XXI 



FUNGOUS DISEASES 



Most of the diseases attacking our fruit trees 

 and commonly known as rusts, blights, rots, mildews, 

 etc., are known to botanists as fungi or closely allied 

 plant growths. As a rule they are minute thread- 

 like plants, that, growing in the tissues of other plants 

 called "host" plants, just as the corn grows in the soil, 

 destroy the parts affected, or so weaken them that 

 the leaves fall off, the fruit is spotted or decays, or the 

 trunks or branches are injured, etc. The plants of this 

 class are often very minute, so much so as to be almost 

 invisible to the naked eye, and propagate or increase by 

 means of minute seedlike bodies called spores. These 

 are so minute as to be invisible to the naked eye, unless 

 in masses, and are carried about by the winds, often for 

 miles at a time, and consequently there are few localities 

 where these spores may not be found, and under favor- 

 able conditions be ready to grow. 



Nearly all of this group of plants grow only under 

 conditions of moisture and high temperature, though 

 some of them, like the apple scab, flourish in rather 

 cool weather. Most, if not all, of these spores require 

 actual water for their germination, as dew or rain, and 

 after becoming rooted in their "host" plant grow in the 

 juices of the cellular tissues, sending their roots from 

 cell to cell, taking up the cell contents for their own 

 development, and the leaves, the branches or other parts 

 attacked become weakened because the food supply is 

 taken up by the parasite, the amount of injury to any 



