234 MANUAL OF FRUIT DISEASES 



The black-rot lesions occur as small dark depressions or 

 cankers on the stems, tendrils, peduncles, petioles and leaf- 

 veins. On the canes the spots rarely extend more than a 

 quarter of the distance around, but the tendrils and leaf-petioles 

 may be nearly or wholly girdled. The affected areas are small, 

 one-twelfth of an inch to one inch long, and are elliptical or 

 considerably elongated in form. 



Cause. 



Black-rot is produced by the fungus Guignardia Bidwellii. 

 The trouble begins in the spring, when both ascospores and 

 pycnospores are liberated from their hibernating quarters. 

 The former spores are discharged with considerable force into 

 the air for some distance. Subsequently they are carried by 

 air currents to a variety of places. Some of the spores per- 

 chance fall on the fruits, leaves or shoots, where black-rot 

 lesions may ultimately be produced. The pycnospores ooze 

 out of the pycnidia which have hibernated on old canes, ten- 

 drils, leaf-petioles or mummies. They are washed by rains 

 to the above-ground susceptible parts, where they may finally 

 induce black-rot areas. Either of these spores ascospores 

 or pycnospores then, may start the trouble in the spring. 

 With the presence of moisture they germinate and the result- 

 ing germtubes penetrate the vine. For some time the fungus 

 is developing a mycelium within the attacked organ, and it 

 may be many days before there is any visible evidence of disease. 

 The length of this period varies with the weather conditions, 

 and with the part affected. When it is hot and dry the period 

 is materially reduced, and with cool weather it is lengthened. 

 In tender, juicy fruits this period is shorter than in stems and 

 leaves. On fruits the period is from 8 to 18 days ; on leaves, 

 10 to 21 days. 



The diseased berries show mycelium both between and 

 within their cells. Eventually the cells are killed, they collapse 

 and the surface becomes sunken. Similar action takes place 



