Control A: 7 



Effect of Soil Nematocldes 



The effect of soil nematocides depends on the kind and amoimt of chemical 

 used. There are now available materials with different ranges of toxic- 

 ity to living organisms. Materials with a broad spectnm of toxicity are 

 conveniently termed soil sterilants. For example, methyl bromide at one 

 pound or more per 100 square feet does a fairly effective sterilization 

 job in the upper 12 inches or so of soilj killing weed seeds, many bac- 

 teria, and fungi as well as the nematodes and soil insects. Chloropicrin 

 at high rates (hOO-^OO pounds per acre) has a similar effect. Vapam and 

 l^lone are sold as being able to control weeds and fungi in addition to 

 nematodes. The other nematocides are intended principally for control 

 of nematodes and do not have as wide a spectrum of toxicity as the soil 

 sterilants, and these materials fall into two groups depending on whether 

 or not the chemical is toxic to plants. This is a very important dis- 

 tinction because phytotoxic nematocides require a waiting period before 

 planting to permit the chemical to be lost from the soil or to be changed 

 to a non-phy to toxic form. 



Two of the most widely-used nematocides are fumigants requiring a wait- 

 ing period before planting. These are ethylene dibromide (EDB) and some 

 form of dichloropropane-dichloropropene (D-D and Telone) or a mixture of 

 both (Dorlone). With the standard application of D-D (20 gallons per 

 acre) or EDB (5U pounds of active ingredient per acre), the usual result 

 is the killing of 80 to 95^ of the nematodes. Effect on weed seeds, 

 bacteria, and fungi seems to be small, though not entirely absent. That 

 there is some effect on bacteria is indicated by reports that the propor- 

 tion of ammonia nitrogen to nitrate nitrogen remains high for some weeks 

 after fumigation, presumably due to destruction of some bacteria. 



Less is known as yet about the toxicity spectrum of the two generally 

 available nematocides reported to be non-toxic to some plants and having 

 low or moderate toxicity to many others. These chemicals are 1,2-dibro- 

 mo-3-chloropropane (Nemagon, Fumazone and V-C13 Nemacide). 



If the soil was heavily infested with nematodes, elimination of a large 

 proportion of the population permits increased root growth with a con- 

 sequent improvement in vigor of the plants. There is often a noticeable 

 increase in uniformity of growth over the field. Yield increases of 2S% 

 to S0% or more, often with improvement in quality, are obtained. 



However, the increased root growth of the plants provides nearly ideal 

 conditions for increase of the remaining nematodes, and the final result 

 may be that there are more nematodes at the end of the season than there 

 were at the start. Thus the effect of soil fumigation lasts only for 

 one season, as a rule, though if conditions are particularly favorable, 

 some effect may be seen the next season. 



