DISEASES DUE TO FUNGI 99 
E and A., described later (page 117) as the cause of a disease 
of cucumber fruits, has been found to cause a spotting of 
the leaves under conditions of exceptionally high humidi- 
ties constantly maintained for long periods. Innumerable, 
small, light-brown, irregular spots are produced on the 
leaves, which are ultimately destroyed. 
Usually the disease does not attack the leaves, but on 
the rare occasions when it is present the attack may be 
checked rapidly by reducing the amount of moisture in 
the atmosphere. 
Alternaria Leaf Spot.—This disease, caused by Alter- 
naria Brassice, var. nigrescens, not infrequently appears 
during the early part of the season. The lowest leaves 
are attacked, and dead, reddish-brown spots, circular in 
shape, are produced on the leaves. Such spots fre- 
quently possess faint concentric growth rings, and are 
not unlike the spots produced by Colletotrichum. 
Powdery Mildew.—This disease, caused by Erysiphe 
polygont D.C., is not infrequently found in cucumber 
houses, but it rarely reaches a serious stage. It appears 
as small, greyish, water-soaked spots, on the upper surface 
of which fungal growths soon appear, giving them a 
white, mealy appearance (Fig. 25). Under suitable con- 
ditions the isolated spots rapidly extend until the whole 
surface of the plant may be covered with a white 
powdery, fungal growth. Excessive watering, insufficient 
ventilation, and irregular temperatures, combined with 
insufficient light, provide favourable conditions for the 
disease. In this country the disease appears chiefly in 
winter and may be controlled by providing suitable 
ventilation so as to dry out the atmosphere of the house, 
reducing the water supply, and improving the conditions 
of lighting. Dusting with sulphur powders is of con- 
siderable assistance in controlling the disease. 
Downy Mildew.—This disease, at present unknown in 
