-lO Bulletin 253. 



green stem. Growth of the tube continues by the absorption of sap and 

 other food material from the vine. The fine thread-like tube branches 

 and spreads out for a short distance on the inside. Cross-walls are now 

 formed and the growth is known as mycelium. The mycelium is the 

 vegetative or growing stage of the fungus and, although rarely seen, is 

 the stage which does the great damage. The threads not only push 

 between the cells of the grape and in that way absorb the sap which 

 would normally flow from one cell to another, but also penetrate directly 

 into the cell and take up food material there. When the mycelium has 

 established itself on the inside and can no longer be prevented from grow- 



FiG. 179 — On the left, black-rot spots on the leaf (natural size); on the right, 

 photomicrograph of a small portion of a single spot showing fruit bodies 



of the fungus. 



ing or be killed by means of a poisonous spray, infection is said to have 

 taken place; the leaf or vine is infected. 



Incubation. 



The mycelial threads grow for a short distance in all directions from 

 the point of infection; never as much as an inch and usually not more 

 than J to J inch. This growth takes place rather slowly and there is no 

 external evidence of it until 12 to 20 days after infection took place. 

 This time is known as the period of incubation. At the end of this time 

 the diseased area changes color and that part of the leaf becomes 

 yellowish brown in color. (See Fig. 179). 



