Insects., etc., Injurious to Currants. 239 



SPREAD OF INFESTATION ix A PLANTATION. 



With regard to the rapidity with which the disease spreads from 

 bush to bush in a plantation, Lewis made the following observations 

 at Wye. In a plantation containing about a thousand bushes 

 (Baldwin variety) there was here and there a bush having one or 

 perhaps two big buds upon it, but never more than two. In January 

 1901, be attached labels to these infested busbes, showing the number 

 of swollen buds and their position on the plant. In the following 

 December he found that the bushes which had been labelled were 

 covered with big bud. In no instance, however, had the unlabelled 

 bushes surrounding them been attacked.* From this it would seem 

 that the infestation in a plantation does not spread rapidly from 

 plant to plant. There is, however, every reason to believe, from 

 direct experiment, that when bushes become covered with big bud, 

 many of the mites in the spring will be compelled to migrate, and 

 the attack may then be spread from one badly infested bush to all the 

 bushes round it. Once these have been attacked, even if only to the 

 extent of one bud (and this is what usually happens, as comparatively 

 few mites out of the thousands compelled to migrate will reach a 

 congenial position), then that bush will be covered with big bud by 

 the end of the following season, and the mites will migrate from it 

 to the surrounding bushes in the ensuing spring. This emphasises 

 the necessity and great importance of pulling up and burning any 

 infested bush in a plantation comparatively free, and this should be 

 done immediately any big bud is detected on a bush. 



This, of course, only applies to where one or two bushes show 

 signs of attack. 



In order to prove that the mites migrate from bushes that are 

 covered with big bud, and to find out how rapidly the infestation is 

 carried to surrounding bushes, Lewis tried the following experiment 

 at Wye. 



In an isolated position far removed from any possible outside 

 source of infestation, three very badly diseased bushes were planted 

 and surrounded by young healthy plants known to be entirely free 

 from disease. These were all surrounded by wire-netting to prevent 

 them from being disturbed in any way and to exclude birds. 



They were put in in December 1900, and in the following 

 November the young bushes, with two exceptions, were infested with 

 big bud, two or three big buds being present on each. 



* These bushes were four years old, planted ten feet between the rows and 

 three feet apart in the rows. 



