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ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 



Tlie Germ 

 The germ causing this disease is named 

 by bacteriologists ''Bacillus amylovorus.'" 

 For our purpose of discussion we may 

 call it the pear-oiight bacillus, the pear- 

 blight germ or the pear-blight microbe, 

 all these terms meaning practically the 

 same thing. These germs are among the 

 most minute of living things. Bacteria 

 or germs are vegetable organisms, and are 

 as truly plants as trees, grasses, etc. How- 

 ever, they are very low down in the scale 

 of the vegetable kingdom since the indi- 

 vidual organism consists of a single cell, 

 which may be elliptical or rod-like in 

 form. They multiply by lengthening a 

 little and then pinching in two. This is 

 the only way in which they may increase 

 their numbers. This process can take 

 place within half an hour or less, and 

 this I have proven by observation in a 

 hanging drop culture under the micro- 

 scope. Although they are extremely 

 minute they may be measured by means 

 of microscopical apparatus. The stan- 

 dard of measurement for these minute 

 objects is the micromillimeter, and a mil- 

 limeter is about one twenty-fifth part of 

 an inch. The pear-blight germ is from 

 two-thirds to three-fourths of a micromil- 

 limeter wide, and from one to one and 



Spitzenburg Infected with Pear Blight. Note 

 the two streams of ooze running down the 

 body. (Original.) 



one-half micromillimeters long when it 

 has reached its mature stage. As another 

 illustration showing their minute size I 

 may say that if 25,000 pear-blight germs 

 were placed end to end they would 

 scarcely measure an inch in length. Cur- 

 iously enough, the young germs are 

 longer than the older ones. When they 

 are growing rapidly their development in 

 length goes on more rapidly than their 

 division. This germ forms no spores, and 

 for this reason can not live over the dry 

 season, as do the germs of anthrax which 

 form spores. The pear-blight germ is 

 very sensitive to drying, and, in fact, is a 

 very short-lived germ. The fact that it 

 does not form spores is highly important, 

 as spore-forming bacteria are capable of 

 living over in dust which may be blown 

 about by the winds. The germ dies rap- 

 idly in the blighted tissues, as soon as 

 the tissues have become fully killed. It 

 cannot withstand drying, usually dying 

 within two weeks or so. It is killed by 

 exposure to direct sunshine in a very few 

 minutes, usually not more than ten min- 

 utes unless protected by the bark or 

 twigs. It rapidly dies when it is washed 

 into the soil, since it can no longer get 

 the necessary food for its existence and 

 multiplication. In fact, the pear-blight 

 germs disappear and die very shortly 

 after they are exuded or washed out by 

 the rains from the twigs and branches. 

 It also dies when the blighted bark dries 

 up. It can only live along the advancing 

 margin of the disease in the thick, fleshy 

 bark or cambium which has been invaded 

 by the bacteria, and which does not have 

 time to dry out until the cool weather 

 approaches. The thick bark of the large 

 limbs, branches and root system remains 

 moist during a long period, especially in 

 the winter. By this method, the impor- 

 tance of which we will show later, the 

 germs are able to carry over, or live over, 

 from one season to another. As a matter 

 of fact, it is only by this means that the 

 pear-blight germ can live over during the 

 dormant period of the trees. The germs 

 are killed by high temperature, they are 

 wholly destroyed when subjected in 

 liquid culture to the temperature of 55 



