NEMATODE DISEASE OF WHEAT. 35 



been adopted. Kiihn's (21) catch-crop method, once thought to be 

 a practicable means of controlling the sugar-beet nematode {Hetero- 

 dera sehachMi) has been proposed. It consists of growing a suscep- 

 tible plant, in this case wheat, on the land, allowing it to become well 

 infected, and then destroying the young plants at the proper time 

 to catch the largest number of nematodes. Then by removing the 

 crop from the land many of the parasites are eliminated. The 

 method is ineffective because only a portion of the nematodes free 

 in the soil enter the seedlings, and it is also impracticable because 

 of the cost involved. Deep plowing is a more successful means of 

 reducing the disease, but this by itself is not adequate to eliminate 

 the nematodes. The deeper the larvae are turned under the soil be- 

 low a depth of a few centimeters, the less opportunity they have for 

 reaching and infecting plants. Marcinowski (22) obtained a higher 

 percentage of infection when the nematodes were placed 3 centi- 

 meters below the surface of the soil than at any other depth, and 

 only a small number were able to reach the wheat seedlings buried at 

 a depth of 30 centimeters. 



SUMMARY. 



A serious disease of wheat known in Europe since 1743 and due 

 to the nematode Tylenchus tritici (Steinbuch) Bastian has been re- 

 cently found causing considerable damage in certain parts of the 

 United States, particularly in Virginia. 



Reports in the literature and an examination of specimens re- 

 ceived from various foreign countries show that the malady is almost 

 world-wide in distribution, having been discovered in all continents 

 except Africa. Within the United States it is known to occur at 

 present in California, Virginia, West Virginia, Maryland, and Geor- 

 gia, and other States also may be found to be infested. 



Samples of harvested wheat collected in Virginia contained from 

 less than 1 per cent to more than 50 per cent of diseased kernels by 

 count, and as much as 40 per cent damage by it to some fields of 

 wheat in the same State has been observed. 



On seedlings the disease. usually causes a decided wrinkling and 

 rolling or distortion of the young leaves, which, if severely attacked, 

 turn yellow, wilt, and die. Occasionally such leaves have one or 

 more small light-colored galls, which may be located on any portion 

 of the leaf and contain the causal organism. Roots are not directly 

 affected, nor do the stems of infected plants near the surface of the 

 ground become swollen, as in the case of tulip-root, another disease 

 of wheat, caused by the nematode Tylenchus dipsaci, which infects 

 stems and crowns. It is on the maturing spikes that the malady 

 produces the most conspicuous symptoms. As a rule, infected heads 



