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



The various chemicais ordinarily used for disinfecting purposes, 

 such as formaldehyde, mercuric chlorid, and copper sulphate, failed 

 to kill the nematodes when used in strengths which did not injure 

 the seed. 



CROP ROTATION. 



Crop rotation, as well as the use of nematode-free seed, is essential 

 in controlling the disease, as the indications are that the parasite 

 may live in the protective gall for one or more years in soil in which 

 no congenial host grows, and at the end of that time constitute a 

 source of infection. If infested land be planted for two or prefer- 

 ably for three consecutive years to nonsusceptible crops, most or 

 all of the nematodes will be eliminated. Although these nematodes 

 are capable of remaining alive, but inactive, in a dried, stiffened state 

 for a number of years, under the usual field conditions the tempera- 

 ture and moisture are such that they become active during the 

 warmer months, and in this motile stage they will starve in less 

 than a year provided no susceptible plant is available. Almost all 

 of the active larvae will have starved in the soil during the first year 

 after a diseased crop has been harvested, but it is advisable not to 

 plant a susceptible crop on infested land until after two or more 

 years. In England, where a 2 to 3 year rotation of crops, including 

 wheat, is now commonly practiced, Ormerod (27) reports that no 

 difficulty in controlling the disease has been met with if uninfected 

 seed wheat is sown. Marcinowski (22) made a similar report of 

 conditions in Germany. 



Until our knowledge concerning the range of the host plants of 

 Tylenchus tritici is more complete, it is highly inadvisable to grow 

 not only any variety of wheat, oats, 'rye, emmer, or spelt but also 

 any of the closely related grasses on land that is to be freed from 

 nematodes. So far as known, all strains or varieties of w T heat are 

 equally susceptible, none having been reported as resistant. All wild 

 grasses, especially those on which flower galls caused by nematodes 

 have been found, should be scrupulously kept off the land, as some 

 of them may be susceptible to the wheat nematode. These grasses, 

 as well as weeds, may be more easily kept down if the rotation crops 

 are planted in rows so as to permit clean cultivation. As there are 

 no plants outside of the grass family on which Tylenchus tritici has 

 been reported, several presumably immune crops exist which can 

 be used in rotations designed to control the disease. Every precau- 

 tion against reinfesting the soil should be taken if success in con- 

 trolling the nematodes by crop rotation is to be obtained. Spread- 

 ing the pest by those means already pointed out should be prevented. 



Other methods of controlling the parasite have been proposed, 

 but because of their inefficiency or impracticability they have not 



