usually no external evidence of the presence of the fungus, but 
if the tissues are examined microscopically an abundance of 
mycelium will be met with. When the haulm and leaves are 
dead or nearly so, the fruit of the fungus appears in abundance ; 
on the leaves it forms minutely velvety blackish-olive patches 
of variable form and size, whereas on the haulm it appears 
under the form of long thin streaks. The difference in superficial 
appearance between the broadly effused patches on the foliage, 
and the long narrow streaks on the stem, has led to the 
supposition that two distinct parasites are present. The sup- 
position is a mistaken one, only one parasite is present ; the 
difference between the appearance on the leaves and stem 
respectively is a matter of mechanics and not of species. On 
the stem the fruit of the fungus can only break through to tin- 
surface between the parallel rows of vascular bundles, hence 
the long, narrow streaks ; whereas the arrangement of the veins 
in the leaf admits of the fruit appearing in extended patches. 
In an acute attack the haulm is stunted from the first, and 
rarely attains to a length of six inches, the leaves also remain 
quite small and are much curled. In such cases no new tubers 
are formed. Sometimes the "sprouts" are killed outright in the 
youngest stage before they appear above-ground. In such an 
instance if the "set" is examined it will be found to be quite 
firm and apparently practically unchanged. 
It has already been explained elsewhere that " leaf-curl " is 
perpetuated from year to year by the presence of mycelium in 
the tuber {Kew Bulletin, 1906, p. 110; Journ. Board A,/rir.. 
September, 1906). When a tuber infested with mycelium is 
planted, the mycelium grows along with the stem and enters the 
leaves ; it also passes into the young tubers. When a tuber is 
only slightly infested with mycelium, it follows the course 
indicated, and its further behaviour as to remaining in a 
subordinate condition in the tissues of the potato plant, and 
doing little harm beyond infecting the new tubers ; or whether 
it assumes a rampant form and destroys the above-ground 
portion of the potato plant, depends almost entirely on weather 
conditions. 
On the other hand when a tuber is badly infested with 
mycelium it is often completely dead before it is planted. If 
not quite killed it may produce the dwarfed growth already 
described. As the fungus mycelium does nor destroy the starch. 
but only the proteid substance, it is praetk-;dly impossible by any 
superficial method to determine whether a tuber i 
This can only be determined by a microscopic — 
Owing to the fact that the conidia of the fungus are only 
produced on dead or dying plants, and most! 
epidemic is not increased by health}- plants bavin- their foliasre 
/est 
„.,. Quite young shoots have been infected, late in the 
season, with conidia obtained from dead plai 
but the infection remained local and did not extend backwards 
down the haulm, hence the voung tubers could not be infected 
with mycelium. 
