110 DISEASES OF GLASSHOUSE PLANTS 
Downy Mildew of the Rose—This disease, while 
common on glasshouse roses, is somewhat difficult to 
detect. Frequently young plants appear to lack vigour 
for no obvious reason, but a careful examination of the 
leaves reveals the presence of minute fungal filaments. 
The causal organism, Peronospora sparsa Berk., is capable 
Fiq. 31. ape J mildew of the rose: (a) Diseased leaves, (b) summer spores, 
(c) a perithecium, (d) ascus containing eight ascospores. 
of attacking all leaves and young shoots. The first 
symptom of the disease is the sudden flagging of young, 
vigorous leaves, which readily fall off the stem if it is 
shaken gently. The shoot itself droops and dies. 
Diseased leaves and shoots possess reddish-purple patches 
bearing a fungus growth. Gradually the disease develops 
until all the green tissues are destroyed. 
