Biology Reference
In-Depth Information
ageratum, amaryllis, azalea, caladium, calendula,
campanula, canna, carnation, cosmos, China
aster, chrysanthemum, dahlia, delphinium,
daphne, duranta, gladiolus, hollyhock, hydran-
gea, iris, jasmine, lemon verbena, lily, lupine,
marigold, morning-glory, myrtle, narcissus,
orchids, phlox, pittosporum, rose, rose-mallow,
rudbeckia, scabiosa, sedum, sweet pea, star-of-
bethlehem, tulip, violet, and zinnia.
The first sign of blight is the formation of
white wefts of mycelium at the base of the stem,
spreading up in somewhat fan-shaped fashion
and sometimes spreading out over the ground in
wet weather. The sclerotia formed in the wefts are
first white, later reddish tan or light brown. They
may be numerous enough to form a crust over the
soil for several inches around a stem, or they may
be somewhat sparse and scattered.
In the white stage, droplets of liquid often
form on the sclerotia, and the oxalic acid in this
liquid is assumed to kill plant cells in advance of
the fungus hyphae. This means that the pathogen
never has to penetrate living tissue and explains
why so many different kinds of plants succumb so
readily to southern blight. Fruits touching the
ground, as well as vegetables with fleshy roots,
like carrots and beets, or plants with bulbs or
rhizomes, like onions, narcissus, and iris, seem
particularly subject to this disease. Low orna-
mentals such as ajuga blight quickly, the whole
plant turning black; tall plants like delphinium rot
at the crown and then die back or topple over;
bulbs have a cheesy interior, with sclerotia
forming on or between the scales.
Control
Remove diseased plants as soon as they
are noticed. Take out surrounding soil, for
6 inches beyond the diseased area, wrapping it
carefully so that none of the sclerotia drop back.
Increasing the organic content of the soil reduces
southern blight, as does the addition of nitroge-
nous fertilizers, such as ammonium nitrate.
Treating narcissus bulbs in hot water for 3 h, as
for nematodes, kills the fungus in all except the
very largest bulbs.
Pellicularia koleroga (
Corticium stevensii
).
Thread Blight
, a southern disease, from North
Carolina to Texas, important on fig and tung,
sometimes defoliating pittosporum, crape myrtle,
roses, and other ornamentals, and some fruits.
The disease is recorded on apple, azalea, banana
shrub, blackberry, boxwood, camphor, cherry
laurel, chinaberry, columbine, crabapple, crape
myrtle, casuarina, currant, dewberry, dogwood,
elderberry, elm, erythrina, euonymus, fig,
flowering almond, flowering quince, goldenrod,
gooseberry, guava, honeysuckle, hibiscus, morn-
ing glory, pear, pecan, pepper vine, persimmon,
pittosporum, plum, pomegranate, quince, rose,
satsuma orange, soapberry, silver maple, sweet
potato,
tievine (
Jacquemontia
),
tung, Virginia
creeper, and viburnum.
The fungus winters as sclerotia on twigs and
leaf petioles, and in May and June produces
threadlike mycelium that grows over lower sur-
face of leaves, killing them and causing prema-
ture defoliation, although often dead leaves hang
on the tree in groups, matted together by thread-
like spider webs. Fruiting patches on leaves are
first white, then buff. The fungus flourishes in
moist weather, temperatures 75
to 90
F.
Control
On figs, one or two applications of tri-
basic copper sulfate, or bordeaux mixture, are
satisfactory until the fruit ripens in July. Pruning
out infected branches may be sufficient on tung
and pecan, but at least one spray of bordeaux
mixture may be required.
Thanatephonus cucumeris (formerly
Pellicularia filamentosa
), teleomorph state of
Rhizoctonia solani
. This is a variable fungus
with some strains or forms causing leaf
blights but best known as cause of Rhizoctonia
rot of potatoes and damping-off of many plants.
▶
Rots
.
Thanatephonus cucumeris (formerly
Pellicularia filamentosa
f. sp.
micro-sclerotia
(
Corticium micro sclerotia
)).
Web Blight
of
snap bean, lima bean, also reported on fig, elder,
hibiscus, hollyhock, tung oil, and phoenix tree,
from Florida to Texas. Many small brown scle-
rotia and abundant weblike mycelium are found
on bean stems, pods, and foliage. Infection starts
with small circular spots that appear water-
soaked or scalded. They enlarge to an inch or
more, become tan with a darker border, are some-
times zonate. The whitish mycelium grows rap-
idly over the leaf blade, killing it, and spreads
