CRUCIFERS

Cabbage - Brassica oleracea L. Cauliflower - B. oleracea L. var. botrytisBrussel Sprouts - B. oleracea var. gemmifera Broccoli - B. oleracea var. botrytisCollard 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: CrucifersPATHOGENS: Erwinia carotovora subsp. carotovora Erwinia carotovora subsp. atroseptica Erwinia carotovora subsp. chrysanthemiThe carotovora subspecies is the most prevalent

pathogen and considered to be the one most commonly involved in soft rot of crucifers.

DISTRIBUTION: WorldwidePATHOGEN DESCRIPTION: BacteriumGram-negative, straight rod, motile, peritrichous

flagclla.

DISEASE SYMPTOMSSoft 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 MEASURESPrimarily 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: CrucifersPATHOGEN: Xanthomonas

campestris pv. campestrisDISTRIBUTION: WorldwidePATHOGEN DESCRIPTION: BacteriumGram-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 rot-resistant cabbage cultivars have been developed.

DISEASE: ClubrootCROP: 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.

SYMPTOMSMost 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: CrucifersPATHOGEN: 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.