PLANT DISEASES. 97 



III. Insecticides and Fungicides and the Chemistry of 



Plant Diseases. 



Rothamsted Experimental Station. 



A series of investigations in progress at the Soil Research 

 Institute at Rothamsted may properly be referred to in this 

 chapter. The work consists mainly in a study of the fundamental 

 nature of insecticides and fungicides. Our present knowledge 

 on this subject is largely empirical, depending on the results of 

 trials of various chemicals, &c., and until we know more of the 

 chemical principles involved and the reasons for the effects 

 observed, further progress cannot be very rapid. 



There is urgent need for work on insecticides, not only to 

 reheve the present scarcity of nicotine, but also, if possible, to dis- 

 cover an insecticide which, while being poisonous to caterpillars, 

 would not be injurious to larger animals. Various plant products 

 are being investigated from this standpoint, in collaboration with 

 the Ministry's Pathological Laboratory. Tuba Root {Derris 

 elliptica), an Eastern plant, has been closely examined in this con- 

 nection and holds great promise. The root has been dealt with by 

 intricate chemical analysis, and found to contain products highly 

 poisonous to insects, and particularly to caterpillars. Different 

 consignments of the root, however, vary greatly in their effici- 

 ency. A simple chemical method has been devised for ascer- 

 taining the relative efficiency of different consignments, thus 

 making grading and blending possible. The poisons contained 

 in the plant have been extracted in a pure state and shown to be 

 at least as poisonous, weight for weight, as nicotine, to cater- 

 pillars, but not so poisonous to aphids. Various methods of 

 extraction are being studied to devise means of securing the 

 poisonous compounds in a concentrated form. 



Another important field of work is the search for an effective 

 soil insecticide. A study of the poisonous effects of various 

 products to wireworms revealed the fact that some of the most 

 commonly used insecticides and partial soil sterilisers are in- 

 effective in certain soils. Thus, naphthalene, which is widely used 

 as a soil insecticide either in a pure state or in the form of a 

 proprietary article, and — in conjunction with other materials — 

 as a partial steriliser, may be decomposed by soil bacteria before 

 its poisonous action reaches the wireworms, and the latter are able 

 to move away from the sphere of influence and so escape death . 

 This is the case with such disinfectants as carbohc acid (phenol, 

 cresol), naphthalene and toluene. In collaboration with the 

 Bacteriological Department of the Institute, some of the soil 

 bacteria which are responsible for this decomposition have been 



