A LEAFBUGHT OF KALMIA LATIFOUA 



By Ella M. A. Enlows 



Scientific Assistant, Laboratory of Plant Pathology, Bureau of Plant Industry, United 

 States Department of Agriculture 



INTRODUCTION 



During the summer of 1914 a bed of mountain laurel {Kahnia latijolia) 

 on the Department grounds became affected by a leafspot or blight. 

 The progress of the disease in this bed was slow, but at the end of about 

 18 months there was not a single plant in the bed which did not show 

 the typical brown margins and areas described below. The disease has 

 also been found by the writer in Rock Creek Park and in the hills about 

 Anacostia, D. C. Cross-sections of the affected leaves showed the pres- 

 ence of a very delicate fungus mycelium within the tissues. Cultural 

 studies were begun and a fungus was isolated, which has been proved to 

 be the cause of the disease. 



DESCRIPTION OF THE DISEASE 



The disease is characterized by a blight or dryrot involving large areas 

 of the leaf blade or the entire leaf. Later, it extends through the petioles 

 into the stems and may eventually kill the entire plant. Those leaves 

 but slightly diseased show small, irregular, dark-brown spots scattered 

 about over the blade. In fact, the disease in its early stages, especially 

 on young, tender leaves, is visible only with a hand lens, appearing as 

 tiny brown specks scattered about over an area of leaf surface which 

 appears slightly lighter, green than that of the surrounding healthy por- 

 tion. These spots increase in size slowly, and finally become visible as 

 tiny brown points in clusters, surroimded by a much lighter portion of 

 green leaf tissue (Pi. 14, B). During dry, cold weather these diseased 

 areas may remain as definitely outlined small spots for three to four 

 v/eeks or longer; but if the air is moist and warm, they soon coalesce, 

 forming the large, seal-brown to Vandyke brown ^ areas (Pi. 14, A, C), 

 which finally involve the entire leaf blade. The incipient stage of the 

 disease is most frequently seen at or near the tip, or along the margin of 

 the leaf. vSometimes it spreads from the point of infection at the tip in 

 rather an even line across the leaf blade, but it may also follow the mid- 

 rib or extend along the margins of the leaf. In the latter case the margins 

 usually curl, the inner portion of the leaf blade then appearing convex. 

 The badly affected leaves, petioles, and stems become very dry and 

 brittle, and such leaves drop off rather easily. 



• The color terms mentioned throughout the text are according to Ridgway's Color Standards (Ridgway, 

 Robert, color standards and color nomenclature. 43 p., 53 col. pi. Washington, D. C.,1912). 



Journal of Agricultural Research, Vol. XIII, No. 3 



Washington, D. C. Apr. 15, 1918 



nb (199) KeyNo. G141 



