DISEASES OF ROSES 87 



can be carried through late summer and autumn prior to the time 

 when firing begins without suffering an epiphytotic of black-spot 

 there is little danger of plants being badly diseased during the 

 winter. No doubt exceptions occur, but the most badly diseased 

 houses noted by the speaker in visits to growers were those where 

 firing was begun late. The natural heat of summer and artificial 

 heat of winter quickly dries off the foliage and must thus be in- 

 strumental in lessening infection. It is very improbable that the 

 ascigerous stage develops in the fallen leaves under glass. 



Out-of-doors, where primary infection is initiated by ascospores 

 formed during the winter in old leaves left lying on the ground, it 

 is obvious that the spring rains are important factors, as moisture 

 is necessary for the discharge of these ascospores from the perithe- 

 cia. Moisture supplied either by rainfall or by dew is probably 

 necessary for the germination of both the ascospores and conidia 

 so that a greater amount of disease may be expected during rainy 

 seasons. It is a matter of common observation that whereas 

 more or less black-spot is present every year, epiphytotics on out- 

 of-doors plants only occur during seasons of heavy rainfall. The 

 precipitation of dew on the foliage during the autumn when cold 

 nights and warm days prevail may account for the increased amount 

 of disease at this time. Lesions on the leaves are more numerous 

 and perhaps larger in rainy, cloudy seasons than in dry seasons. 

 When conditions favorable to black-spot are known a big step will 

 have been taken toward the control of this disease, especially under 

 glass. 



Control. 



Smtitation. Since the fungus lives over winter in fallen leaves, 

 where the ascospores are produced which serve as the source of 

 primary infection in the spring for out-of-doors roses, it follows that 

 these should be carefully collected and burned late in the autumn. 

 It is also advisable to keep the benches free from old leaves affected 

 with the disease, for they bear the summer spores and thus serve 

 as sources of infection. Where a rose garden consists of only a 

 few plants much may be accomplished by picking and burning 

 every leaf as soon as it shows signs of disease. 



