THE PERFECT STAGE OF ACTINONEMA ROSAE 1 



Frederick A. Wolf 



I 



(with plate xiii) 



Perhaps no plant disease has been more widely observed or 

 is more generally known, both in Europe and the United States, 

 than the black spot of roses caused by the parasitic fungus Acti- 

 nonema rosae (Lib.) Fries. The spots, which are more or less 

 circular in outline, are characterized by a very irregular, fibrillose 

 border. This fibrillose character is due to the radiating strands 

 of mycelium which occur beneath the cuticle. Appearing among 

 the mycelial strands are numerous dark specks, the fruit bodies 

 of the fungus. The spots may be isolated and confluent, or so 

 numerous as to involve the entire upper surface of the leaf. Plants 

 which are attacked become defoliated early in the season, and the 

 leaf buds, which should remain dormant till the next year, often 

 open late in the s.eason. As a result, the plant is weakened so that 

 it blossoms poorly or not at all in the following season. 



Since very little is known concerning the life history of the 

 fungus and the development of the Actinonema stage, an attempt 

 has been made by cultures on artificial media and on the host to 

 furnish a more satisfactory knowledge of this interesting organism. 

 Before giving an account of this study it may be well to state 

 briefly the characters of the vegetative and fruiting structures of 

 the rose Actinonema. 



The vegetative body of the fungus consists of two parts, the 

 subcuticular mycelium and the internal mycelium. The sub- 

 cuticular mycelium is immediately underneath the cuticle, being 

 above the outer wall of the epidermal cells. It consists of branched, 

 radiating strands of mycelium which anastomose, making a net- 

 work. Each strand consists of several filaments united together, 

 either side by side or sometimes superimposed. At the right oi 

 the acervulus in fig. i is shown a cross-section of one of these 

 strands. The internal mycelium penetrates the mesophyll of the 



1 Contribution from the Department of Botany, Cornell University. No. H3* 



Botanical 



[218 



