Letters on the Diseases of JPlants. 43 



likely that land will be given up for sucli a length of time, especially when it 

 is remembered that it must be kept clear of vegetation at considerable 

 expense ; but the fact that the worms may be starved out utterly, leads to 

 the question whether or not, by a proper rotation of crops, they may not 

 become so reduced by partial starvation as to become comparatively harmless. 

 The gall-worm evil, like most evils, is endurable so long as it does not 

 become too great. It is only when the soil swarms with larvae that serious 

 damage is effected. Now, certain crops — for instance, maize and the cereals 

 generally — are but little affected by root-gall. If land badly infested be 

 planted with maize, it is reasonable to suppose that the effect will be much 

 the same as if it stood idle and bare ; but it is impossible as yet to say 

 what value this method may have. Of course, this plan contemplates keeping 

 the land absolutely free of weeds while it is bearing maize. The tough root 

 of maize is not easily penetrated by the worms, and, finding no other plant 

 to feed upon, they must, it would seem, of necessity starve. 



7. Use of non-infected Soil. — When trees are to be set in infected land they 

 may be filled in with earth taken from a depth of at least 2 feet. Such 

 earth is practically free from gall-worms, and, if mixed with some artificial 

 fertiliser known to be also free from them, will be found to answer the 

 purpose well. This is a method recommended by Dr. Neal, though he cautions 

 the gardener to beware using too much nitrogenous fertiliser, saying the 

 vigorous growth thereby promoted is unusually subject to root-gall, because 

 the root tissues are tender and therefore easily penetrated by the worms. 



8. Artificial Barriers. — Another precaution which is of service in protect- 

 ing young trees from root-gall is the use of artificial subterranean barriers. 

 Having sterilised a spot of ground for a tree by means of fire, or by the 

 substitution of uninfected subsoil, it becomes a question whether this now 

 uninfested spot cannot in future be kept free from gall-worms, even though 

 the surrounding land be infested. A means towards this end is the placing 

 in the soil of a barrier or obstruction all around each tree, at a distance of 

 (say) 2 feet from it. The barrier may be of staves placed close together in 

 a circle, or, better, of old scraps of iron, such as old kerosene tins or old 

 galvanised sheet-iron. Bark is an excellent and cheap material for the 

 purpose. The barrier should be vertical, or slope from the tree so as to 

 leave room for unobstructed root-growth. The purpose of such a barrier is 

 evident at once. It prevents to a certain extent the entrance of worms from 

 the outside infested soil. It will be effective in proportion as it is tight. 

 Staves would therefore be less effective than old tin or iron in moderately 

 large pieces. The latter, however, are much less likely to be at hand in 

 sufficient quantity. Bark is probably about as available as any material. 

 The pieces of bark should overlap each other. Of whatever material the 

 barrier is made it should extend from a little above the surface of the soil 

 downward at least 18 inches — better, 2 feet. Such an obstruction will, even 

 if made of wood or bark, which will decay in the course of a few years, 

 protect the young tree until it has attained considerable size and sent its 

 roots deep into the ground. After that it is comparatively safe. Old trees 

 with tough roots sinking deep into the ground suffer but little from the 

 gall-worm. 



Of course these barriers, even if watertight, will not prevent the entrance 

 of the gall-worms spattered into the enclosed area during rains, or blown in 

 as dust in dry weather. This fact points towards the usefulness of a mulch 

 (uninfested, of course). 



How does the root-gall spread, and at what rate ? The disease will spread 

 from a centre of infection at the rate of a few rods each year. In such 



