[Vol. 9 



340 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



General Description of the Disease 



The symptoms of the disease vary somewhat with host and 

 considerably with conditions under which infection takes place. 

 The pathogen grows best at relatively high temperatures, so that 

 other conditions being favorable, at a temperature of around 90 

 F. the first signs of infection will occur in 24 to 48 hours. Under 

 very moist conditions the spots appear water-soaked at first and 

 as the atmosphere becomes dry they appear as light brown or 

 grayish brown spots or streaks. On foxtail and on Chaetochloa 

 genicalata these spots may be bordered by a distinct brown or 

 reddish brown area (see pi. 23, fig. 1). Under favorable condi- 

 tions of infection attacked areas often appear grayish green, 

 withered, looking as if scalded, and surrounded by a somewhat 

 indefinite yellowish halo. The spots on foxtail as they occur 

 naturally in the field, in a dry atmosphere, are usually of two 

 types, (1), small, reddish brown, round or oval-shaped areas en- 

 closing lighter brown centers, and (2), light or dark brown or 

 blackish streaks, usually starting at the tip of the leaf and often 

 including a large part of leaf area. The spots may appear on 

 any part above ground, including leaf-sheath as well as blade, 

 rachis, and glume. Under natural conditions spots are more 

 often to be observed on the leaf blades. 



On oats the spots vary from light yellow, somewhat indefinite 

 areas to grayish green, markedly withered areas. Often there 

 are marked tinges of red 1 in attacked areas, particularly when a 

 number of infections have coalesced. Attacks may occur on 

 seedlings or on plants that are in head. Numerous artificial 

 inoculations have been made on various varieties of oat seedlings, 

 and under favorable conditions of infection not a single oat plant 

 out of a large number inoculated would remain alive 4 days after 

 inoculation (see pi. 26, fig. 1). It is of course evident that a 

 seedling with a limited surface is much more readily killed than 



a larger plant. 



Attacks on wheat, rye, barley, and corn do not vary consid- 

 erably from that on oats. There is usually very little red dis- 

 coloration, the spots vary in size and shape, and the predomi- 

 nant discolorations are grayish green, light yellow, or brownish. 



ir The writer is satisfied that various agents can cause reddening of oat leaves. 

 Under certain conditions of light and temperature a mere sharp crease in an oat 

 blade may produce a reddening in the part above the crease. It is well known 

 that the production of anthocyans may take place at low temperatures and when 

 the transfer of foods is cut off, resulting in an accumulation of sugars. Attacks 

 by aphids often appear as yellow or red discolorations. 



