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TEXTBOOK OF BOTANY 



spores continue to be produced for three or four months. 

 In the meantime the knot has been growing in size and 

 has turned almost black, and small rounded swellings have 

 appeared on the surface ; these swellings contain a second 

 type of spore. During the late 

 summer another set of rounded 

 bodies appear on the knot ; these 

 remain over the winter, and in the 

 spring they are found to contain 

 many small sacs, in each of which 

 are eight spores. The only known 

 means of controlling this disease is 

 to cut out and destroy the knots. 

 This means is successful only if 

 there is cooperation on the part of 

 growers, not only in cleaning out 

 their orchards but also in carefully 

 watching the wild varieties for the 

 appearance of knots. 



388. Black Rot of Grapes. — This 

 disease causes great damage to the 

 grape crop if weather conditions are 

 favorable. It is most serious if the 

 days are warm and moist, and its 

 ravages are greatly checked by a 

 short spell of cool, clear days. Al- 

 though the greater hann comes from 

 the injury to the fruit, the disease is 

 found on the leaves, and from these, under favorable con- 

 ditions, it may soon spread to the fruit. On the leaves 

 small brown spots appear, which rapidly grow in size and 

 number, giving the leaf a wrinkled appearance. Upon the 

 fruit a small spot, purplish in color, is first noticed. This 

 grows, and finally the whole fniit decays and shrivels. 

 Spores are produced in abundance upon the affected parts, 



Fig. 218. — Portions of 

 two branches of the choke- 

 cherry attacked by the black 

 knot. The fungus grows in 

 the tissues of the host, pro- 

 ducing large, irregular swell- 

 ings which will eventually 

 encircle the stem and kill 

 all beyond the diseased 

 portion. 



