THE FUNGI WHICH CAUSE PLANT DISEASE 



219 



Fig. 156.— p. morbosa; host 

 showing opportunity for 

 lodgement for spores in a 

 crotch. After Lodeman. 



3 dm. long; perithecia scattered, often entirely suppressed; asci 

 about 120 n long; spores variously arranged in the ascus, 16-20 x 

 8-10 n, ovate, the cells usually unequal; paraphyses filiform. 



Conidia (= Cladosporium sp.) pro- 

 duced upon greenish areas on the young 

 stromata; conidiophores erect, flexuose, 

 septate, simple, 40-60 x 4-5 n; conidia 

 borne singly at the apex of the conidio- 

 phore, obovate, unicellular, light brown, 

 about 6-8 X 2-5 n. 



Hosts: Cultivated sour cherry and 

 plum, wild red and yellow plum, 

 Chickasaw plum, choke cherry, wild 

 red cherry and wild black cherry. 

 Found only in America. 



The mycelium invades the cambium 

 of twigs and from it grows outward into the bark region causing 

 the bark elements to overgrow and the twig to swell slightly dur- 

 ing the first summer. With the renewed growth of the following 

 spring the swelling proceeds rapidly. During May to June the 

 mycelium ruptures the bark which is soon lost and a dense fun- 

 gous pseudoparenchyma is formed. From this the conidiophores 

 appear, forming a velvety growth of olivaceous color. At this 

 period the knot consists largely of a fimgous stroma with an ad- 

 mixture of bark elements and even some wood cells. 



Later in the season conidiophores cease to form and the knot 

 turns to a black hard stroma. Perithecia now become easily 

 visible in this black stroma and in January or later the asci mature. 

 Farlow has described "stylospores" (a form named Hendersonula 

 morbosa by Saccardo the connection of which to P. morbosa is 

 in some doubt) and spermogonia and pycnidia. Humphrey ^^ 

 from ascospores, in artificial media, raised a pycnidial form which 

 seemed to be distinct from any of these. That the fungus is 

 the actual cause of the black knot was first demonstrated by 

 Farlow ^** in 1876, though the fungus was described as early as 

 1821 by Schweintiz.i^ 



Lodeman *^^ considered that infection is favored by cracks 

 existing at crotches of the tree. Fig. 156. 



