35 



the numbers of both bacteria and protozoa decrease rapidly for 

 the first two or three days, after which there is a slow but 

 steady fall for periods exceeding three months. No explanation 

 can as yet be offered. 



Wart Disease of Potatoes. — An important case of control of 

 a soil micro-organism has been investigated by Dr. Brierley, Mr. 

 Crowther, Miss Glynne and Mr. Roach. Wart disease, one of 

 the worst potato troubles in this country, is caused by an organism 

 having a remarkable power of persisting in the soil so that it 

 cannot be eliminated by the ordinary method of ceasing tem- 

 porarily to grow potatoes. The direct method of studying the 

 effect of various chemicals on the organism is inapplicable owing 

 to the difficulty of germinating the winter sporangia : pot 

 experiments failed owing to difficulties of obtaining infection in 

 pots, till Miss Glynne showed how this could be brought about. 

 Direct field experiments were the only satisfactory method of 

 procedure, and these, while tedious and costly, showed that heat 

 (which owing to obvious practical difficulties was tried only 

 in pot experiments), formaldehyde and sulphur were all 

 effective in dealing with the disease. Heat is too expensive, so 

 also is formaldehyde at present, and possibly for a long time to 

 come, but sulphur is relatively cheap. Mr. Roach overcame 

 the earlier failures by using the Simar cultivator, and so ensuring 

 a better mixture of the sulphur with the soil. There is evidence 

 that on light soils, such as are generally used for potatoes, an 

 application of 12 cwts. per aGre of sulphur eliminates wart disease. 

 A large scale trial is now being made to test the practicability 

 and effectiveness of the treatment. Heavier soils apparently 

 require bigger doses of sulphur. 



On the other hand,' it does not appear that the " scab M 

 of potatoes caused by the fungus Spongospora subterranea is 

 amenable to treatment by sulphur, although in America, positive 

 results are said to have been obtained. 



INSECTICIDES. 



The Staff of the Department of insecticides, fungicides, and 

 partial sterilising agents, under Mr. F. Tattersfield, have for 

 the past three years been engaged in a search for a substitute 

 for nicotine. The seeds and leaves of a tropical plant, 

 Tephrosia vogelii, have been found to possess approximately 

 the same toxicity as nicotine ; these could readily be obtained 

 should the need arise. Special attention has been directed to 

 the possibility of using synthetic substances, since these can be 

 made to any desired standard of purity, and in any quantity. 

 The work is done on systematic lines, the effects of the various 

 groups being studied as they are substituted in a relatively 

 simple molecule such as benzene. Thus it is found that the 

 introduction of a nitro (N0 2 ) group into the benzene molecule 

 considerably increases the toxicity, while the methyl (CH 3 ) group 

 has less effect than any of those tested, the order being : — 

 N0 8 >NH 2 >0H>C1>CH 3 



