414 REPORT OF STATE BOARD OF HORTICULTURE. 



PEAR BLIGHT. 



By Dk. B. T. Galloway. 



In parts of Califoi'nia. Oregon, and Washington where this trouble has 

 shown itself, and, fortunately, they are not many, the following explicit 

 account of pear blight by Dr. B. T. Galloway, Chief of the Division of 

 Vegetable Physiology and Pathology of the Department of Agriculture, 

 will be read with much interest. In pear orchards, where the disease is 

 unknown, it should be always looked for, and the grower will be glad to 

 know what to look for. 



What the hlight is — Pear blight is a contagious bacterial disease of the 

 pear and allied fruit trees. It attacks and rapidly kills the blossoms, young 

 fruits, and new twig growths, runs down in the living bark to the larger 

 limbs and thence to the trunk. While the bacteria themselves rarely kill 

 the leaves, at most only occasionally attacking the stems and midribs of the 

 youngest ones, all the foliage on the blighted branches must, of course, 

 eventually die. The leaves usually succumb in from one to two weeks after 

 the branch on which they grow is killed, but remain attached and are the 

 most striking and prominent feature of the disease. 



Hoiv it acts — The most important parts of the tree killed by the blight 

 are the inner bark and cambium layer of the limbs and trunk. Of course, 

 when the bark of a limb is killed, the whole limb soon dies, but where the 

 limb is simply girdled by the disease, it may send out leaves again the next 

 season and then die. All the parts of the tree below the blight are healthy, 

 no more injury resulting to the unaffected parts of the tree than if the 

 blighted parts had been killed by fire or girdling. 



The cause — The blight is caused by a very minute microbe of the class 

 Bacteria. This microbe was discovered by Prof. T. J. Burrill in 1879 and is 

 known to science as Bacillus amylovoms. The following are the principal 

 proofs that it caused the disease : 



1. The microbes are found in immense numbers in freshly blighted twigs. 



2. They can be taken from an affected tree and cultivated in pure 

 cultures, and in this way can be kept for months at a time. 



3. By inoculating a suitable healthy tree with these cultures the disease 

 is produced. 



4. In a tree so inoculated the microbes are again found in abundance. 

 Treatment — The treatment of the disease may be classed under two heads : 



1. Methods which aim to put the tree in a condition to resist blight or 

 to render it less liable to the disease. 



2. Methods for exterminating the microbe itself, which is of first impor- 

 tance, for, if carried out fully, there can be no blight. 



The methods under the first head must, unfortunately, be directed more 

 or less to checking the growth of the tree, and, therefore, are undesirable. 



