i8 95 . AN EELWORM DISEASE OE HOPS. 193 



the soil, but it is important that no crop should be grown which is 

 known to be subject to attacks of Hetevodera schachtii, or Tylenchus 

 devastatrix, until the ground has been thoroughly cleaned in this 

 manner. The evil effects of planting hops on the site of old gardens 

 which have been grubbed only a year previously on account of the 

 disease, I have witnessed more than once. Both the theory and 

 practice of cleaning the ground by Kuhn's method are good, but 

 much of its success depends upon the estimation of the correct time 

 at which the destruction of the " trap plants " should take place, and 

 this can only be determined by an examination of the root, which 

 requires a certain amount of skill in the use of the microscope. 



The mature parasites are not more than a twenty-fifth part of 

 an inch long, and the larvae and eggs very much smaller. The 

 minute character of the pest, coupled with its great reproductive 

 power, makes it very difficult to deal with by direct application of 

 poisonous chemical substances. Much of the life is passed inside the 

 delicate roots of the plant. Moreover, the eggs are protected during 

 their development by the dead leathery chitinous body of the female, 

 and would not be injured by solutions strong enough to destroy the 

 free larvae in the soil. 



The addition of chemical substances, such as carbon disulphide 

 and gas-water, to the soil, apart from the cost of saturating the latter 

 to any appreciable depth, would not be suitable in the case of 

 growing hops suffering from the disease, since these substances are 

 injurious to plants. Application of alkaline solutions and alkaline 

 salts to the soil as manures has met with some success at the 

 experimental stations in France and Germany, and these are to be 

 recommended. Top-dressing in spring with muriate of potash or 

 sulphate of potash has been found most beneficial ; kainite, which is 

 a hydrous magnesium sulphate with potassium chloride, has not been 

 so satisfactory. Liming, in some instances, and working in of salt 

 have diminished the evil effects of the parasites, and the former 

 process should be tried, especially where the soil is full of humus 

 from the indiscriminate use of large quantities of organic manures. 

 As soon as an affected plant is seen in a garden it should be grubbed 

 and burnt on the spot, and quicklime applied to the ground in the 

 immediate neighbourhood. All implements used in cultivating 

 infected gardens should be properly cleaned before being used among 

 healthy crops, and the planting of "sets" taken from infected gardens 

 should be discontinued. 



It is difficult to determine the rate at which the parasites would 

 naturally spread if left to themselves ; there is no doubt that the 

 parasites are at present largely carried from " hill " to " hill " by the 

 incessant cultivation which goes on in the hop-garden. 



Owing to the great extent of the root-system of the hop-plant 

 and its rapidly growing and recuperative powers, it takes a con- 

 siderable time before a plant is totally disabled and rendered useless ; 



p 



