22 - Farmers' Bulletin 1126.
chance has been afforded for cross-pollination. The great need of special efforts to keep Sudan grass pure is illustrated by the pres- ent condition of the sweet sorghums, very few fields of which are
to be found anywhere that are pure as to variety. They are very commonly mixed not only with other varieties of sorgo, but also with the grain sorghums. Unless seedsmen and growers unite in an effort to keep their seed fields free from sorghum hybrids, Sudan grass will lose much of its distinctiveness within the next 20 years.
DISEASES OF SUDAN GRASS.
RED-SPOT. The worst disease of Sudan grass is the so-called sorghum blight,7
more appropriately . designated as red-spot. - This is a bacterial disease, characterized by the appearance of distinct reddish spots on the leaves, which gradually spread until the leaves turn brown and die. (Fig. 11.) Its effect on the plant is much the same as rast, and, like rust, it is most destructive in warm, humid regions. It is prevalent in the Great Plains, but does little damage there except in wet seasons. Along the South Atlantic and Gulf coasts it almost entirely prevents the profitable production of Sudan grass. (See map, Fig. 3.)
Xo remedy or preventive of red-spot has been found other than the uge of resistant varieties. Rotation with other crops not sub- ject to the disease probably holds it in check. In localities where sorghums are regularly grown, however, they furnish a continuous source of infection.
KERNEL SMUT.
Sudan grass is subject to the kernel smut of sorghum. This disease changes the individual kernels into a mass of dark spores covered by a grayish membrane. These spore masses look like an elongated seed, fully twice as large as the healthy kernels.
As in the case of red-spot, the sorghums furnish a source of infec- tion for the kernel smut.' This disease of the seed is not of any great
importance in the production of Sudan grass for hay and pasture.
Kernel smut can be controlled very effectively by treating the seed with formaldehyde.8 One pound of full-strength formaldehyde is mixed with 30 gallons of water. The sacks of seed are immersed for
7 Kellerman, W. A., and Swingle, W. T. Sorghum blight. In 1st Ann. Rpt., Kans. Agr. Exp. Sta., 1888, p. 281-302. 1889.
Bnrrill, T. J. A disease of broom corn and sorghum. In Proc. 8th Ann., Soc. Prom. Agr. Sei., 1887,. p. 30-36. 1887.
Radais, Maxime. On the blight of sorghum. In Bot. Gaz., vol. 28, no. 1, p. 65-68. 1899. . s Humphrey, Harry B., and Potter, Alden A. Cereal smuts and the disinfection of seed grain. U. S. Dept. Agr., Farmers' Bui. 939, 28 p., 16 fig. 1918.