DISEASES OF CULTIVATED PLANTS 473 



forming an abundance of gum when injured in any manner. On 

 this account, if any active sap movement from the roots is started 

 early in the season by warm weather or an abundance of moisture 

 and this activity be checked again by less stimulating weather con- 

 ditions, trouble is very apt to follow. The sap becomes stagnant in 

 the tissues, full of sugars and other easily fermentable substances, 

 gum begins to form, sun burn may also occur and very often se- 

 vere injury take place in the tissues of the sap wood and cambium 

 layer. (Cal. E. 8. B. 218.) 



Other Apricot Diseases. Yellows (See page 482) ; Blight 

 (See page 464). 



PLUM DISEASES. 



Black Knot. The black knot is a rough wart-like outgrowth 

 from the bark of twigs and branches in severe cases extending along 

 the trunk for several feet. The first symptom is the swelling of the 

 tissue just beneath the bark. This enlargement increases during the 

 fall or growing season until the bark is ruptured. This exposed por- 

 tion of the twig is soon covered with an olive green velvety coating 

 composed of the reproductive hyphae of the fungus. A microscopi- 

 cal examination of the diseased portion at this time would show nu- 

 merous erect hyphae bearing spores the so-called summer spores. 

 These are readily carried about by the wind and other agencies and 

 serve to spread the disease during the growing season. Later in the 

 season the production of summer spores ceases and the velvety cover- 

 ing gradually disappears. The surface of the wart gradually be- 

 comes hardened and altered in color to a dark brown and finally to 

 a dead black. Pimples may be seen late in the fall covering the 

 wart. In these are developed the winter spores called ascospores. 

 These ascospores only develop during the winter and are capable of 

 germination in February and March. (Ala. E. S. B. 132.) 



The diseased twigs should be removed after leaf-fall or before 

 the winter spores are formed. Twigs that show in the spring or early 

 summer the beginning of a "knot should be removed and burned 

 to prevent the formation of the summer spores. 



Bacterial Black Spot. This trouble, apparently, occurs only on 

 the Japanese plums, but may attack any of the varieties of these, 

 according to growers. The green plums show conspicuous black- 

 purple spots which are often slightly sunken. These spots vary in 

 size up to half an inch in diameter. There are not many on a single 

 plum and these are usually isolated. The diseased tissue does not 

 extend much below the skin, so the injury is quite superficial. Usu- 

 ally only a few plums scattered over the tree show the trouble, but 

 occurring on the green fruit one is apt to fear that later it will de- 

 velop into a very serious pest. This does not happen, as the trouble 

 becomes less conspicuous and vigorous on the ripening fruit and fails 

 to spread further. Specimens of the nearly grown but green fruit 

 when placed in a moist chamber in the laboratory did not show any 

 further progress of the disease though kept for some time under ob- 

 servation, Cultures made from diseased tissue from the interior uni- 



