MOSAIC OF SUGAR CANE ' AND OTHER GRASSES. 9 



Among the cane varieties commonly grown in Louisiana and other 

 Southern States, some rather constant differences occur in the expres- 

 sion of the mosaic disease. In L 511 it will be noticed that streaks 

 are rather scant in newly invaded leaves and on account of their light 

 color make a great contrast with the normal areas. They are bluntly 

 pointed and range from one-sixteenth to three-sixteenths of an inch 

 wide and from one-fourth of an inch to 3 or 4 inches long (PI. I, fig. 4) . 

 Later, the light areas or streaks are more numerous and in most 

 cases tend to become confluent in well-defined bands of light tissue 

 extending across the leaf at right angles to the midrib and alternating 

 with bands where the light streaks remain isolated. These bands are 

 from 1| to 2 inches long. The above condition is typical of the 

 disease as it appears in L 511, but does not invariably occur. 



In D 74 the streaks are not usually isolated, even at first, so that 

 very quickly the coalesced light areas are predominant and the nor- 

 mal areas appear as irregular, elongated islands 1 thirty-second to 

 three-eighths of an inch wide and of varying length, from one-fourth 

 of an inch to several inches, as shown in Plate I, fig. 5. Affected 

 areas are light green at first, but the tendency for the whole leaf 

 to become opaque yellow is pronounced. 



In purple cane the light areas are elongate and isolated at first, but 

 later they predominate and coalesce and the normal green shows as 

 irregular elongated islands, as illustrated in Plate I, fig. 5. The islands 

 are not of uniform width or length. 



In the youngest leaves of Ribbon cane, the light areas are in the 

 shape of attenuated streaks, usually about one-eighth of an inch wide 

 and one-half of an inch to 1J inches long, but the size varies greatly, 

 some streaks being very minute, and others, by running together at 

 the ends, form continuous stripes 6 inches or more in length. In 

 general, the streaks are isolated from one another and uniformly dis- 

 tributed on the leaf blade as in Plate I, fig. 4. The amount of nor- 

 mal-colored tissue greatly exceeds the light tissue at this time. Excep- 

 tionally, the light streaks may be confluent from the first, and this is 

 more frequently seen near the midrib, leaving the margin normal in color 

 or with a f ewscattered pale streaks. In slightly older leaves, by growth 

 and confluence of the light-colored areas the latter becomes predomi- 

 nant and the whole leaf becomes pallid or even yellow in its general ap- 

 pearance. The dark-green or normal areas are now very scant, and 

 they appear as elongated streaks in the pale green, just the reverse of 

 the condition in young leaves, except that the dark-green streaks 

 are less regular in outline. The individual streaks vary considerably 

 in width and direction throughout their extent, streaks perhaps 

 three-eighths of an inch wide at one end becoming constricted to 

 1 thirty-second of an inch, then alternately widening and narrow- 

 ing or becoming oblique with the midrib, with no apparent forces 

 142481°— 19— Bull. 829 2 



