92 DISEASES OF GLASSHOUSE PLANTS 
lesions are light yellow water-soaked areas at first, turn- 
ing dark brown later. The epidermis becomes loosened 
and changes from dark brown to a light brown colour. 
Minute black pycnidia are produced on the light brown 
areas. In some cases only one side of the graft is attacked 
and the plants do not appear to have suffered, but when 
taken into rose houses they are instrumental in spreading 
the disease. If possible, resistant varieties should be 
grown. | 
(b) LEAF DISEASES 
Leaf Diseases of the Cucumber.—Under the name 
‘* Spot Disease ” the commercial cucumber grower groups 
all the leaf diseases of that plant. The most important 
of these in this country are those caused by Cercospora 
melonis Cke., and Colletotrichum oligochetum Cav., while 
under conditions of abnormally high humidity Clad- 
sporium cucumerinum KEK and A may be a destructive 
parasite. In spring Alternaria Brassice, var. nigrescens, 
may damage the lowest leaves, and Hrysiphe polygons 
D.C. may be destructive at low temperatures. 
Cercospora Leaf Spot.—During the years 1896 to 1907 
Cercospora melonis appeared in the Lea Valley in this 
country and caused considerable destruction, but the 
advent of the immune variety, Butcher’s Disease Resister 
in 1903, and the application of methods of soil steriliza- 
tion, led to its ultimate disappearance. At the present 
time the Cercospora disease exists only in isolated parts 
of Great Britain. 
Instances are recorded of the entire crop in houses 
one hundred feet long containing ninety-six plants per 
house being completely destroyed in four to six days 
after the disease made its first appearance. 
The first sign of the disease is the appearance on the 
upper surface of the leaves of tiny pale green, water- 
soaked spots. These rapidly increase in size and number 
and coalesce, turning grey at first and afterwards becom- 
