192 



Blight in Pear Trees. 



Vol. IX. 



IS to turn it to a viscid, unctuous state. It 

 assumes a reddish, brown colour; becomes 

 black by exposure to the air; is poisonous 

 to vegetables, even when applied upon the 

 leaf. Whether in some measure this fol- 

 lows all degrees of congelation, or only un- 

 der certain conditions, I have no means of 

 knowing. 



The effect of freezing and thawing upon 

 the tissues and sap vessels is better known. 

 Congelation is accompanied with expansion; 

 the tender vessels are either burst or lace- 

 rated ; the excitability of the parts is im- 

 paired or destroyed; the air is expelled from 

 the asriferous cavities, and forced into the 

 passages for fluids; and lastly, the tubes for 

 the conveyance of fluids are obstructed by a 

 thickening of tiieir sides.* The fruit trees 

 in the fali of 184.3, were then brought into 

 a morbid state— the sap thickened and dis- 

 eased ; the passages lacerated, obstructed, 

 and probably, in many instances, burst. The 

 sap, elaborated, and now passing down in an 

 mju red state, would descend slowly, by rea- 

 son of its inspissation, the torpidity of the 

 parts, and the injured condition of the ves- 

 sels. The grosser parts, naturally the most 

 sluggish, would tend to lodge and gradually 

 collect at the junction of fruit spurs, the 

 forks of branches, or wherever the condition 

 of the sap-vessels favoured a lodgement. In 

 some cases the passages are wholly obstruct- 

 ed ; in others, only in part. 



At length the spring approaches. In 

 enr'.y pruning, the cultivator v.'ill find in 

 those trees which will ere lono- develope 

 blight, that the knife is followed "by an unc- 

 tuous sap, and that the liber is of a green- 

 ish yellow colour. These will be the first 

 Signs, and the practised eye may detect 

 them long before a leaf is put forth. 



When tlie season is advanced sufficientlv 

 to excite the tree- to action, the sap will, as 

 usual, ascend by the alburnum, which has, 

 probably, been but little injured; the leaf 

 puts out, and no outward sign of disease 

 appears; nor will it appear until the leaf 

 prepares the downward current. May, 

 June, and July, are the months when the 

 growth is most rapid, and when the tree re- 

 quires the most elaborated sap; and in these 

 months tlie blight is fully developed. When 

 the descending fluid reaches the point where, 

 m the previous fall, a total obstruction had 

 taken place, it is as effectually stooped as if 

 the branch were girdled. For the sap which 

 had lodged there, would, by the winds and 

 sun, be entirely dried. This would not be 

 the case if the sap was good and the vitality 

 of the wood unimpaired ; but where the sap 



* Lindley's Horticulture, 81, 82. 



and vessels are both diseased, the sun affects 

 the branch on the tree just as it would if 

 severed and lying on the ground. There 

 will, therefore, be found on the tree, branches 

 v>'ith spots where the bark is dead and shrunk 

 away below the level of the surrounding bark ; 

 and at these points the current downward is 

 wholly stopped. Only the outward part, 

 however, is dead, while the alburnum, or 

 sap-wood, is but jjartially injured. Through 

 the alburnum, then, the sap from the roots 

 passes up, enters the leaf, and men are as- 

 tonished to see a branch, seemingly dead in 

 the middle, growing thriftily at its extremi- 

 ty. No insect-theory can account for this 

 case; yet it is perfectly plain and simple, 

 when we consider tiiat there are two cur- 

 rents of sap, one of which may be destroyed 

 — and the other, for a limited time, go on. 

 The blight, under this aspect, is nothing but 

 ringing or decortication effected by diseased 

 sap, destroying the parts in which it lodo-es, 

 and then itself drying up. The branch will 

 grow, fruit will set, and frequently become 

 larger and finer flavoured than usual. 



But in a seond class of cases, the down- 

 v.;ard current comes to a point where the 

 diseased sap had effected only a partial 

 lodgement. The vitality of the neighbour- 

 ing parts was preserved, and the diseased 

 fluids have been undried by wind or sun, 

 and remain more or less inspissated. The 

 descending current meets and takes up more 

 or less of this diseased matter, according to 

 the particular condition of the sap. Where- 

 ever the elaborated sap passes, after touch- 

 ing this diseased region, it will carry its 

 poison along with it, down the trunk, and 

 by the lateral vessels, in toward the pith. 

 We may suppose tliat a violence, which 

 would destroy the health of the outer parts, 

 would, to some degree, rupture the inner 

 sap vessels. By this, or by some unknown 

 way, the diseased sap is taken into the in- 

 ner, upward current, and goes into the gen- 

 eral circulation. If it be in a diluted state, 

 or in small quantities, languor and decline 

 will be the result; if in large quantities, 

 and concentrated, the branch will die sud- 

 denly, and the odor of it will be that of 

 frostbitten vegetation. All the different de- 

 grees of mortality result from the quantity 

 and quality of the diseased sap which is 

 taken into circulation. In conclusion, then, 

 where, in one class of cases, the feculent 

 matter was, in the fall, so virulent as to de- 

 stroy the parts where it lodged, and was 

 then dried by exposure to wind and sun, the 

 branch above will live, even through the 

 summer, but perish the next winter; and 

 the spring afterwards, standing bare amid 

 green branches, the cultivator may suppose 



