DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 541 



the hypertrophies are black in color. The knot begins as a slight 

 swelling of the branch, and as the swelling increases in size the bark 

 is cracked (Fig. 194). 



The mycelium of the fungus occupies the cambium and bast areas 

 of the stem, extending throughout the cortex also. The knot consists 

 of dense areas of the fungus and tissue elements of the host. Bast 

 fibers, parenchyma cells and even vessels may be found in the gall 

 tissue. In the spring, small greenish areas may be noticed on the 

 surface of the knot, and later, the hyphae break through the bark in 

 all directions and form a pseudoparenchymatous layer. This stomatic 

 layer gives rise to the conidiospores, which are fiexuous and septate. 

 The conidiophores are 40 to 6o/i by 4 to 5)U and the conidiospores 

 abstricted off are light brown in color. Conidiospores are formed 

 from Spring to late midsummer. They are simple and light brown 

 in color. The fungous stromata is covered with papillae which locate 

 the opening of the perithecia which include the asci with eight asco- 

 spores, that ripen during midwinter, or later. Each ascus is 120/i in 

 length and the ascospores measure 16 to 20 fx by 8 to lo/j.. Between 

 the asci are paraphyses. 



Since the conidial stage is produced during late Spring and early 

 Summer pruning out the developing knots is found an efficient remedy 

 in most cases against black knot. 



Plum Pockets {Exoascus Pruni, Fckl.).^ — The plum pocket fungus is 

 widely distributed over the United States and Europe and its etiology 

 of the disease it produces is somewhat similar to that of the peach leaf 

 curl. The mycelium lives in the flower buds and causes remarkable 

 changes in the ovaries, as they develop into fruits. The hyphae are 

 found in the mesocarp, the cells of which are stimulated to form a 

 spongy growth, so that the plum fruit becomes swollen and somewhat 

 distorted. As a result of the fungus attack, the endocarp which nor- 

 mally would develop a putamen, or stone, fails to do so, and no stone, 

 or seed, is formed, but in their place a cavity appears which gives the 

 common name to the disease. The mycelium is probably perennial in 

 the twigs of the plum tree and is, therefore, in a position to grow out 

 into the young ovaries of the next succeeding crop of flowers. The 

 ascogenous cells develop beneath the cuticle of the well-formed fruits 

 and finally rupture the latter, appearing as a velvety layer. The asci 

 are 30 to 6o/i by 7 to 12^1, although Robinson notes a certain dimor- 



