Letters on the Diseases of Plants. 41 



Consequently, in any trial of Kiilin's remedy in dealing with the Australian 

 gall-worm, it will be best, in the present state of our knowledge, to keep 

 well within his limits and allow (say) three weeks before ploughing up the 

 trap-roots. Mangels will make the Ijest trap-root, and they should be sown 

 thickly. Cow peas may also be tried. 



2. Gall-proof Varieties. — Dr. Neal recommends, as a practically gall-proof 

 stock for the orange, the hardy bitter-sweet or sour species, and, with some 

 qualification. Citrus trifoliata, and the Japanese Unshin, or Satsuma ; as a 

 nearly gall-proof stock for the peach, seedling American wild plum or one 

 of the Japanese plums Kelsey, Satsuma, or Ogru ; as a stock for grapes 

 perhaps the cordifolia or vulpina races. Por other plants subject to root- 

 gall he found no resistant stocks. 



Fire is a 'powerful destructive agent which may, in certain cases, be 

 brought into play in combating root-gall. The larvae of the gall-worm infest 

 the soil to a depth of at least 2 feet, but by far the greater number are 



Fig. 53. — Moulting larva of a gall-worm, Pig. 54. — Neck aud head of a gall-worm, 



X 100 ; c, old skin ; s, spear ; v, x 200 ; s, spear ; b, median bulb ; p, 



ventral gland that emptied through excretory pore, 

 the pore p ; b, median bulb ; o, rudi- 

 mentary sexual organ. 



found within a few inches of the surface. The heat of a large fire will 

 penetrate to this depth in sufficient degree to destroy life. This fact may 

 be applied in setting out those trees particularly subject to root-gall, such as 

 the peach, apricot, almond, plum, and fig. Nothing short of a large fire 

 lasting several hours, will kill the worms. The tree must be set in the midst 

 of the burnt area, aud no soil or fertilizer used except such as is known to 

 be free from gall-worms. 



3. Use of destructive chemicals. — Experiments looking towards the use of 

 some fertiliser or chemical destructive to gall-worms have been made, and 

 the results may be summed up as on the whole negative. No thorough- 

 going chemical remedy for Eiibenmildigkeit has ever been discovered, 

 although multitudes have been tried. Similar negative results were obtained 

 by Dr. Neal in the case of his T. arenariiis. Still the results are interesting, as 

 pointing out what positively will not succeed ; furthermore, they may be 

 made the basis of plans for further trials. 



Of all the vermicides yet tried, lime receives the highest commendation. 

 It must be used in large quantities to be effective. (The same is true of the 

 muriates and sulphates of potash and ammonium.) One to two tons of lime 

 to the acre, applied, not all at once, but part in June and the remainder in 

 October or November, may be recommended. The result is the destruction 

 of a large number of worms ; but many eggs, protected by the tissues in which 

 they are being incubated, doubtless escape destruction, and live to propagate 

 the disease. I have noticed that when the disease is combated with 



