126 REPORT OF THE COMMISSIONER OP AGRICULTURE. 



other bacteria, whatever may be their capacity for inciting disease 

 in other plants or animals, are debarred and harmless so far as the 

 pear tree is concerned. Why it is that only this one kind can suc- 

 cessfully overcome the forces in the tree and break down its structure 

 is not definitely known, but it is usually accepted that the acidity of 

 the plant juices, being in general unfavorable to bacterial develoi)- 

 ment, but not affecting this species, is to be accounted one reason, 

 although it may not be the principal one. 



Knowing that but one kind of germ can set up the disease, it bo- 

 comes evident that its discontinuous appearance m the same locality 

 is explainable on the same grounds that govern epidemic diseases of 

 animals. It requires a conjunction of favorable circumstances not 

 likely to be maintained long at a time, to permit maximum develop- 

 ment, thus giving rise to occasional severe visitations, with interme- 

 diate years of feeble manifestation or total disappearance. 



It is also found by means of inoculation tests that not all trees are 

 susceptible to this disease, but only those embraced in the Pome fam- 

 ily, such as the pear, apple, crab-api^le, quince, hawthorn, and sev- 

 eral other thorns, mountain ash, &c., and of these the pear is so much 

 the greatest sufferer that the disease is usually called Pear Blight, 

 although the descriptive term Fire Blight is also in common use. 



Besides the safeguards already mentioned, each tree is shielded 

 against the invasion of germs by a dry cuticle or bark which envelops 

 the aerial portion, and when fully mature and unbroken proves an 

 effective barrier. As the roots are protected from air infection by the 

 soil, the tree in winter, under ordinary circumstances, is abundantly 

 guarded at every point. In summer, however, the leaves, with their 

 innumerable open stomata and tender watery tissues, would naturally 

 be supposed to offer ample opportunity for the entrance of germs from 

 the air, if such came in contact with them. It is a curious fact, how- 

 ever, that has yet received no explanation, but which is substantiated 

 by both experiment and observation, that the blight bacteria will not 

 grow in the leaves, whether naturally or artificially introduced ; the 

 death of the leaves on a blighting tree is brought about by want of 

 nourishment, the supply being cut off when the conducting power of 

 the limb on which they are seated is destroyed by the disease. 



While the tree is well protected over most of its surface, there are, 

 nevertheless, certain vulnerable points, and none more so than the 

 inside of the flower cup. The surface at the base of the styles is 

 moist and uncuticularized, and the germs which touch it are securely 

 held, find no difficulty in penetrating and develox)ing in the soft tis- 

 sues of the thickened base of the flower, and by passing along the 

 pedicel convey the disease to the twigs and branches. The short 

 period that the flowers are open is time enough to seal the doom of 

 many limbs, and even whole trees. The symptoms of the disease are 

 not observable for days, or more usually weeks, and frequently the 

 first knowledge of impending danger is seen in the blackening leaves 

 near the flowers. If an early examination is made, the dead flowers, 

 or flower-sets, no larger or but little larger than when they first 

 opened some time before, will usually be sufiicient evidence, if taken 

 in connection with the time that has elapsed since flowering and with 

 the present development of the foliage, to show that the germs passed 

 in tlirough this channel. We have in this fact a satisfactory explana- 

 tion of the common ol:»servation of orchardists that trees are far more 

 likely to succumb to the blight when they come into bearing than at 

 any previous time. 



