Biology Reference
In-Depth Information
Spilocaea pomi formerly
Fusicladium
dendriticum
). Conidial stage of the apple-scab
fungus.
dissolving cuticle and killing cells. When the
food supply diminishes, the plasmodium again
breaks up into spore balls.
Control
Avoid low soggy ground; if such soil
must be used, acidify it with sulfur as for common
scab.
Venturia inaequalis
.
Spilocaea pyracanthae (formerly
Fusicladium
eriobotryae
).
Loquat Scab
, widespread on
leaves, stems, fruit of loquat. This is similar to
pear and apple scab. Dark velvety spots cause
more or less deformation of fruit, but the disease
is seldom important.
Spilocaea pyracanthae (formerly
Fusicladium
pyracantha
).
Pyracantha Scab
,widespreadon
leaves and fruit. The unsightly black scabs
spoil the appearance of bright berries. The
fungus winters in the mycelial state in
attached leaves. Frequent spraying with bor-
deaux mixture controls scab but causes some
defoliation.
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Streptomyces
Rots
.
Streptomyces acidiscabies
Acid Scab
on beet,
carrot, radish, parsley and turnip.
Streptomyces scabies (Syn.
Actinomyces sca-
bies
).
Common Scab
of potatoes,
Beet Scab
,
Corky Scab
,
Actinomycosis
, general on potatoes,
widespread on beets, also reported on carrot,
parsnip, radish, rutabaga, and turnip. This disease
may have been in America as long as potatoes
have been grown, but the causal organism was
not described until 1890. Scabby potatoes, by
lowering the market grade, mean an annual loss
of several million dollars. Chief symptoms are
the tuber lesions, starting as minute brown specks
and progressing to scabs that are warty or with
corky ridges, or are pitted and depressed with the
skin cracking open. Such potatoes can be eaten,
but have poor customer appeal and are wasteful
because of the deep peeling required. On beets,
the scabs are similar but more bulging. The path-
ogen can be found even in virgin soil. It invades
young tubers and may sometimes be seen as
a grayish coating on freshly dug potatoes. It is
most destructive in soils with pH 5.7 and over,
with its activity sharply limited in soils slightly
more acid. Although its optimum temperature is
72
to 86
F, the fungus can withstand great
extremes of temperature and moisture and can
pass through the digestive tract of animals,
returning to the field in manure.
Control
Seed tubers have been treated with for-
malin, but the organism is so prevalent in potato
soils that such treatment may have little result.
Soils already slightly acid may be further acidi-
fied with sulfur. Enough sulfur to acidify highly
alkaline soil would be too expensive and too
injurious to potatoes. Alkaline materials - time,
wood ashes, and manure - should not be
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Spongospora
Plasmodiophoromycetes,
Plasmodiophorales
Spores in a hollow sphere with several openings;
zoosporangia formed; zoospores anteriorly bi-
flagellate; sexual fusion of myxamoebae.
Spongospora subterranea
Powdery Scab
of
potatoes,
Canker
,
Spongy Scab
. Indigenous to
South America and introduced into Europe
more than a century ago, potato scab was not
noticed in North America before 1913, in
Maine. Ordinarily not important, it causes eco-
nomic loss in some seasons. Slightly raised pim-
ples appear on tubers when they are less than an
inch in diameter; they are varying shades of
brown on the surface, faintly purple underneath.
The epidermis, not growing as fast as the pimple,
breaks and curls back over the pustule, which, by
this time, is a brown powdery mass of spore balls
and decomposed plant tissue. The lesions are
often “corked off,” but under favorable condi-
tions large, depressed cankers may form. The
fungus winters on stored tubers or in soil,
remaining viable for many years. In the presence
of a potato tuber and enough moisture, each
spore in the ball germinates by swarmspores,
which stay grouped together in a plasmodium,
