DISEASES OF ROSES 93 



might be considered as evidence favoring the claim that the 

 mycelium is perennial. In countries of warm climates the fungus 

 is doubtless carried throughout the year in the asexual stage as is 

 true on roses grown vmder glass where the temperature does not 

 fall sufficiently low to kill the spores and mycelium. 



There are, then, two and possibly three sources of primary 

 infection in the spring. First and of primary importance is the 

 production of ascospores which live over winter in perithecia on 

 plants grown out-of-doors. These spores are distributed by the 

 wind, rain, man, and other agents and, under proper conditions of 

 temperature, moisture and position, germinate and produce in- 

 fection. The second source of inoculum for roses in the open is 

 the distribution of asexual spores formed throughout the year on 

 roses under glass. These spores are very light and might readily 

 be carried by the wind for great distances. Growers frequently 

 buy pot-grown roses in the spring to plant in their gardens. Some 

 of these plants may be affected and often the fungus spreads 

 quickly to other bushes. The third possible source of inoculum is 

 the production of conidia by mycelium which has overwintered on 

 plants in the open. Some doubt exists, as stated above, as to 

 whether or not the mycelium is perennial in sections having rela- 

 tively cold winters. 



Spores, then, either ascospores or summer spores, are carried to 

 rose plants in the spring where under proper conditions they germi- 

 nate. The germ-tube coming from the spore quickly elongates, 

 branches and soon establishes a food relation with the host by 

 sending haustoria into the epidermal cells. Very soon thousands 

 of new spores are produced which when mature are carried by the 

 slightest air currents to other parts of the plant and to other bushes. 



Many florists believe that rose mildew is caused by drafts, having 

 noticed the initial appearance of the disease in the areas in the 

 greenhouse near doors or broken panes of glass. Needless to say 

 mildew cannot develop without the presence of the fungus, the 

 drafts serving as bearers of spores and possibly bringing about 

 favorable conditions for infection, either by its effect on the host 

 or on the fungus, or on both. 



