11] Potato Disease and Allied Diseases 23 
the tuber. Later on however the browning may spread 
throughout the tuber. These patches may show them- 
selves soon after the leaves have become diseased, 
especially on tubers near the surface. This stage of 
the fungus has given it the name of “dry rot.” 
Tubers which are apparently healthy have been 
sown, and the plants from them grown under conditions 
rendering external infection impossible, and still these 
plants have become diseased. Careful examination of 
a number of these apparently healthy tubers showed 
them to contain portions of the mycelium of the potato 
disease fungus. The disease then starts from the 
tuber. When the tuber sprouts and produces shoots 
the fungus mycelium also grows and makes its way 
into the stems and leaves. We have seen how the 
disease reaches the leaves and spreads from one plant 
to another, but we have still to account for the in- 
fection of the new tubers which carry the fungus on 
to the following season. 
We should expect to find that the fungus mycelium 
grows down the stem and into the new tubers. The 
evidence on this point is not very clear. It is supposed 
that the new tubers are capable of being infected from 
the conidia produced on the leaves. These conidia fall 
on the damp earth and there produce zoospores which 
are washed down to the tubers. The zoospores 
germinate and penetrate the tuber, the mycelium 
formed causing the brown patches so characteristic of 
the disease. In favour of this hypothesis are the facts 
(1) that the diseased patches appear as frequently at 
the end of the tuber away from the stem as at the end 
which is attached to the stem; (2) that the upper 
surface is often more diseased than the lower; (3) that 
