10 U. S. DEPARTMENT OF AGRICULTURE. 



of the fungus, Colletotrichum lagenarium, penetrate the cell 

 walls and feed upon the protoplasm. As a result, the 

 affected cells die and lose their water, consequently one of 

 the first symptoms of the disease is a water-soaked spot on 

 the fruit. Since the water evaporates rapidly from this spot, 

 it dries out and becomes sunken. As more cells are killed 

 and the lesion enlarges, cavities result from the shrinkage 

 and rupture of the killed tissue. In other diseases, such as 

 late blight of the potato, the fungus grows between the cells 

 and sends suckers into the cells, thereby securing its nour- 

 ishment, and eventually killing the host protoplasm. 



The activity of the bacteria and fungi, like that of other 

 plants, is greatly influenced by temperature. Low tempera- 

 tures inhibit the germination and growth of these parasites, 

 consequently are useful in controlling many diseases. Low 

 temperatures also retard digestion and respiration in these 

 organisms, consequently decrease their destructive activities. 

 These facts are fundamental reasons for the refrigeration 

 of fruits and vegetables. Control of the life processes of 

 these organisms by low temperatures probably is more 

 important than the effect of refrigeration upon the life 

 activities of the stored fruits or vegetables. Some fungi and 

 bacteria can grow well at moderate temperatures and conse- 

 quently are very active even in iced cars. Scletotinia, for 

 example, grows well at moderate temperatures and there- 

 fore causes a rot of celery in iced cars. 



Active bacterial and fungous cells do not possess the effec- 

 tive protective tissues of higher plants, consequently their 

 protoplasm is very subject to drying out. In the desiccated 

 state these organisms are inactive or resting, and cannot 

 resume growth or develop so long as they are dry. In com- 

 bating diseases caused by parasites, it is fundamentally 

 important to recognize the fact that a moist environment is 

 essential for their growth and spread. Dried or slightly 

 wilted plant tissues, when shipped or stored in a well aer- 

 ated place, are not attacked readily by bacteria and fungi. 

 Loading or storing wet fruits or vegetables often leads to 

 disastrous results because of the action of micro-organisms. 



Inception of Diseases Due to Bacteria and Fungi. 



While bacterial cells and fungous spores are everywhere 

 present in great numbers on the exposed surfaces of all 

 normal uninjured plant parts, the interior tissue is usually 

 absolutely sterile ; that is, free from all foreign organisms. 



Before disease can occur infection must take place. By 

 infection we mean the entrance of the disease-producing 

 organism into the host tissue and its development therein. 

 Plants are protected against the entrance of foreign organ- 

 isms by their epidermis, by their cork layers, and by the 

 nature of their protoplasm. Foreign organisms may gain 

 entrance through the unbroken skin, or through natural 

 openings in the epidermis such as stomata and water pores. 



