THE CAUSAL PARASITE. 

 Table II. — Differences between Heterodera schachtii and II. radidcola. 



37 



That these nematodes are not the same is readily seen wlien they 

 occur on the sugar beet. The one causes no conspicuous galls wliile 

 the other makes the galls so characteristic of root-knot (PL II, fig. 1). 

 Both are very destructive pests of this host, and there is not much 

 choice as to which is the more harmful. Another difference not 

 mentioned above is that H. scJiachtii, perhaps by virtue of its more 

 powerful spear, is able to thrive and spread in stiff er soils than does 

 H. radicicola. In Plate I the figures for the larvae of Heterodera 

 radicicola (figs. 3 and 4) and H. schacJitii (figs. 18 and 19) are drawn 

 to the same scale, respectively. The difference between the two 

 species was emphasized in tabular form by Voigt in 1890. 



METHODS OF SPREAD. 



The larva of Heterodera radicicola is capable of active movement 

 in the soil, and in tliis manner doubtless the disease is slowly spread. 

 From some experiments made by Frank ^ he estimated the rate of 

 progress at about 3 cm. per week. Tliis would amount, during the 

 warm weather, in which infection occurs, say May 1 to September 

 15, to about 75 cm., or about 30 inches. These figures are probably 

 far too low. However, it is not tlirough their own efforts that these 

 nematodes are mainly spread. There are many means of transporta- 

 tion at their disposal. A very frequent one is running water. Thus, 

 a field previously free from the pest sometimes shows its presence 

 in those spots where surface water at a time of heavy rains has 

 deposited a lot of soil from an infested field lying higher up. In this 

 way the pest has been carried from infested fields even to uncultivated 

 woods, as observed by the writer at one place. It has been suggested 

 that heavy winds carrjang large quantities of soil from one field to 

 another may also transfer the nematodes, but in view of their sus- 

 ceptibility to injury by drying, this seems little likely. Especially is 



» Frank. 1885. 



217 



