84 MASSACHUSETTS HORTICULTURAL SOCIETY 



leaves. The more or less circular spots may reach a diameter of a 

 centimeter or more, are black in color and are characterized by 

 radiate-fibrillose margins. They usually appear late in the spring 

 or early in the summer and occur only on the upper surface of the 

 leaf. The spots are small at first but increase in size as the disease 

 progresses. Often a number of them coalesce and in severe attacks 

 the entire leaf may be covered with large dark patches. In the 

 latter part of the season the spots frequently grow light in color and 

 dry in the center showing this part of the leaf to be entirely dead. 



Very commonly the leaf tissue adjacent to the black spots 

 becomes chlorotic before the leaves fall from the plant, and not 

 uncommonly all of the uninvaded tissue becomes yellow before 

 defoliation occurs. The leaflets may turn yellow in spots, while 

 sometimes the yellow area is limited to a band outside the black 

 spot. Commonly, and especially during the autumn the yellow 

 color appears at the apex of the leaflets whence it spreads downward 

 and is succeeded by brown. A leaf with a green base and brown 

 tip with a yellow band between is very characteristic of this disease. 



Premature defoliation is one of the most pronounced charac- 

 teristics of this disease. Affected leaves may fall before they turn 

 yellow, the slightest jar or breeze often causing them to drop in 

 great numbers. Diseased plants usually have a partially defoliated 

 appearance. 



The size and shape of the black spots, the rapidity, and the extent 

 of defoliation of plants, seem to ^•ary with the variety. No 

 reports of observations on these points are to be found in literature. 



Etiology. 



Black-spot of the rose is caused by a fungous parasite, Diplo- 

 carpon rosae Wolf, long known under the name of Actinonema 

 rosae (Lib.) Fries. 



Life History of the Parasite. 



Diplocarpon rosae has two phases in its life cycle — an actively 

 parasitic phase developed during the summer and a saprophytic 



