Biology Reference
In-Depth Information
pathogens at home. For 200 years potatoes
thrived in Europe as the main source of carbohy-
drate food, but in August 1845, the Gardener's
Chronicle reported: “A fatal malady has broken
out amongst the potato crop. On all sides we hear
of destruction. In Belgium the fields are said to
have been completely desolated. There is hardly
a sound sample in Covent Garden Market.”The
editor went on to describe the decay and to say:
“As to cure for this distemper there is none. One
of our correspondents is today angry with us for
not telling the public how to stop it; but he ought
to consider that Man has no power to arrest the
dispensations of Providence. We are visited by
a great calamity which we must bear.” And in
1946 American gardeners were again blaming the
editor, for lack of information on tomato blight.
In 1845 the weather was continued gloom and
fog, with below-average temperatures. The Gar-
dener's Chronicle editor was sure blight was due
to potatoes being overladen with water. The Rev.
M. J. Berkeley disagreed. He insisted blight was
due to a fungus, with the weather contributing to
spread of a moisture-loving parasite. The argu-
ment raged, for this was long before Pasteur and
his germ theory, and the first time anyone
believed a fungus could be the cause and not the
consequence of plant disease. A French scientist,
Montagne, named the fungus Botrytis infestans ,
but the first really good description of it was
published by Berkeley, and it remained for the
German de Bary, in 1876, actually to prove
the pathogenic nature of the fungus and to
erect the new genus Phytophthora to include it.
Meanwhile the disease was making history.
The loss of the potato crop in 1845 and 1846
killed off a million people and caused another
million and a half to emigrate; the first Govern-
ment Relief program was instigated; and the
English Corn Laws were repealed with a change
to a policy of free trade and unbounded expansion
of commerce.
Fig. 4 Late Blight on Potato
weather, first on lower leaves. As a spot enlarges
the center is shriveled, dry, dark brown to black,
and a downy, whitish growth appears on the
underside of leaves. Similar lesions are formed
on stems and petioles, and there is a characteristic
strong odor as tops are blighted. On tubers, first
symptoms are small brown to purple discolor-
ations of skin on upper side, changing to
depressed pits when tubers are removed from
soil and put in storage (see Fig. 4 ). On cutting
through the potato, a reddish brown dry rot is
seen.
Life History The primary cycle starts with
infected tubers, which have harbored mycelium
in the dry rot patches over winter. If infected seed
pieces are planted, the fungus grows systemically
into the shoots and finally fruits by sending spo-
rangiophores out through the stomata on lower
leaf surfaces (see Fig. 5 ). These swell at the tips
into ovoid bodies, sporangia, then branch and
produce successively more sporangia. The latter
may function as conidia, putting out a germ tube,
but more often are differentiated into a number of
swarmspores (zoospores), which have cilia
enabling them to swim about after they are
splashed by rain to another leaf. Eventually they
stop swimming and send a germ tube in through
the leaf cuticle or enter through a stoma. Initial
infection in the field also comes from conidia
blown over from sprouts produced on infected
tubers in cull piles. Blighting follows rapidly,
with first symptoms 5 days or less from the time
of infection and with the fungus fruiting again in
a whitish layer on the underside of leaves.
Late Blight of Potato
Symptoms After blossoming, large, dark green,
water-soaked spots appear on leaves in wet
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