136 PLANT DISEASES 



spermatia, which escape through the mouth of the spermo- 

 gonium as a mucilaginous mass, the component spermatia 

 becoming hberated by dew or rain. 



Spermatia continue to be produced until the autumn. 



In the autumn perithecia are also developed in the 

 stroma, their complete condition being attained during the 

 winter after the leaves have fallen. The spores produced 

 in the perithecia inoculate the young leaves of the host the 

 following spring. 



Preventive Means. — The injury occasioned by this 

 parasite is in exact proportion to the amount of leaf surface 

 it occupies, the mycelium not extending beyond the margin 

 of the stroma. These patches, however, are sometimes of 

 large size, covering the greater portion of the surface of 

 the leaf, and, as frequently nearly all the leaves on a tree 

 are attacked, ripening of fruit and maturing of the wood 

 become impossible. The most effective means of pre- 

 venting a repetition of the disease is by collecting and 

 burning the diseased fallen leaves. 



Frank, Ber. der Deutsch. Bot. GeselL, vol. i. p. 58 (1883). 

 De Bary, Fungi, Mycetozoa, and Bacteria (Engl, ed.), 

 p. 215. 



BLACK KNOT 



(Plowrightia morbosa, Sacc.) 



Undoubtedly the very worst fungous disease attacking 

 cultivated plum and cherry trees known in the United 

 States, to which country it is at present confined. Accord- 

 ing to Dr. Halsted it also attacks the following wild trees 

 in that country : — Prunus chicasa, Mich., Prunus mari- 



