Alan
McNab
Professor of Plant Pathology |
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| ONION
DISEASES |
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Purple
Blotch, caused by the fungu Alternaria porri, may develop where
white specks associated with Botrytis blight are present on leaves.
Purple blotch begins as water-soaked spots which rapidly turn brown.
As spots expand they become zonate, purplish with a darker margin
surrounded by a yellow zone, and may attain a size of 1 by 2 inches.
In moist weather spots become covered with a brown dusty-appearing
mold. Affected leaves and seed stalks may break over where large spots
are present. The fungus overwinters in residue form diseased plants.
Long periods of rain or heavy dew promote disease |
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Downy
Mildew, caused by the fungus Peronospora destructor, infrequently
appearson young plants as white specks resembling Botrytis blight.
Symptoms usually appear after leaves are 6 inches long and on oldest
leaves first. On these leaves, a white to purplish mold develops on
elongated leaf spots during cool moist weather. The spots become pale
green, then change to white or tan. As leaves are affected down to
the leaf sheath, they drop over and dry up. The downy mildew fungus
overwinters in onion bulbs and sets and in residue from diseased plants.
Fungus spores (microscopic fungus "seeds") are spread by
air currents. Cool moist weather promotes disease development. |
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Botrytis
Leaf Blight (Blast) symptoms, caused by Botrytis spp., appear
first as numerous white specks on the leaves. As spots expand, the
leaves die from the tips and turn brown. Plant tops may be killed
and topple over within a week' all plants in a field may be affected.
Botrytis spp. are present in all fields where plants are grown. Fungus
spores (microscopic dry reproductive structures) are spread by air
currents and wind. Leaf blight frequently develops where leaf tissue
has been injured by thrips, mildew, ozone, sand blasting, or other
agents. |
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White
Rot, caused by the fungus Sclerotium cepivorum, appears first
on leaves. Yellowing and die-black progress from leaf tips downward.
Below ground, a wet rot develops on roots and at the bulb base. Affected
plants may be pulled easily from the soil because most roots are destroyed.
A white superficial fluffy mold develops on and in affected bulbs.
Tiny black sclerotia (fungus reproductive structures), acout 1/50
of an inch in diameter, form on the mold and in the bulb. Sclerotia
persist in soil for many years and can be spread with soil in running
water, on implements, and with seedlings. Disease is most prevalent
in cool seasons and in poorly drained areas. |
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Fusarium
Basal Rot, caused by Fusarium spp. of fungi, appears in the
field and in storage. The fungus can attack crowns and roots in the
field following injury or infection by other disease-causing organisms.
When this occurs, tops may turn yellowish and wilt. Diseased bulbs
may not have visible outward symptoms; dry outer scales appear normal
but the inner neck tissue may feel soft. One or more inner scales
may be water-soaked and later turn brown; white mold develops between
diseased scales during moist conditions. When conditions are dry,
diseased tissue is firm, dried, and shriveled. When conditions are
moist, other organisms may cause a soft watery rot. Symptoms develops
slowly and although most infection likely occurs in the field, symptoms
may not appear until storage. The basal rot Fusarium spp. persist
in soil and are spread in water, soil, air, and on insects and implements.
The fungus usually enters plants through areas injured by maggots,
burned by fetilizer, bruised during harvest, or infected by fungi
which cause smut or pink rot. Disease is most severe when onions are
grown in poorly drained soil and stored where humidity is high. |
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Botrytis
Neck Rot, caused by Botrytis spp. of fungi, is seldom seen
in the field; symptoms usually are first noticed in storage. The neck
tissue may shrivel. In cross section, the scales appear as if they
have been cooked; tissue is soft and brownish. A gray powdery-appearing
mold may appear between the scales and small black sclerotia (hard
pea-like structures) may for on dried scales. Soft rot bacteria may
follow neck rot and cause the onion to become soft, watery, and foul-smelling.
Botrytis spp. are present wherever plants are grown. Neck rot develops
only on onions that are injured or diseased. Infection is promoted
by prolonged wet conditions. |
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Bacterial
Soft Rot, caused by Erwinia carotovora and Pseudomonas spp.
of bacteria, usually begins in the field but appears in storage. The
bacteria entered onion neck tissue through dead or senescent leaves
and move down through individual scales to the bulb. Water may drip
from the neck when affected tissue is pressed. Affected tissue is
wet, slimy, and foul-smelling. Soft rot bacteria live in soil and
can be spred by maggots, implements or workers to the injured or weakened
onion tissue during the growing season. Infection occurs most frequently
and disease development is most rapid in moist conditions. |
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Smudge,
caused by the fungus Colletotrichum circinans, usually appears as
a dark green or black smudge, up to 1 inch in diameter, on the bulb
or neck. The spot may be uniformly black but usually is composed of
dark concentric rings. The dar portion is covered with stiff bristles.
The fungus overwinters on sets and in soil. Spores, produced by the
fungus when it is actively growing, are spread by wint, splashing
water, and on tools and clothing. Disease development is promoted
by fairly wet soil and temperatures of 75 to 85 ° F. |
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Smut,
caused by the fungus Urocystis cepulae, appears as gray-colored streaks
on leaves, leaf sheathes, and bulbs. These streaks are filled with
dusty fungus spores. Affected leaves twist and bend and seedlings
may be killed,. Only immature tissue is susceptible. This includes
young initial leaves and also young tissue protected within the sheath
at the base of the plant. Immature tissue enclosed in the sheath can
become infected only from an outer scale that is diseased. Therefore,
smut-free sets and seedlings will remain disease-free, even in smut-infested
soil. The smut fungus overwinters in smut-infested soil. Disease development
is promoted by cool soil conditions. |
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Pink
Root is caused by the soil-inhabiting fungus Pyrenochaeta terrestris.
Affected roots turn pink, shrivel, and die. New roots continue to
be killed. Plant tops may be little affected but bulbs are reduced
in size. The fungus lives indefinitely in soil and is spread with
soil. Disease is able to develop under a wide range of moisture and
temperature conditions. |
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