48 ON WHITE RUST. 



IV. — Observations on a Form of White Rust in Pear Trees. 

 By the Rev. M. J. Berkeley, M.A., F.L.S. 



Every cultivator of Pears has of late years, in addition to the 

 well known TEcidium cancellatum, which in certain localities has 

 long been the pest of the Pear orchard, had to struggle against 

 another less seemly intruder (Helmiuthosporium pyrorura, Desm.) 

 which not only impairs the health and beauty of his trees, but renders 

 the fruit, which at first perhaps gave fair promise of remuneration, 

 perfectly unsaleable. During the early part of the past summer an 

 enemy belonging to the animal world, under the form of a minute 

 Acarus, was more than usually abundant, affecting not only the 

 leaves but distorting the fruit, as represented in the Gardeners 

 Chronicle, 1853, p. 420. In the course of some investigations of 

 the mode of operation of the Acarus, which had been hard at 

 work at my own pear trees, I observed, especially in the Marie 

 Louise and Glout Morceau, that a great quantity of the leaves 

 were blistered somewhat after the fashion of peach leaves when 

 suffering from chilly weather, before their perfect expansion, but 

 with the blisters in general more neatly defined. In many cases 

 these blisters formed two parallel lines on either side of the 

 midrib, but sometimes, especially when larger, they were 

 irregularly scattered over the frond. In some cases the blistered 

 part had become black, and in others the portion of tlie leaf 

 which had protruded had fallen out so as to leave a regularly 

 defined aperture. On inspecting the under side of the distorted 

 leaves, the cavities were found to be lined with a thin white 

 stratum consisting of myriads of confluent white specks, of a 

 waxy rather than powdery appearance, and in an early stage of 

 growth, covered with the cuticle which is loose and more or less 

 ruptured, showing the subjacent organisms through the fissures. 

 On examining the substance more closely, it appeared first that the 

 vesicles of which it was composed did not occupy exactly their 

 original position, but that the tips of the little heaps of spores 

 had collapsed more or less from moisture, so as to produce a more 

 or less uniform stratum like that of a Corticium. The mass 

 itself consisted of ovate or elliptic hyaline bodies about 4-0V0 of an 

 inch long, without any apparent sporophores, though the greatest 

 pains were taken to discover them both b^^ myself and Mr. 

 Broome, to whom I had communicated fresh specimens. Here 

 and there a spore might be discovered with a little projecting 



