J^eio South Wales. 183 



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

 fertilizer Itnown 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 fertilizer, 

 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 sterilized 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 laud 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 bo of staves placed close together in. 

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

 galvanized 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 iu 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 obstruc- 

 tion will, even if made of wood or bark, which will decay in the cour.se of a 

 few years, protect the young tree until it has attained considerable size and 

 sent its roots deeji 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 toward 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 cases its progress is through the soil, and may be marked by 

 its effects on roots. But infection does not always occur in this manner. 

 During a spell of dry weather the eggs and dried up larva) exposed 

 on the surface of cultivated ground may be whirled aloft by the wind and 

 scattered for miles over adjacent territory. The disease may thus unseen 

 spread by leaps, making itself felt however in the new localities only after 

 some years have elapsed, and the worms have become abundant by n.itural 

 increase from the few eggs or larvit) deposited by the wind. These facts 

 indicate sufficiently the rate at which root-gall may spread. The facts and 

 rate are much the same as for T. deoastatrix. 



The different modes by which the disease may pass from one piece of laud 

 to another deserve careful consideration, for upon them are based a number 

 of useful precautions. Some of these modes nave already been mentioned 

 incidentally, but the importance of the subject will justify dwelling upon 

 them at greater length, even at the risk of some repetition. The migrations 

 tluo to the animal's own muscular powers are not rapid or great. In fact 



