54 DISEASES OF CULTIVATED PLANTS 



Nut gall mite (Eriophyes avellanae, Napela) sometimes 

 proves destructive to cobnuts and filberts. It forms big 

 buds similar to those made by the black currant mite, 

 which either shrivel and die or expand into a mass of 

 deformed, stunted leaves. Such buds are very conspicuous 

 on the bushes in winter. 



The belief is current in some districts that the nut mite and 

 the black currant mite are the same species, and that the nut 

 mite can infect the black currant, and vice versa. This idea 

 is a mistaken one ; the two mites are distinct species, and 

 each one can only infect its own host-plant. However, 

 prejudices die hard, and an object lesson was considered to 

 be the only means of convincing sceptics. With this object 

 in view, I had a long row of badly-galled ' Baldwin ' black 

 currant bushes planted in the experimental ground at Wisley, 

 and alternating with the currant bushes, perfectly clean hazel 

 bushes were planted. The bushes were intentionally crowded 

 so that the branches of nut and currant were in contact. 

 This row remained growing for some years, but the hazels 

 remained perfectly free from disease. On the other hand, 

 black currant bushes, free from 'big bud.' were planted in 

 close proximity to diseased hazel bushes. The currant bushes 

 remained free from disease. Sceptics were not convinced. 



Pear leaf blister mite {Eriopliyes piri, Nal.) is a well- 

 known pest in Europe and the United States. In this country 

 the pear is more especially attacked, the apple rarely, but in 

 the United States the apple suffers more severely than the 

 pear. Among other hosts of the parasite are the service 

 berry {Amelanchier vulgaris); cotoneaster (Cotoneaster vul- 

 garis)', white beam (Sorl>i/s aria); mountain ash (Sor/'us 

 ancuparia) ; service tree (Sorbus terminalis). 



On pear leaves and young fruit the mite at first forms 

 minute greenish warts or blisters, which as a rule soon change 

 to a red colour, and at a later stage become brown or black. 

 When the minute galls are numerous they often coalesce and 

 form diffused red, then black patches ; in such cases the leaves 

 fall early in the season. The mites hibernate in winter in 

 the buds, and as the leaves expand the mites emerge from the 

 galls and enter other points of the same or other leaves 

 through the stomata, and thus the disease spreads until near 

 the end of the growing season. When the leaves become old 

 the mites migrate and enter the newly-formed buds, where 

 they remain until the following spring. 



