36 BULLETIN 842, U. S. DEPARTMENT OF AGRICULTURE. 



retain their green color longer than normal spikes. They are reduced 

 in length and have their glumes spread out at greater angles than 

 uninfected spikes. In place of normal kernels the glumes contain 

 dark, hard galls, which are shorter and frequently thicker than 

 wheat grains and which may be seen sometimes partly exposed in 

 diseased spikes. Because of a general similarity, these galls often 

 have been mistaken for the seed of cockle (Agrostemma githago), a 

 weed which occurs commonly in wheat fields, for kernels affected by 

 stinking smut due to Tilletia tritici, and for bin-burnt wheat caused 

 by overheating the stored grain. 



The disease is caused by a minute nematode, Tylenchus tritici 

 (Steinbuch) Bastian, the larval stage of which in great numbers 

 may be seen with the unaided eye when a gall is teased apart in 

 water. These larva? in mass appear milky white in color. Indi- 

 vidually they are threadlike animals, nearly 1 millimeter in length 

 and capable of an eellike movement. Under favorable conditions 

 of moisture and temperature they escape from the galls which have 

 fallen to the ground or have been sown along with wheat seed, and 

 by their own movement reach the young seedlings. They finally be- 

 come located between the leaf sheaths near the terminal stem bud 

 and in this region are passively elevated to the inflorescence. Occa- 

 sionally they penetrate the leaves and induce small swellings, but 

 usually they enter only the embryonic flowers, which develop into 

 galls instead of kernels. In these places the larvae, after several 

 metamorphoses, develop into about equal numbers of male and 

 female adults, the former about 2 mm. in length and the latter 

 from 3 to 5 mm. long. Each female may lay more than 2,000 eggs. 

 Measuring approximately 37 p- wide by 85 \l long, these eggs are 

 elongate, ellipsoidal, granular bodies, which in a short time after ovi- 

 position develop into larva? and in this manner complete the life his- 

 tory of the organism. At the maturity of the plant the larva? within 

 the gall become inactive, dried, and motionless, in which lethargic 

 condition they can remain alive for many years. 



The larva? inclosed in galls were killed by an immersion in water 

 at temperatures of 50°, 52°, 54°, and 56° C. for 30, 20, 10, and 5 

 minutes, respectively, provided the galls were thoroughly moistened 

 before the treatment. Much longer immersions at the same tem- 

 peratures are necessary to kill them if the galls are dry when 

 treated. Free from the protective gall, the larva? succumb to tem- 

 peratures of 50° and 52° C. in 15 and 10 minutes, respectively; that 

 is, in half the time required to kill them within the galls. Larva? 

 either inside or outside the protective gall are highly resistant to 

 the toxic action of formaldehyde, mercuric chlorid, copper sulphate, 

 and sulphuric acid. They live after long submersion in concentra- 



