24 



when phenol is added to the soil. But there is also an unexpected 

 chemical decomposition which has been studied in the Chemical 

 Department by Mr. Sen Gupta, under Mr. Pag-e ; it appears that 

 the small quantity of manganese oxide in the soil plays an im- 

 portant part here. 



Serious efforts are also being made to control wart disease of 

 potatoes. Sterilising- agents have been found capable of destroy- 

 ing the organisms in a badly infested plot of land so that perfectly 

 clean tubers could be grown ; the various problems arising out of 

 the practical application of the method are being- studied by Dr. 

 W. B. Brierley, Mr. VV. A. Roach and Miss Cilynne en plots of 

 land at Ormskirk and at Hatfield. 



THE PLANT IN DISEASE. 



(ENTOMOLOGICAL, MYCOLOGICAL, INSECTICIDE AND 

 FUNGICIDE DEPARTMENTS.) 



Much damage to crops is caused by the attacks of insects and 

 fungi. These pests can often be kept in check by spraying, but 

 on the farm it would usually be cheaper, where possible, to enable 

 the plant itself to resist the attacks. Both methods are being 

 studied. 



In the case of one disease — the Wart Disease of Potatoes — 

 certain varieties are absolutely immune. Attempts are being made 

 to find out the reason for this. Immunity might be due to some- 

 thing made in the leaf and distributed throughout the plant, or, 

 on the other hand, it might result from some special characteristic 

 of the lower part of the plant. In order to test these possibilities, 

 Mr. Roach is building up new varieties of potatoes by grafting one 

 sort on to another ; he has grafted immunes on to susceptibles and 

 vice versa ; the resulting plants are then grown in infested soil. 

 So far the substitution of a top from a susceptible plant on to an 

 immune variety has caused no loss of immunity, nor has the substi- 

 tution of the top from an immune to a susceptible variety conferred 

 immunity. It does not appear, therefore, that immunity is the 

 result of any action in the leaf. 



Considerable attention has been paid by Dr. Davidson to the 

 aphids attacking broad beans. It is shown that the rate of 

 multiplication of the insect on the plant differs for the different 

 varieties of bean, though unfortunately the most resistant of the 

 beans has little commercial value. Attempts are therefore being 

 made to breed a variety of high resistance and at the same time 

 having a value to the farmer comparable with that of the present 

 kinds. Even with the same variety, however, the power of 

 resistance is affected by the dissolved substances in the plant 

 tissues, and this can be modified by changes in the nutrients 

 supplied to the plant. In both directions there seem to be possi- 

 bilities of the control of this troublesome pest. 



The usual history of this particular pest is that the asexual 

 forms (which do the damage to the crop) continue throughout the 

 Summer, and are then followed by sexual forms in October which 

 produce eggs that lie dormant through the Winter and hatch 

 out in the following April. Dr. Davidson has, however, shown 



