74 
the disease is present at all, there is a risk of a proportion 
of the tubers becoming diseased; further, the attack of 
the fungus on the leaves invariably results in a diminished 
crop. The first outbreak of the disease in the season is 
usually dependent upon the weather conditions. In this 
country a few days of close, moist weather about the 
middle of July are often followed by an outbreak of the 
disease, and, if such weather persists, an epidemic fre- 
quently results. 
The disease is easily detected by the characteristic spots 
on the leaves. These often appear near the margin or tip 
of the leaf and they usually present a dull, water-soaked 
appearance. As the disease progresses the infected spots 
increase in size, become darker in colour, and may rapidly 
involve the whole leaf-blade, extending from the latter 
to the petiole and even to the stem. Ultimately the Icaves 
hang limp and the whole plant becomes moist and 
blackened. Such diseased plants emit a peculiar, offensive 
odour which is very characteristic of the disease. If the 
attack is a severe one, it is generally found on lifting the 
tubers that they also are diseased. If in the earliest stages 
of the disease, the dark spots on the leaves are closely 
examined on the under surface, a delicate, white, mildew- 
like growth will be evident round the margin of the dis- 
coloured spot. This growth follows the advancing 
margin as the diseased area increases in size. Microscopic 
examination shows the mildew appearance to be produced 
by filaments of the. fungus Phytophthora, which project 
from the lower surface of the leaf. The growth as a rule 
has a somewhat powdery appearance produced by large 
numbers of spore-like bodies, which readily fall free from 
the filaments bearing them. Thin sections through a 
diseased spot show that at this stage the threads of fungus 
run between and within the cells of the leaf. Many of 
these cells are shrivelled and dead, and the contents 
having turned brown, they contrast strangely with the 
taut living cells lined with colourless protoplasm and 
green granules of chlorophyll. 
In many places branches from the filaments of the 
fungus pass out through the air pores or stomata, which 
are especially abundant on the lower surface of the leaf. 
These aerial filaments become branched and bear small 
lemon-shaped bodies, called conidia, on the tips of the 
branches. The production of such conidia enables the 
