140 Journal of Agricultural Research vo1.xix,no.4 



the leaf tissue, and in a day or two forms a gray or brown dry tissue from 

 I to several millimeters in diameter, evident on both sides of the leaf 

 blade. The halolike margin spreads rapidly, becoming uniformly 

 lighter green to yellow or showing concentric markings (PI. 26) of 

 different shades of green and yellow. Occasionally these halolike 

 margins are prolonged at one end into points (PI. C) from i to several 

 centimeters long. They may extend as yellow streaks through the 

 center or along the margin to the tip of the leaf, but ordinarily they 

 appear as oval spots, measuring i cm. or more in diameter. Marginal 

 infections are common, forming crescent-shaped lesions. These halo- 

 like lesions are conspicuous and characteristic. Except in the central 

 infection area the tissues remain turgid and have a normal appearance 

 except for the paler yellowish color. There is no water-soaked margin 

 about the halo as described by Wolf and Foster {loY for similar lesions 

 of the wildfire disease of tobacco, and the spots do not fall out of the 

 leaves. Exudate does not occur in connection with the lesions. When 

 several lesions occur on the same leaf they often coalesce and produce a 

 general yellowing followed by a breaking across of leaf blades (Pi. C) or 

 a shriveling and drying of tips and margins. During periods of warm, 

 dry weather yellow haloed leaf tissue loses its turgidity and color and 

 forms oval, gray-brown dead spots which on some leaves have narrow, 

 brown margins and on others narrow, yellow halolike margins. Very 

 rarely the dead tissue may assume a pinkish or reddish brown color. In 

 separate lesions the oval outline of the dead halo persists, and even when 

 the whole leaf becomes dry and brown, the original halo outlines may 

 still be distinguished. 



PREVALENCE AND GEOGRAPHICAL DISTRIBUTION 



Personal observations in Wisconsin and specimens of diseased plants 

 from Ohio, Illinois, Indiana, Minnesota, Tennessee, California, and 

 Virginia have led to the conclusion that halo-blight is present in oat fields 

 every season, scattered lesions occurring on the lower leaves more or less 

 throughout the season and occasionally attacking the panicles. These 

 lesions on the lower leaves are more or less hidden by the fresher upper 

 leaves and so escape observation. Only under particularly favorable 

 weather conditions does the blight develop sufficiently to attract 

 attention or to do serious damage. 



FIELD WORK IN 1918 



During the season of 191 8 weather conditions favorable to halo-blight 

 prevailed in Wisconsin and parts of adjoining States, causing an un- 

 usually severe bacterial blighting. In the experimental plots, halo 

 lesions began to appear on from i to 25 per cent of the young plants 

 about the middle of May. By May 25 practically every plant showed 



1 Reference is made by number (italic) to " Lireratiiie cited," p. 17a. 



