100 DISEASES OF GLASSHOUSE PLANTS 
this country but important in America, is caused by 
Pseudoperonospora cubensis (B and C) Rost. When the 
air is very moist and warm the disease appears on the 
leaves as ill-defined yellow spots, which rapidly run 
together and cause the leaves to turn yellow and die. 
At low temperatures the progress of the disease is con- 
siderably less rapid. The disease attacks the old leaves 
first, but rapidly spreads over the entire plant, which 
produces only a few small, misshapen fruit. 
When the disease appears all overhead damping 
should cease and the foliage should be kept dry. Spraying 
with Bordeaux Mixture is recommended in America as 
being an efficient means of control. 
Tomato Leaf Mould.—The most important leaf disease 
of the tomato in this country is that caused by Clado- 
sporium fulvuum Cke., and variously named “ mildew,” 
“rust ’ and “leaf mould” (Fig. 26). The term “ mil- 
dew,’ which is most commonly used, is unfortunate, as 
most mildews are whitish in appearance, and the most 
suitable name would seem to be “leaf mould,” for 
diseased leaves possess a typical mouldy appearance. 
The first sign of the disease is the appearance of a 
pale olive-buff, downy growth in local spots on the under 
surface of the leaf. A little later the top surface of the 
leaf immediately above the diseased spot turns a pale 
yellow colour which merges gradually into the green 
colour round the spot. As the disease progresses, the 
yellow colour of the top surface turns a deep ochre-yellow, 
finally becoming reddish-brown when the leaf tissue 
is killed. 
The fungus growth on the lower surface of the leaf 
changes colour as the disease progresses. From the 
original pale olive-buff it becomes tawny-olive and finally 
purple when the leaf tissues are dead. The infected 
areas soon begin to die, and the fungus growth covering 
the dead tissues assumes a violet-purple colour. 
As the first infected parts of the leaf are the first to 
