MISS 8. LONGMAN ON THE DRY-ROT OF POTATOES. 121 
potatoes; after Christmas, and, indeed, when the conditions of storage are 
defective, dry-rot may cause considerable loss. The earliest outward sign of 
the disease is the wrinkling of the skin as the potato shrinks. The wrinkles 
are often first seen round the place of infection, and spread from this point in 
more or less concentric rings until the whole tuber is dry and shrunken. 
Before the potato begins to wrinkle, however, changes take place within. 
The presence of the fungus inside the tuber is shown by a brown staining, 
which appears in the vascular bundles and spreads into the flesh, which then 
begins to dry and shrink, and becomes powdery owing to the fungus leaving 
the starch untouched. 
If the fungus has not killed the whole potato before the spring, the eyes on 
the sound parts may sprout in a normal way; but as soon as the disease 
reaches the shoots their stems shrink at the base, their leaflets blacken, and 
the shoots die. 
The fungus in some cases does not break through the skin of the potato 
until it is covered with wrinkles, but in other cases the fungus appears on. 
the surface of the potato at a comparatively early stage. 
It appears on the surface of the potato either in white patches (often 
covering wounds) or else in the form of small pustules breaking through the 
skin at the place where it first wrinkled. As the wrinkling progresses the 
pustules increase in number; but they never appear on the sound part nor 
within 1 cm. of the margin of the wrinkled part of the potato. Saprophytie 
fungi often follow on the dead part of the potato amongst the oldest pustules, 
but they never mix with the younger ones nor precede them. At Reading, 
by far the commonest saprophyte following thus in the wake of F. Solani was 
a species of Monosporium. 
Phases in the Development of Fusarium. 
The mycelium is septate and much branched, and varies a good deal at: 
different stages of growth in colour, thickness, and number of transverse 
septa. The hyphæ sometimes intertwine, forming rope-like strands. Such 
strands occur both in cultures growing on living potatoes and also on chunks 
of sterilized potato in Buchner-tubes. The first spores to appear, whether 
borne on patches of mycelium or in pustules, are the typical Fusarium spores, 
although on young mycelium they are often poorly developed. The typical 
spores are transparent, colourless, and usually sickle-shaped though oceasion- 
ally spindle-shaped, with 0-7 transverse walls. The spores are very variable 
in size ; the average of 160 measured being 35 y. 
On mature mycelium, spores having three transverse walls are the 
commonest ; but young mycelium often produces smaller spores with one 
LINN. JOURN.—BOTANY, VOL. XXXIX. K 
