304 THE PARASITIC DISEASES OF PLANTS. 



The Bacterioses and Rots. A single illustration of this 

 type must suffice. 



The Fire Blight of the pear, quince, apple, etc., (B. amylovorus.) 

 This bacterium attacks various members of the apple family and 

 a number of other plants as well. The disease has been known 

 for over a century and almost every conceivable explanation has 

 been given for it. That it is caused by a bacterium has been finally 

 demonstrated by the isolation of the organism and the reproduction 

 of the disease by inoculation experiments. In the form known as 

 the twig gall, the first indication of the disease is commonly seen in a 

 browning or blackening of the leaves of the young shoots, which soon 

 die. It then extends into the stem by the way of the inner bark, causing 

 it to become blackened. The whole of this tissue is destroyed by the 

 bacterium, causing a girdling of the tree. Then it extends down the 

 stem, sometimes going at the rate of an inch a day, and eventually 

 causing great injury or complete destruction. It particularly 

 attacks the stored starch, converting it into a gummy substance. 

 The diseased area may extend for a distance down the stem causing 

 a patch of "canker", and if checked in its growth by the onset of 

 winter, it remains alive in the stem till warm weather, when it once 

 more begins its work of destruction. In moist weather a viscid 

 mass extends from the canker spots, containing bacteria. .Little 

 is known of its means of distribution, although it is thought that 

 it may find entrance through the flowers and be carried to them by 

 bees. It may, however, enter through wounds in the bark elsewhere. 

 The only feasible method of fighting it of any value is to cut away the 

 diseased parts as soon as the trouble appears, great care being taken 

 to be thorough in the pruning and to cut away every bit of diseased 

 wood. 



Other examples of this type of bacterial diseases are the following. 



Bean blight, produced by Bact. Phaseoli. 



Cotton bacteriosis, produced by Bact. malveacarum. 



Walnut bacteriosis, produced by Pseud, juglandis. 



Mulberry blight, produced by B. cubonianus. 



Black spot of plum, produced by Pseud. Pruni. 



Wakker's disease of hyacinth, produced by Bact. Hyacinthi. 



