ECONOMIC IMPORTANCE OF FUNGI 309 



290. Bacteria and Plant Diseases. — In addition to the 

 plant diseases caused by fungi, as mentioned above, a 

 number are known to be caused by bacteria. The "wilt" 

 of sweet corn, a disease first discovered on Long Island, 

 is caused by bacteria, as is also the crown gall, a tumorous 

 or cancerous-like disease common in the rose family 

 (peaches, apples, roses, raspberries), and the walnut, 

 grape, and willow (Fig. 226). The " bean blight " and pear 

 blight, the soft rot of the calla-lily, and the "wilt" of 

 cucumbers and melons, are also caused each by its own 

 peculiar kind of bacteria. On account of their nature 

 these diseases may all be transmitted from one plant to 

 another of the same kind. 



291. Contagious and Infectious Diseases. — The ease 

 with which such tiny organisms as bacteria can be trans- 

 ferred from one place to another makes the diseases they 

 cause easily transmissible or "catching." We actually 

 do "catch cold"; that is, our all too common "colds" are 

 due to the presence of " cold "-producing germs. Arctic 

 explorers testify to the fact that, notwithstanding the 

 great exposures to which they are subjected, they never 

 "catch cold." This is explained by the absence of the 

 "cold" germs that cause colds in other climates or 

 regions. 



When the members of the Peary arctic expedition of 

 1908-09 were in the field away from the heat and infective 

 dust of the ship, they were practically immune from colds 

 and respiratory troubles. "The fact that colds are due 

 to bacteria was clearly demonstrated in the Arctic. 

 We might be precipitated into icy water with the air many 

 degrees below zero ; our clothing saturated with moisture 



