38 REPORT OP THE COMMISSIONER OF AGRICULTURE. 



merous on plants of any kind, their presence is indicated by tlic curling 

 and shrinking of the leaf. Leaf blister is produced by fanRi, and is dis- 

 tinguished by the leaves becoming thickened and swollen into blisters, 

 which are whitish or faintly reddish on the upper and hollow on the 

 under surface. The leaves thus attacked fall off in a few weeks and new 

 and healthy leaves are at once produced, and usually with but little 

 otfect upon the quality or quantity of the crop. From the circumstance 

 that these diseased leaves are sometimes attacked by aphides, the blis- 

 ters forming an agreeable asylum for these lice, it is frequntly stated 

 tliat the blister is caused by insects, and the terms curl and blister have 

 therefore come to be considered as synonymous by those who are not 

 observant enough to perceive the difference between cause and effect in 

 this case. Close observation would show that the blistered leaves are 

 most frequently found to be entirely free from any of the aphis family. 



The predisposing cause to leaf blister has long been recognized, and, 

 unlike some other diseases of the peach — the yellows, for instance — ^it is 

 perhaps universal wherever climatic conditions may prove favorable to 

 its existence. 



Its cause is entirely atmospheric, and it may be looked for, with a 

 certainty of linding it, wherever sudden extreme changes of tempera- 

 ture occur when the leaves of the tree are in a young state, or, in some 

 varieties, about the time of flowering. 



In Britain, where the peach trees are generally trained on walls, and 

 where mild winters are oftentimes followed by frosty spring weather, 

 the tendency to leaf blister is so common that measures are usually 

 taken to prevent it. Fifty years ago it was a common practice to cover 

 the face of the wall upon which the trees were trained with evergreen 

 boughs early in spring in order to guard the young leaves against injury 

 from cold. Trees which were not protected in this manner would be 

 often severely injured from blister on the foliage, and even one-half of a 

 tree not protected would be attacked while the protected half would be 

 perfectly clean and healthy. Portable glass coverings are now largely 

 employed for this purpose. 



A change of 30° in forty-eight hours in the early stages of growth will 

 produce peach-leaf blister. The most notable and most severe case 

 which we have met was a faU of 40° in twenty-four hours, with a cold 

 northeast breeze ; its effects upon a row of peach trees planted about 6 

 feet from, and on the west side of, a board fence, were, that in a few days 

 the exposed tops were severely attacked by blister, while the lower por- 

 tions, protected in some measure by the fence, were all but uninjured. 



It has been stated that this disease is contagious, and the advice has 

 been given to cut down and burn every tree thus attacked to prevent 

 its spread and the total destruction of peach trees. This disease, ho^\- 

 ever, is not contagious, and it is probable that none of these leaf diseases 

 are so ; at least we have not had reason to consider them so from a Ion-; 

 observance of fungoid growth on plants. 



Many years ago an experiment was made hero with peach trees in pots 

 and tubs which were grown in the orchard house. For the purpose of 

 exemplifying the origin of leaf blister several trees were removed to the 

 outer air from an average temperature of 60° to 65°, varying from 7;")° 

 to 80° during the day, to 50° or 55° during the night, and exposed until 

 the temperature fell to 38°. They were then again placed in the house. 

 In the course of a few days the leaves showed the effect of the cold and 

 became badly blistered, but no blister appeared on other trees in the 

 house, although the branches having blistered leaves were purposely 

 intermingled with those which had not been exposed to cold. 



