77 



watchfulness. They verify the truth of a maxim 

 of WilHam Wirt, that "There is no excellence with- 

 out great labor." 



For the consideration of the horticultural profes- 

 sion, Mr. Editor, I now venture to suggest a hypoth- 

 esis relative to the cause of blight, which, to me, seems 

 to explain most, if not all the phenomena usually at- 

 tending the disease. Blight manifests itself during 

 the growing season only, and during, or just after, 

 warm and rainy or damp weather; but more fre- 

 quently just after rains, preceded by several weeks 

 of very dry weather. Alternately wet and dry warm 

 weather seems favorable to its development. The 

 spores of most species of fungi are more or less dif- 

 fused throughout the atmosphere at all times and 

 places, though in greater abundance in damp 

 weather, or in the vicinity of soils adapted to their 

 production. They are inhaled and absorbed by the 

 Pear tree through its stomata (and possibly roots), 

 much in the same manner that miasm is inhaled by 

 man, — and, getting thus into the circulation, they 

 remain there for a longer or shorter period, either 

 fixed or in motion, awaiting the proper conditions 

 'for development. If the tree continues healthy, 

 and its normal condition remains undisturbed, the 

 spores are probably thrown off by evaporation, or 

 remain dormant, as in the case of spores of the ague 

 plant in the healthy human system. But if de- 

 rangement, from whatever cause, occurs in the cir- 

 culation of the tree, whereby it becomes predisposed 

 to disease form outward sources, the spores in the 

 circulation and tissues at once germinate and de- 

 velop into poisonous plants, producing the disease. 

 Seeds of the more common plants, it is well known, 

 often lie imbedded in the ground for years before 

 an opportunity to germinate occurs. 



Ordinary conditions, favorable to the development 

 of blight, occur where the weather has been dry and 

 warm for several weeks, and is followed closely by 

 wet or damp weather. 



During the dry weather all the functions of the 

 tree are actively employed in supplying sap to meet 

 the demand created by constant evaporation from 

 the fohage. While this is going on, a sudden 

 change of weather immerses the tree in a damp, 

 humid atmosphere. At once the active vessels of 

 the tree inhale and absorb voraciously, and become 

 gorged with moisture and crude sap. This gorging 

 deranges the healthy condition, and incipient de- 

 composition in the tissues takes place. The spores 

 of the fungi (either just introduced or remaining 

 dormant in the tree,) at once seize upon the decom- 

 posing matter, and generate into plants, which send 

 their poisonous mycelium rapidly through and along 



the cambium of the tree, and, eventually, into the 

 cells and inter-cellular spaces of the heart wood, ex- 

 tracting the juices and destroying succulent shoots, 

 branches, or whole trees, according as predisposed 

 for the attack. Fungus, it is said, acts as one of 

 nature's scavengers, seeking the removal of all de- 

 composing substances. 



A large majority of Pear trees have health and 

 capacity to resist the derangement referred to, unless 

 there is great concentration of the exciting cause, — 

 just as a large majority of men have capacity to re- 

 sist derangements in their systems, — and, hence, 

 blight usually attacks only here and tjiere a branch 

 or tree. 



The functional derangement of the tree may be 

 styled the internal, and the fungi the external cause 

 of the disease. Why some trees of an orchard are 

 predisposed to attack, and others not, although 

 equally exposed, seems incomprehensible. The mi- 

 croscope, it is hoped, may, ere long, determine the 

 reason. 



Of late years, the use of that instrument has led 

 many learned men to the belief that several animal, 

 as well as vegetable diseases, are of cryptogamous 

 origin. Not only is it now claimed that cholera is 

 of fungus origin, but that measles, certain skin dis- 

 eases, some kind of poisoning, (from the common 

 Ivy vine, for instance,) and intermittent and remit- 

 tent fevers, are of like origin. In the vegetable 

 kingdom, not only blight in the Pear, Apple, Peach 

 and Quince, but the Potato disease, the Plum rot, 

 smut in wheat and corn, rust of wheat, ergot of rye, 

 mildew of Grapes, and other kindred affections, are 

 confidently claimed to be of like origin. 



During the past season I made several examina- 

 tions, under the microscope, of diseased leaves and 

 blighted branches of Pear trees, and, though inex- 

 perienced, had no difficulty in recognizing upon the 

 leaves a most beautiful variety of tungus, with nu- 

 merous pearl white threads anastomising over the 

 entire surface, drawing, no doubt, their support 

 from the tissues of the leaf, and causing the 

 deadened appearance of a leaf in autumn. On ex- 

 amining very thin sections of the blighted wood, the 

 mj'celium of fungi were plainly visible in the cells 

 and spaces between ; showing, beyond question, the 

 existence of fungi in blighted trees. In some in- 

 stances, the fuf.gi are thought to be a consequence 

 of the disease, though, ordinarily, they are claimed 

 as a cause. 



I submit the foregoing theory of blight, not as my 

 own, but as that of Dr. Salisbury substantially, be- 

 lieving it affords a more rational solution of the 

 " vexed " question than any yet coming under mY 



