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 wheat 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 
