Rose Diseases 325 



chlorotic, long before the affected leaves drop off. 

 As the spots become old, minute specks appear 

 within. These are the fruiting bodies of the causal 

 organism. 



The Organism. The fungus of black spot has two 

 spore stages. The summer stage (fig. 66, b) is 

 known as Actinonema ros<z (Lib.) Fr. The pycnidia 

 are tuberculoid in shape, scattered, black. The co- 

 nidia (fig. 66, c) are oblong, constricted, and are 

 borne oil short conidiophores. The ascus or winter 

 stage (fig. 66, f and g) was discovered by Wolf,* 

 who named it Diplocarpon rosez Wolf. The winter 

 stage matures on dead and fallen leaves which have 

 wintered over. The mature asci are oblong. The 

 ascospores are discharged from an apical pore and 

 pile up in whitish masses in the opened perithecia. 

 The ascospores are not so strongly constricted at the 

 septum as is the case with the conidia or summer 

 spores, both of which are hyaline in color. 



Control. There seems to be a difference in the 

 susceptibility of some varieties to the disease. It 

 seems that the bushy sorts are more susceptible than 

 the climbing varieties. The thin-leaved varieties, 

 too, seem to possess less resistance than those with 

 thick leaves. 



Spraying is often recommended for the control of 

 black spot. The more recent investigations by 

 Massey t show that ammoniacal copper carbonate 

 is not as efficient as Bordeaux mixture for the con- 



•Wolf, F. A^ Science N. S. 33: 151, 191Z. 



t Massey, L. M., The American Rose Mann: 67-71, 1918. 



