86 Diseases of Economic Plants 



fruit, per tree, in rows treated with the most effective Bor- 

 deaux mixture ranged as high as $6.20 above that in ad- 

 joining untreated rows, or the equivalent of a net gain of 

 $427.80 per acre. Over one thousand per cent net gain in the 

 set fruit has resuhed from the use of some of the more effec- 

 tive sprays. 



The trees should be sprayed each season, since experi- 

 ments prove that treatment one season will not prevent the 

 disease the following year. Spraying should he done even 

 though the trees are not expected to liear, since the loss of 

 the crop of leaves is as great a drain upon the trees as is the 

 maturing of one-half to two-thirds of a crop of fruit. 



Die-back^'* (Valsa leucostoma (Pers.) Fr., Cytospora). — 

 This disease of limbs, trunk, and twigs was first fully de- 

 scribed in the United States by Rolfs of Missouri in 1907 who 

 noted it upon peach and Japanese plum. It is present also in 

 Europe and Australia. 



Infection occurs upon buds or wounds during the growing 

 season, and in early winter and spring the tips of young 

 branches, especially water sprouts, are killed back from 2.5 

 to 46 cm. As many as 300 such dead twigs have been noted 

 on a single tree. It develops most rapidly in a warm spell 

 following freezing weather in the spring, and is more serious 

 upon trees that have been weakened by other causes. 



Twigs killed during the winter show at first a dark pur- 

 plish skin, changing later to leathery, scarlet, or purple, 

 finally drab. Then the skin loosens and wrinkles. At this 

 time black pycnidia appear under the skin. These soon 

 push out a white cap through a transverse slit in the skin, 

 and in wet weather exude very fine, red threads of spores. 

 Gum flow usually accompanies the constriction, marking 

 the juncture of dead and healthy wood. During summer, 

 leaves on infected twigs frequently wilt, owing to the gir- 

 dling of the stem. Upon the trunk large wounds, often 

 regarded as sun scald, are produced, while upon young limbs 

 the wounds appear as enlargements or "knots." Large 

 limbs or even whole trees succumb. 



