| CRUCIFER DISEASES |
|
|
Wirestem,
Bottom Rot, and Head Rot are caused by Rhizoctonia solani.
This fungus also is a common cause of damping-off. Wirestem is characterized
by stems that are darkened and girdled near the soil line. Affected
plants are weak, produce small heads, and sometimes wilt and die.
Bottom rot develops on plants after they have been transplanted into
the field. Dark slightly sunken spots develop on basal leaves near
the soil. In moist conditions and in storage, rot spreads to adjacent
leaves and causes a head rot. The causal fungus is present in all
field soil. Disease is promoted by moist conditions. |
|
|
White
Rot is caused by the fungus Sclerotinia sclerotiorum. Symptoms
can appear on leaves or petioles nearest the ground or on the top
of cabbage heads. Infected areas first appear as water-soaked spots;
these soon enlarge to irregular-shaped areas which become covered
by white mold. The fungus grows upward over the maturing plant, often
producing a soft water-soaked mass. Numerous black sclerotia (seed-like
fungus reproductive structures) form on and in diseased parts. Severely
affected plants may wilt or topple. Sclerotia produced by the fungus
can survive in soil for many years. |
|
|
Alternaria
Leaf Spot, caused by the fungus Alternaria brassicae, is characterized
by distinct spots with concentric rings on the lower leaves; the dark
dusty fungus growth develops on these spots during moise periods.
During storage, spots enlarge. Soft rot bacteria may enter through
dead leaf spots. The fungus overwinters in seed and in residue from
diseased plants. Wet conditions promote disease development |
|
|
Black
Leg is caused by the fungus Phoma (Plenodomus) lingam. Symptoms
begin as dark sunken cankers at the base of the stem and as light
brown circular leaf spots. Stem cankers enlarge and girdle stems,
causing plants to wilt. A diagnostic feature of black leg is the presence
of distinct black pycnidia (speck-size fungus reproductive structures)
within stem cankers and leaf spots. The causal fungus overwinters
on seed and in residue from diseased plants; the fungus can persist
in residue for 2 to 3 years. The fungus can be carried on seed and
on transplants; it can be spread within fields when diseased and healthy
plants are dipped in the same water, when workers and implements move
through fields that include diseased plants, and by splashing and
running contaminated water. |
|
|
Downy
Mildew, caused by the fungus Peronospora parasitica, is most
serious in seedbeds and appears as small leaf spots which first are
yellow and later turn brown with bluish-black lace-like markings.
In moist weather, a white downy mold develops on the underside of
the leaf spots. Vascular tissue becomes discolored. In turnip and
radish, roots also can be affected. They become discolored internally
and during storage dry out more rapidly than do roots from plants
free of mildew. The causal fungus overwinters on seed, in crucifer
weeds, and perhaps in soil. Disease is promoted by cool wet weather
in spring and fall. Downy mildew can predispose plants to bacterial
soft rot. |
|
|
Black
Rot, caused by the bacterium Xanthomonas campestris, affects
young as well as mature plants. Affected seedlings turn yellow and
die. On older plants, yellow wedge-shaped areas appear at leaf margins
and expand toward the center of the leaf; affected areas later turn
brown and die. Vascular tissue is black in veins within affected areas.
This discoloration develops from leaf margins toward the base of the
plant. Heads are dwarfed and lower leaves fall off. Frequently symptoms
are most severe on one side of the head. Soft rot often develops on
affected heads. The black rot bacterium overwinters on seed and in
residue from diseased plants; it persists in residue for from diseased
plants; it persists in residue for 1 to 2 years. As with the black
leg fungus, the black rot bacterium is seed-borne and is spread on
seedlings and by movement of contaminated water. |
|
|
Fusarium
Yellows, caused by the fungus Fusarium oxysporum f. sp. conglutinans,
is most severe on susceptible cabbage varieties but also affects other
crucifers. Affected plants have a sickly, dwarfed, yellow appearance.
In affected leaves, edges frequently become purple and bases become
brown. Lower leaves drop one by one. Vascular tissue in veins turns
dark in affected sides of leaves and plants. This discoloration develops
from the base of the plant toward leaf margins. The causal fungus
can persist in soil for many years. Disease development is promoted
by high soil temperatures. |
|
|
Clubroot,
caused by the fungus Plasmodiophora brassicae causes wilting and yellowing
of above-ground parts. The diagnostic symptom is the presence of large
spindle-shaped galls on roots. The causal fungus is soil-borne and
persists in soil for at least 7 years. Soil pH of less than 7.2 favors
disease development. |
|
|
Edema
appears as small brownish gray wart-like growths on the leaf surface.
These growths are through to be initiated by leaf injury caused by
sand or insects. Edema usually develops during cool nights following
warm muggy days. Under these conditions water uptake is faster than
water loss; consequently, the leaf epidermis bursts and expanding
leaf cells are exposed and become corky. |
|
|
Bacterial
Soft Rot is caused by Erwinia caratovora and some other bacteria.
Affected areas appear to be water-soaked, develop a soft decay, and
have a foul distinctive odor. Affected cabbage and cauliflower heads
decay rapidly and turn dark. The bacteria usually infect plants through
surface areas injured by insects, cold, or mechanical means. Warm
wet conditions promote disease development. |
)
)
)
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}