Download Slide 1

CRUCIFERS
Cabbage - Brassica oleracea L.
Cauliflower - B. oleracea L. var. botrytis
Brussel Sprouts - B. oleracea var. gemmifera
Broccoli - B. oleracea var. botrytis
Collard and Kale - B. oleracea var. acephala
Turnip - B. rapa
Mustard (black) - B. nigra
Rutabaga - B. campestris L. var. napobrassica
Radish - Raphanus sativus L.
CRUCIFERS
These crops grow best in a cool climate. They are
grown in the southern United States during the
fall, winter, and spring. Most can stand some
frost, and some can stand a variable amount of
actual freezing depending upon condition of the
crop and duration of the freeze.
Summer crops are grown in the northern U.S.,
frequently from plants started in the South.
Crucifer Diseases
• Soft Rot
• Black rot
• Clubroot
• Sclerotina white mold
DISEASE: Soft Rot
CROP: Crucifers
PATHOGENS:
Erwinia carotovora subsp. carotovora
Erwinia carotovora subsp. atroseptica
Erwinia carotovora subsp. chrysanthemi
The carotovora subspecies is the most prevalent pathogen
and considered to be the one most commonly involved
in soft rot of crucifers.
DISTRIBUTION: Worldwide
PATHOGEN DESCRIPTION: Bacterium
Gram-negative, straight rod, motile, peritrichous flagclla.
DISEASE SYMPTOMS
Soft rot occurs on all crucifers, particularly damaging to
Chinese cabbage and common cabbage.
Infected tissues first develop a water-soaked lesion which
enlarges rapidly in diameter and depth.
The affected area becomes soft and mushy and turns a dark
color in advanced stages of disease.
Soft rot-infected cruciferous plants almost always give off
an offensive odor, perhaps in part due to invasion by
secondary organisms.
Disease losses from soft rot may occur in the field, transit,
or storage.
SYMPTOMS
CONDITIONS FOR DISEASE
DEVELOPMENT
• Soft rot bacteria persist in infected plant debris,
in association with plant roots, in low numbers
in the soil, and in association with several
insects.
• Wounds such as leaf scars, insect injury,
mechanical injury, lesions caused by other
pathogens, etc. are the primary avenues for soft
rot bacterial invasion.
• Infection in the field is enhanced by rainfall and
high temperatures.
CONDITIONS FOR DISEASE
DEVELOPMENT
• Transit and storage infection may develop from
bacterial contamination that occurred in the
field or during post harvest from handling
equipment and storage containers.
• Soft rot bacteria can grow over a temp. range of
5 - 37°C with an optimum temperature of
about 22°C.
CONTROL MEASURES
Primarily sanitation and cultural practices.
Sufficient time should be allowed for crop residues
to breakdown before planting a second crop.
Vegetable crops should be rotated with cereals or
other non-susceptible crops.
Fields should be well drained to reduce soil
surface moisture and plants should be spaced to
allow ventilation.
Rain shelters prevent soil splash and foliage
welting.
DISEASE: Black Rot
CROP: Crucifers
PATHOGEN: Xanthomonas campestris
pv. campestris
DISTRIBUTION: Worldwide
PATHOGEN DESCRIPTION: Bacterium
Gram-negative, straight rod, motile, single
polar flagellum.
SYMPTOMS
• Black rot occurs on all cultivated crucifers and
several wild species.
• Lesions typically begin at the leaf margin and
progress inward forming V-shaped chlorotic
lesions.
• With time the lesions begin to dry and become
necrotic. Infection normally occurs at the
hydathodes, but it can occur at any place on the
leaf lamina where insect or mechanical wounds
allow for bacterial entry.
SYMPTOMS
SYMPTOMS
The bacterium is a vascular invader and can
move systemically in the plant.
Invaded vascular tissue turns black in color
which can be seen as dark veins in lesions or by
observing the vascular bundles in cross
sections of the leaf midrib, petiole, or main
stem of infected plants.
Black rot infections can serve as avenues for soft
rot entry.
SYMPTOMS
CONDITIONS FOR DISEASE
DEVELOPMENT
The bacterium persists in debris from infected plants, but
it does not survive in the soil after the debris has
deteriorated.
The bacterium can also survive on other crucifer crop and
weed plants and it is seedborne.
Bacteria are spread within a crop primarily by wind-blown
and splashing water and by workers, machinery, and
occasionally insects.
Bacteria can survive on leaf surfaces for several days
until dispersed to hydathodes or wounds where
infection can occur.
