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 
