NEMATODE DISEASE OF WHEAT. 31 



It is evident that the larvae are most frequently introduced into 

 new localities while contained in the galls. The latter may be trans- 

 ported along with seed wheat in which they are mixed, or in soil, or 

 by many other means. If conditions are unfavorable for the larvae 

 to escape from these galls, the parasite is able to remain in a lethar- 

 gic dried state for many years and afterwards to produce the dis- 

 ease. The organism may also be transferred long distances outside 

 of the protective galls, in which case it probably could not remain 

 alive and active for more than one year unless a host plant were 

 found. In this free state the larvae are most likely to be carried in 

 soil, which is practically the only place where they normally occur 

 outside the host. Although the spreading of free larvae may take 

 place, it is, of course, almost entirely by means of those contained 

 in galls that the organism reaches widely separated areas. In the 

 latter condition it has been repeatedly introduced into this country 

 with importations of seed wheat. In all probability such introduc- 

 tions would have been entirely prevented or long delayed if careful 

 inspection of all shipments had been made. In the future every pre- 

 caution should be taken to prevent its further importation. 



Once introduced into a locality the larvae are capable of traveling 

 by their own movements only very limited distances in the soil. 

 Haberlandt (15) states that he obtained infection in seedlings which 

 were growing in the ground 20 cm. distant from an unopened gall. 

 The writer has found abundant infection in seedlings growing 30 

 cm. away from unopened galls which were buried a few inches be- 

 low the surface of the ground. When it is considered that this move- 

 ment must have taken place within a short time, probably less than 

 a month, the rate of travel for so small an animal is fairly rapid. 

 Probably during the course of a growing season, in which infection 

 may take place at any time if young plants are available, the organ- 

 ism might be able to cover several feet or even a few yards. It is, 

 however, not through their own efforts that these larvae mainly are 

 spread. Many other means are at their disposal. Either within or 

 without the galls, they may be distributed by running water or water 

 resulting from heavy rains. They will not drown after months of 

 submersion, and as drying for a long period does not kill them they 

 may be carried alive in small particles of desiccated soil in various 

 ways, such as on the feet of men and animals, and by agricultural 

 implements. Fortunately, there usually is but little tramping over 

 fields while they contain growing wheat, so that this is an unim- 

 portant means of spreading the parasite except when no crop is 

 present. 



Of course, the chief manner of spreading the parasite within a 

 locality is by the sowing of seed wheat which is mixed with nema- 



