. Affected plants 

 were first observed during the summer of 1910, but owing to the 

 pressure of other duties at that time the investigation was only 

 superficial and failed to reveal the true nature of the trouble. 

 At the beginning of the present season however, it was made a 

 subject of special investigation which has brought out clearly the 

 nature of the disease and all of the salient features in the life 

 history of the causal organism. A few points which as yet are 

 not fully determined will be more carefully followed this coming 

 season. 



Symptoms of the Disease 



The disease affects the aerial portions of the plant and pro- 

 duces numerous hypertrophied areas of the internodes. These 

 areas are not localized on any particular portion of the stem but 

 occur at irregular intervals beginning near the base and extending 

 to the tip. The internode which bears the inflorescence, and mid- 

 rib of the leaf are frequently affected. The swollen areas vary 

 in length from one to three centimeters and usually completely 

 encircle the stem. The hypertrophy is not uniform but occurs 

 as distinct ridges opposite the peripheral steles, which in this spe- 

 cies of Dianthera are six in number (Plate LIX, fig. i). 



As the disease develops the outer tissue of the stem is rup- 

 tured by a longitudinal fissure and exposes the developing fungus 

 which presents a smooth grayish surface over a dark background 



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