i82 DISEASES OF CROP-PLANTS 



evidence of infection from tree to tree. When a tree dies the 

 soil in its neighbourhood must become heavily infested, and it 

 is reasonable to assume from analogy with the known habits 

 of similar parasites that it remains so for a considerable period. 



It cannot be otherwise than desirable to prevent as far as 

 possible the worms in infested trees from reaching the soil, and 

 for this reason trees recognised to be affected should be dealt 

 with as promptly as possible, before the infestation of the roots, 

 which cannot well be dealt with, has become general. It is 

 necessary not merely to cut the tree across at the base, but to 

 dig round it, cut the roots, and turn it out — which is not a very 

 difficult operation in most soils. When the leaves have dried an 

 attempt should be made to roast the stem sufficiently to kill 

 the worms. Planters may have ideas of their own as to the best 

 way of doing this. A slow fire would be most likely to penetrate 

 to the required depth, and it is suggested that the stern be sawn 

 or chopped into convenient lengths, piled in the excavation 

 with the chopped up leaves and added fuel, and banked by 

 scraping up the loose soil and any chips that have been scattered, 

 so as to give something approaching to the structure of a charcoal 

 pit. It is recognised that the problem of disposing of such large 

 masses of material in a satisfactory way is a difficult one, and fire 

 in some form would seem to be its only solution. The worms 

 have been found to be alive in large numbers several months 

 later in a stem which had been merely scorched. 



It is conceivable and sufficiently probable to be the basis of 

 precaution until the facts are known, that the worms maybe 

 harboured in the crevices of the husk of fallen nuts and be trans- 

 ferred with these when they are used for planting. It would, 

 therefore, be safer to plant nuts which come from areas with no 

 record of infestation, or otherwise to use them only when taken 

 direct from the trees and not picked up from the ground. 



It is not believed that any method of soil disinfection with 

 chemicals is practicable. Experiments are in progress to test 

 the protection of trees by banding and by placing a slowly 

 diffusible repellent substance in the top. The common practice 

 of putting lime or salt among the leaves may possibly be of 

 some use in this direction, though nematodes in general are 

 highly resistant to chemical poisons. It is considered possible, 

 on the theory that the worms travel on the outside of the stem, 

 that a band of crude oil may hinder their ascent by producing 

 a quickly drying surface. 



Observations of the spread of the disease suggest that drainage 

 trenches act as obstacles to its extension and it is advised that 

 the sites of eariy cases should be dealt with by the method 

 described in Chapter XV for the isolation of contacts where 

 Rosellinia disease occurs. 



