HETERODERA RADICICOLA. 



415 



slaked and unslaked, calcium carbide, and other moderately 

 inexpensive chemicals are of little avail. But it has been observed 

 that trees and plants with a good ramifying root system do not, 

 as a rule, show any obvious signs of disorder and consequently 

 where valuable orchards are at stake, the use of phosphates and 

 other fertilisers employed so as to stimulate an extensive root 

 system and to induce deep-root growth, would be most beneficial. 

 For deciduous orchards the heavy application of foimaldehyde (one 

 part commercial formalin to one hundred parts water) poured 

 into a small trench round the tree when it is in a dormant state, is 

 said to be advantageous. Other chemicals, such as carbon bisul- 

 phide, magnesium sulphate, sodium cyanide, sodium chloride, have 

 been tried, but must be used in such concentrations that uhey 

 become fatal to the young roots and often kill the tiee. According 

 to Professor Kuhn their use is not to be counselled. 



Inundating the land for some time has been advocated by 

 some authorities, but in view of the fact that the larvae are 

 capable of surviving for over a month in a water medium little 

 hope of success can be attached to the mode of treatment, even 

 if it were practicable in South Africa. Steaming the soil by 

 means of a system of underground pipes under a pressure of 401bs. 

 per square inch is said to have beneficial results, but here again 

 practical economy will restrict this procedure to the hothouses 

 and to small valuable plots. 



The "trap-crop" method of controlling the sugar-beet parasite 

 //. schachtii as devised by Kuhn at Halle is well worthy of con- 

 sideration. Essentially this method was based on the researches 

 of Strubell on the life-history of II. schachtii. A highly sus- 

 ceptible "catch-crop," in this case a species of mustard, is thickly 

 sown in an infested field in early spring. The development of the 

 worm within the roots is constantly followed by microscopic 

 examination, and the infected crop is uprooted before the new 

 generation of larvae have had time to escape from the female 

 (three to four weeks). This procedure perceptibly diminishes the 

 number of parasites in the soil, and after two or three trap crops 

 have been grown and destroyed the land can again be profitably 

 employed for the growing of sugar beet. A considerable measure 

 of success was obtained by the employment of this method in 

 Europe, but when tested by Bessey for //. radicicola in America 

 it does not seem to have yielded such favourable results. As yet 

 our knowledge here 'in South Africa of susceptible and resistant 

 crops is not sufficiently complete to allow of experimenting to the 

 best advantage, but nevertheless this method is worthy of trial. 



The principle of starving cut the parasite in the soil also 

 suggests itself, and conditions in South Africa in many cases 

 would render it fairly simple. The infested land should be cleared 

 of all crops and weeds that might act as hosts for the organism 

 and either be allowed to lie fallow, or where possible, sown with 

 a crop, such as the "iron" variety of cowpea, winter oats, or pea- 

 nuts, that is immune to the parasite. Wheat (Triticum 

 aestivum), according to Sorauer, is somewhat susceptible, and I 