CONDITIONS FOR DISEASE
DEVELOPMENT
Bacteria enter leaves through hydathodes when water
exuded through these pores at the leaf margin during
the night is drawn back into the plant in the morning.
Bacteria can enter leaves in 8 to 10 hours, and wilt
symptoms are visible as soon as 5 to 15 hours later.
Wounds, including those made by insects feeding on leaves
and by mechanical injury to roots during transplanting,
also provide entry sites. Wounds on roots are most
important when transplants are dipped in water or the
soil becomes saturated.
High temperatures and periods extensive rainfall favor
black rot development.
CONTROL MEASURES
Rotate with non-cruciferous crops to allow time for
debris from the previous crucifer crop to
decompose.
Use pathogen-free seed produced in an arid climate
with no overhead irrigation. Avoid working in the
fields when the foliage is wet.
Use resistant cultivars when available; black rotresistant cabbage cultivars have been developed.
DISEASE: Clubroot
CROP: Crucifers
PATHOGEN:
Plasmodiophora brassicae
DISTRIBUTION: Worldwide, most damaging in
temperate regions and tropical highlands.
DISEASE: Clubroot
PATHOGEN DESCRIPTION: Fungal-like
organism, that is an endoparasitic slime mold. It
forms cysts (resting spores), zoospores,
plasmodia, and zoosporangia.
SYMPTOMS
Most crucifer are susceptible to clubroot; cabbage,
Chinese cabbage, and Brussels sprouts are highly
susceptible.
Enlargement of fine roots, secondary roots, and the tap
root results in the formation of spindle-shaped clubs.
The clubs are generally widest in the middle and taper
toward the ends.
Severly clubed systems are impaired and cannot absorb
water and nutrients, therefore the plants are stunted
and may wilt with only slight water stress.
Lower leaves frequently turn yellow in advanced stages
of disease development.
SYMPTOMS
SYMPTOMS
SYMPTOMS
CONDITIONS FOR DISEASE
DEVELOPMENT
Resting spores can survive in the soil for at least 10 yrs.
Can persist on cruciferous weeds indefinitely.
The pathogen can be disseminated by soil equipment, in soil
water, and infected transplants.
The disease is favored by acidic soils and infection can occur
at soil temperatures between 10 and 32°C with an
optimum between 20 and 25°C.
The pathogen is composed of numerous pathotypes which
have hampered efforts to breed cultivars with durable
resistance.
Resisting spores in cells
CONTROL MEASURES
Use of disease-free transplants is most important in a
disease management program.
Crop rotation is impractical because the resting spores
survive for such a long time and weed hosts can
maintain the inoculum.
Raising the soil pH to 7.2 is very effective in reducing
disease development.
Use of fungicides in transplant water can also reduce
disease development.
Resistant cultivars must be tested in each location
because of the many pathotypes of the pathogen.
DISEASE: White Mold
CROP: Crucifers
PATHOGEN:
Sclerotinia sclerotiorum
Sclerotinia minor
DISTRIBUTION: Worldwide
PATHOGEN DESCRIPTION
These fungi produce a white cottony mycelium
and black sclerotia from which apothecia
develop and produce ascospores. The sclerotial
diameter range is 2.5-6 for S. sclerotiorum and
0.5-1 mm for 5. minor.
SYMPTOMS AND SIGNS
• The disease affects all crucifers as well as
many other vegetables.
• On cabbage, rapidly developing water soaked
areas develop on the stem, leaves, or head.
• Soon a white fluffy mycelial growth develops on
the surface of the infected tissue which may
encompass the entire head.
SYMPTOMS AND SIGNS
SYMPTOMS AND SIGNS
Hard, black sclerotia develop in the surface
mycelial growth.
Disease development can occur in the field or
during transit and storage.
Crucifer plants with stem infections may be girdled
and killed.
Sclerotia frequently form in the pith of these
stems.
CONDITIONS FOR DISEASE
DEVELOPMENT
Sclerotia can persist in crop debris, on many
alternate hosts, or as sclerotia in the soil.
Cool, moist conditions are favorable for disease
spread and development.
Apothecia are produced by sclerotia under these
conditions and discharge ascospores which is the
primary means of dissemination of the pathogen.
Infection may occur at temperature from 0 to
28°C, but the optimum temperature range is 16
to 21°C.
CONTROL MEASURES:
Avoid fields known to have a high incidence of
while mold.
Rotate crucifer crops with less susceptible crops
such as cereals, grasses, or onion.
Deep plow to bury the sclerotia so as to reduce
apothecial formation.
Flooding has a deleterious effect on sclerotia,
therefore rotations with flooded rice should be of
benefit.
Fungicidal sprays provide some protection against
infection.