22 Potato Disease and Allied Diseases [cH. 
Every potato grower knows that the disease is very 
bad in some years which become known as potato 
disease years and much less harmful in others. The 
disease is bad when dull, muggy weather prevails in 
July and August. Farmers formerly held the idea 
that the dull weather caused it; indirectly it does, for 
damp conditions are favourable to the growth and 
spread of the fungus which produces the disease. 
Potatoes grown in a shady place with a moist atmo- 
sphere readily succumb to the disease, whereas others 
in a dry sunny place may show no signs of it. 
We are now in a position to understand why the 
disease should be most marked in dull weather. The 
conidia and zoospores by which it is distributed can only 
germinate when a certain amount of water is present 
and at certain temperatures. The optimum tempera- 
ture has been found to be about 70°-74° F., and this is 
about the temperature of dull weather in July. 
If the weather is bright and hot the brown patches 
formed on the leaves dry up and curl, and the disease 
may not spread at all or only very slowly. If, however, 
the weather is dull the brown patches rapidly spread 
over the whole leaf. The formation of conidiophores 
on the under surface accompanies the browning of the 
tissues in its spread over the leaves. It is at this stage 
that the casual observer usually notices the disease. 
Brown patches also appear on the stem and in bad 
attacks all the leaves are killed. A diseased field has 
a very bare appearance, similar to that of a healthy 
field when the leaves have died normally. 
The tubers of diseased plants are marked in a 
characteristic manner. They have sunken brownish 
patches which at first do not extend very deep into 
