11. 



Estimation of nematode damage 



pcfin^ft J^K^^'^''^^ "^^^^ u^ nematode damage tne damage was rrostly 



heau'v ol^tr^Hr^f.''' J".'^' °' '^' ^^"^^'^ ^^°P ^^th that Jf the 

 ,^^,,^?f P^^\^- H^^lthy and diseased plants could be distinguished by 

 specific symptoms of attack caused by the nematodes. 



l«.. ti^^^°^^^ ^^^ nematodes damage to plants is accompanied by more or 

 less specific symptoms the reactions of plant roots to certain Pratv- 

 S^lc^'t"'" r? ectoparasites is not so ciear cut as, l^r e:.^, 

 in rootknot, cyst foming nematodes, or sting nematodes. Moreover, 

 as all soils contain one or more ectoparasitic species, the role of 

 nematodes in plant growth can only be evaluated if plants in soil with 

 and without nematodes can be compared. Some nematologists used gentle 

 heat or a treatment with a nematicide, preferably DD, as a means to 

 Obtain eelwom free soil. Many estimates of eelwom'dLaJe also S 

 Snt of^the soil ^"^^^^-^^ "P°" S"""^^ improvement found after DD treat- 

 Unfortunately the hope, sometimes an unshakeable belief, of these 

 nematologists that DD or gentle heating would not do anything but kill 

 nematodes has not come true in Europe. Both treatments appeLed to 

 result in growth improvement sometdjnes also under conditions where 

 damage by nematodes in the untreated soil would be a rather far fetched 

 explanation of this phenomenon. (PITCHER, 1957, GOFFAKT and HElLlNG. 

 ^!nf ; n^\ f'"' ^"^ °^^^''^ ^^^° "°^^^e^ ^ type of growth iinprove- 

 to th^^«??.^r^^'"'"i ^" ^^'^^ experinents which was remarkably simlar 

 to the effect of a dressing with nitrogen fertilizer. 



f >,. /°^f°^er some diseases appeared to be reduced considerably by DD 

 treatment whicn had no relation to nematodes. VOGEL 1958 found that 

 fouS'irr^' infestation of a soil was strongly reduced. The writer 

 ilZt 1/ ^^'^^\^ ^ carrots, which shows no relation to the occur- 

 rence of any nematode, was controlled very effectively by DD in the 

 first crop of carrots after treatment but not in the second, whereas 

 the nematodes still were very low in number then. 



Therefore, in the preceding pages only those cases have been 

 mentioned where evidence of a more reliable nature than growth im- 

 provement after a nematicidal treatment of the soil was available. 



