Biology Reference
In-Depth Information
Damping-Off
Damping-off is the destruction of young seed-
lings by soil organisms. There are two types.
Pre-emergence damping-off rots the sprouting
seed before it breaks through the soil; it is recog-
nized by bare spaces in what should be uniform
rows. Such a poor stand may be due to poor
viability of seed, but more often it is due to soil
fungi functioning in cold, wet soils when germi-
nation is slow. Post-emergence damping-off is
the rotting or wilting of seedlings soon after
they emerge from the soil. Succulent stems have
a water-soaked, then necrotic and sunken, zone at
ground level; the little herbaceous plants fall over
on the ground or, in woody seedlings, wilt and
remain upright. Root decay follows. This type of
damping-off is most common in greenhouses or
outdoors in warm humid weather and where seed-
lings are too crowded. Tree seedlings in nursery
rows are subject to this type of damping-off, and
so are perennial flowers started in late summer for
the next year.
Many fungi living saprophytically in the upper
layers of soil can cause damping-off.
Pythium
debaryanum, P. mastophorum
and
Rhizoctonia
solani
are probably most common, but other
species of these two genera and
Aphanomyces,
Botrytis
,
Cylindrocladium, Diplodia, Fusarium,
Macrophomina, Helminthosporium, Sclerotium
rolfsii, Fusarium equiseti
, and
Phytophthora
may be important on occasion. A synergistic
interaction of
Pythium myriotylum, Fusarium
solani
, and
Meloidogyne arenaria
causes
damping-off of peanut which has been reported
in Florida. See under Rots for details.
Also,
Caloscypha fulgens
(anamorph state,
Geniculodendron pyriforme
) causes damping-off
of spruce seed,
Colletotrichum gloeosporioides
of
papaya,
Colletotrichum acutatum
of flowering dog-
wood and
Fusarium moniliforme
var.
intermedia
of pine.
Damping-off is prevented by starting seed in
a sterile medium, such as vermiculite, perlite, or
sphagnummoss, or by treating the soil or the seed
before planting. Commercial operators treat soil
with steam or electricity.
Seed treatment, the coating of seed with
a protectant dust, is crop insurance. In some sea-
sons, good stands can be obtained without it, but
it scarcely pays to take a chance. Seed disinfec-
tion is used to kill organisms of anthracnose
and other specific diseases carried on seed. The
damping-off organisms are in the soil, not on the
seed, and coating the seed with a chemical is
intended to kill or inhibit fungi in the soil imme-
diately surrounding the seed and so provide
temporary protection during germination.
