85 
attacked is also produced. In the latter case, Bacellus 
solanacearum is the cause. The Septoria disease of the 
leaves produced by Septoria lycopersici is the only other 
malady likely to be confused with the leaf-rust, but in 
this case the spots are always small and concentric, and 
the spores are produced in minute black bodies scattered 
over the patches. 
The Leaf-Blotch of the Cucumber caused by Cercospora 
melonis is another destructive disease of leaves. It was 
first described in this country by Dr. M. C. Cooke in 1896, 
and since then it has become so wide-spread that many 
horticulturists have been compelled to cease growing 
cucumbers. Once the fungus appears in greenhouses it is 
extremely difficult to eradicate. The leaves are most often 
attacked, but the fungus frequently spreads to the fruit. 
An outbreak of the disease is usually first indicated by 
the appearance of pale, scattered spots on the leaves. 
These spots gradually increase in size, become brown, 
and the leaves are so rapidly killed that death of the 
plant may soon result. 
Microscopic examination shows that, in the region of 
the spots, the tissues of the leaf are occupied by filaments 
of the fungus, that the chlorophyll bodies are pale 
in colour and many of the cells of the leaf are shrivelling 
and dying. From the fungus within the leaf stiff branched 
filaments grow out and stand more or less erect from the 
surface. These aerial threads are dark in colour, and 
bear numbers of large conidia which fall free as they 
mature. Each somewhat spindle-shaped conidium is 
divided into about seven or eight cells, and may germi- 
nate in a warm, moist atmosphere by sending out fila- 
mentous germ-tubes from any of the cells. The germ- 
tubes may then produce new infections by growing through 
the stomata into the healthy tissues of the leaf. This 
spreading by means of conidia that germinate imme- 
diately, takes place very rapidly under favourable con- 
ditions. In addition to producing large conidia on the 
leaves the fungus is said to grow as a saprophyte on decay- 
ing leaves and damp soil producing myriads of smaller 
spores which also rapidly spread the disease. If the con- 
ditions are unfavourable to the germination of the spores, 
and especially at the end of the season, the large conidia 
persist alive as resting spores. Filaments of the fungus 
are also able to pass into a resting condition in the soil, 
