8 U. S. DEPARTMENT OF AGRICULTURE. 



growth of mold and no rot, while others may show the mold 

 and black masses of the fungus. The presence of all of 

 these symptoms in a package or car, however, furnish the 

 basis for a correct diagnosis of the rot as watery soft rot due 

 to Sclerotinia. 



Very frequently the symptoms of several diseases appear 

 on the same specimen or in the same lot. A single potato 

 may show symptoms of black scurf, black heart, and Fusa- 

 rium tuber rot at the same time. Sometimes the symptoms 

 of one disease are followed immediately by those of another, 

 as in black heart of potatoes followed by slimy soft rot. 



Diseases Due to Living, Parasitic Agencies. 



Most plant diseases are caused by one of two types of 

 minute plant life, commonly known as germs and molds and 

 often referred to collectively as "microorganisms." 



Host and Parasite. 



The plant attacked and at whose expense the bacteria 

 or fungi feed, and upon and in which they live, is known as 

 the host. Bacteria or fungi living upon and at the expense 

 of a living plant or animal are known as parasites. When 

 these live upon dead plant or animal matter they are classed 

 as saprophytes. Some bacteria and fungi are restricted to 

 a single host; others can attack a variety of hosts. Diseases 

 due to the latter type of organisms are more likely to origi- 

 nate in transit and storage than those due to the former 

 type. 



Bacteria and Fungi. 



Bacteria are exceedingly small, single-celled, non-green 

 plants which multiply by simple division. Under proper 

 conditions of temperature, moisture, and food supply, bac- 

 teria multiply with astounding rapidity. 



Some fungi also are composed of single cells, but most of 

 them consist of many cells united end to end to form threads. 

 A single thread is known as a hypha (plural hyphae) and 

 the mass of hyphae of a fungus is known as the mycelium. 

 Generally the mycelium grows within the tissues of the 

 host and is invisible, although under favorable conditions it 

 may also grow out on the surface. At times the mycelium 

 of some fungi form thickened, hardened masses known as 

 sclerotia. 



Fungi generally reproduce by the formation and libera- 

 tion of special cells or groups of cells called spores. In 

 function, these spores are the equivalent of the seed of 

 higher plants, and are distributed as these are, by man, 

 animals, wind, and water. They are, of course, distributed 

 very much more readily and widely because they are so 

 small and light. The spores of some fungi, such as "blue 

 mold" or "bread mold" are found everywhere. 



