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JOURNAL OF AG1TOCML RESEARCH 



Vol. XX Washington, D. C, February 15, 1921 No. 10 



A TRANSMISSIBLE MOSAIC DISEASE OF LETTUCE 



By Ivan C. Jagger 



Pathologist, Office 0/ Cotton, Truck, and Forage Crop Disease Investigations, Bureau 

 of Plant Industry, United States Department of Agriculture 



During January, 1920, Romaine lettuce (variety Paris White Cos) in a 

 field of several acres at Sanford, Fla., developed a condition very sugges- 

 tive of a transmissible mosaic disease. The first symptom of disease was 

 a yellowish discoloration along the smaller veins of the younger expanding 

 leaves. This symptom was usually evident for only a few days, giving 

 way to a general yellowish, discolored appearance of the whole plant. 

 All gradations of discoloration occurred, from very marked to conditions 

 not distinguishable with certainty from normal. Close examination 

 usually revealed irregular blotches of an approximately normal green 

 color, which were usually located along the larger leaf veins. The 

 blotching varied from a few barely perceptible green areas on a yellowish 

 leaf to numerous pronounced green spots giving a marked mottled ap- 

 pearance to an occasional plant (Pi. 87, A). The leaves of diseased 

 plants generally seemed to be rather more wrinkled than those of normal 

 plants. Where plants became diseased only after reaching considerable 

 size, the older leaves, which were fully expanded on the first appearance 

 of disease symptoms, frequently continued to appear perfectly normal, 

 while all younger leaves developed the disease symptoms. 



At the same time head lettuce (variety Big Boston) in a neighboring 

 field developed a similar diseased condition. The general yellowish, 

 discolored appearance of whole plants was frequently pronounced, but 

 in most cases the blotching was less marked than in the Romaine lettuce, 

 and a decided mottled appearance was never observed. 



In general, diseased plants made a stunted growth. In severe cases 

 the plants were decidedly undersized, and occasionally the leaves formed 

 only a rosette, with no indications of a folding together of the tips to form 

 a head. Usually loose heads of poor quality were formed, although all 

 gradations of development, including occasional heads of practically 

 normal size and hardness, occurred. Often plants that showed marked 

 discoloration, mottling, and stunting soon after becoming diseased would 

 later seem to recover in part and to make a more or less normal growth 

 with only slight discoloration and mottling. 



Journal of Agricultural Research, Vol. XX, No. xo 



Washington, D. C Feb. 15, 1921 



wt Key No. G-219 



(737) 



