The Black-rot of the Grape, and its Control. 



209 



ascospore 



which is widest at the middle, were trying to squeeze through a tight 

 place. At first, it moves slowly, but eventually the widest part passes 

 the constriction and the spore is snapped into the air. It is often thus 

 discharged for a distance of more than a centimetre (J inch). The next 

 spore is pushed up and follows closely after the first, so that all 

 eight spores may be discharged into the air in the course of ten 

 minutes. 



Critical stage. 



Now comes the most critical stage for the fungus. In order to con- 

 tinue its existence the spore must fall upon some green part of the vine, 

 either stem, tendril, leaf, or young cluster. The only plant, other than 

 the grape, on which it 

 is known' to grow is 

 the Virginia creeper or 

 5-leaved ivy. It will 

 not grow on weeds, 

 grass, posts, wires, or 

 on any dead material 

 except by careful nurs- 

 ing in the laboratory. 

 If the cluster of mum- 

 mies is clinging to the 

 vine, it is easy for the 

 spores to fall down and 



lodge on a leaf or even 



, 1 , , Fig. 178 — Diagrammatic section of a perithecium. contain- 



De Diown across to ^^^^ winter-spores. The spores do not all mature at the 



other vines. Most of sam.e time. Germination of the spores can be seen at 



the mummied berries, '^' ''^^^- ^^''""'^y '^^S'"'/^^^-) 



however, are knocked off in pruning and are lying on the ground. The 



fungus is therefore largely dependent on the wind to blow the spores 



to the leaves or fruits, and for this reason a comparatively small 



number ever survive. 



qerminaTion 



Infection. 



The spore that lodges upon a green part of the vine must have a drop 

 of water, though it may be very small, in which to germinate and grow. 

 Germination rarely takes place in less than 36 hours. In 36 to 48 hours 

 a small bud or protrusion (germ tube) appears on one side, see (Fig. 178), 

 and as it grows the tip makes its way through the surface of the leaf or 



