44 Letters on the Diseases of 'Plants. 



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 larvse 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 natural increase from the 

 few eggs or larvaj 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. devastatrix. 



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

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

 of useful precautions. Some of these modes have 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 

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

 they are so slight that I think it may be questionable whether they would 

 account for any but the very slowest spread of the disease. Even when the 

 worms pass from plant to plant in the same paddock it is questionable 

 whether the movement is not due to transportation by some of the numerous 

 agencies constantly at work in their neighbourhood. Almost everything 

 that moves either in or upon the soil may transport the minute eggs and 

 larvae of gall-worms. Air, water, animals are all agents in disseminating the 

 disease. The manner in which winds may act has already been alluded to, 

 and some precautions, such as mulching, suggested. Under the head of 

 drainage we have seen how necessary it is in combating root-gall to have an 

 eye to the surface currents which during rains may pick the disease up as it 

 were, and deposit it in mass elsewhere. The action of subterranean water 

 has also been lightly touched upon. The general lay of the land determines 

 largely the nature and direction of the currents in the soil. These doubtless 

 have something to do with the spread of the disease. Here very little can 

 be suggested beyond a proper system of drainage. 



One set of agencies in the spread of root-gall, and a most important one 

 too, has not yet received consideration. I refer to other animals. Insects, 

 earth-worms, birds, domestic animals, man himself, are all factors in the life 

 history of the gall-worm. Let the farmer who is fresh from cultivating his 

 infested paddock show me his boot, and the chances are that I shall be 

 able to remove from it small clumps of earth containing larvae of the gall- 

 worm. The hoofs of his horses are in a similar condition. If the weather 

 is damp, it only needs a gun to demonstrate that even the feet of the magpie 

 that followed the plough repeat on a smaller scale the same conditions. The 

 insect that burrows in the ground and brings to the surface subterranean 

 material is active in aiding the gall-worm in finding new pastures. The egg 

 or larva leaves the mandibles of the insect only to be seized up by the wind, 

 or be pressed with other matter into some crevice in boot or hoof, and thus, 

 it may be, travel miles before being again set down. This is far from being 

 a fancy sketch ; every statement rests on the most unimpeachable observation. 

 Even the hands when soiled from field work may carry enough material to 

 start a thriving colony of gall-worms. It only needs to be washed off, and 

 thrown with the water around roots of some favourite plant (to help it along, 

 poor thing!), to form a nucleus for a new infested area. But enough has 

 been said on this head to put those interested on their guard. The thorough 

 cleansing of boots and hoofs before passing from infested to uninfested land 

 is too obvious a precaution to need mentioning. There could be no more 



