20 H. L. Russell. 



Reference must again be made to the pear-blight germ (B. amylo- 

 vorus), as its biology is the most thoroughly investigated of any of 

 the bacterial plant-diseases. So far as is known, this malady is con- 

 fined strictly to the Rosacea?, and almost without exception to the 

 sub-order Pomese. It has been reported to have been found in the 

 young fruit of the Kelsey plum 1 (Primus sp.), where it was traced to 

 the sting of a curculio, when the eggs were deposited, and also in a 

 new disease of rasp- and blackberry. 2 However, this last exception 

 has not yet been thoroughly demonstrated. Arthur also reports that 

 he was able to produce a slight infection in the succulent shoots of 

 the peach, but not at all in other non-rosaceous fruits, as mulberry 

 and grape. 3 While the disease afflicts several different species of 

 Pomea3, its commonest host is the cultivated pear. The early history 

 of this disease shows that it was at first more or less restricted in its 

 development on this species, attacking only certain varieties, but in 

 the wider range of the malady in later years, it seems to have 

 acquired the ability of successfully attacking other varieties, until 

 now we know of no variety that is absolutely immune, in the state 

 of nature, from the disease. Although not wholly immune, many 

 horticultural strains possess immunity in a partial degree, as is evi- 

 denced by the fact that under like conditions certain varieties yield 

 much more readily to the disease than others. By some peculiarity 

 in the structure of the plant, possibly merely mechanical in its 

 nature, one variety is able to successfully resist the attacks of the 

 parasite to a larger extent, and thus possesses a partial natural 

 immunity from the disease. 



This immunity can often be overcome, however, if the germs are 

 able to gain access to the tissues in some manner, as by wounds from 

 insect stings, etc. 



This is well shown by the following greenhouse experiment : 



Set 1. Two pear trees (Japan seedlings 4 ), 2 years old, were inocu- 



1 Personal communication from Mr. M. B. Waite, to whom I am indebted for a 

 number of facts bearing on this topic. 



2 Detmers: Ohio Bull. Exp. Stat., No. 6, Oct. 1891. 



3 Arthur: N. Y. Ann. Agri. Exp. Stat., 1884, 362. 



4 This stock, lately introduced from Japan, is in great favor with nurserymen on 

 account of its vigorous and luxuriant growth, and its seeming refractory qualities, 

 under ordinary conditions, toward the blight. Growers have, however, not had 

 experience with it long enough to determine whether its seeming good qualities are 

 of a permanent nature or not. 



