Cole Crops:Diagnostics

COLE CROP PESTS AND NUTRITIONAL DISORDERS

ABIOTIC DISORDERS

‘‘‘Abiotic Disorders:’‘‘ Environmental conditions, agricultural chemicals, and nutrient deficiencies can cause lettuce and cole crop plants to develop symptoms similar to those caused by pathogens, insects, or other pests including reduced yields and tipburn.

ALTERNARIA LEAFSPOT (ALTERNARIA SPP)

‘‘‘Alternaria Leafspot (Alternaria spp.) ‘‘‘ During wet seasons, a brown velvety spore-bearing lesion appears on the older leaves. Leaf spots begin as a small dark spot and enlarge to form a large circular lesion forming a bull's-eye pattern. The best form of control is to apply fungicides.

AMMONIA INJURY

‘‘‘Ammonia Injury:’‘‘ Ammonia injury is sometimes associated with cool, waterlogged, and compacted soil. High ammonia concentrations near the plant roots will dehydrate the roots and free ammonia damages roots, killing small root hairs and lateral roots.. Warm, aerobic conditions are necessary for the nitrification of ammonium into the usable nontoxic form of nitrogen. To prevent ammonia injury, carefully place ammonium forms of nitrogen fertilizers keeping them out of the seed row or where roots may touch them.

ANTHRACNOSE (PSEUDOMONAS SYRINGAE)

‘‘‘Anthracnose (Pseudomonas syringae) ‘‘‘ Small, yellow water-soaked spots develop on the lower leaves and enlarge, dry up, and the center falls out when plants are infected with this fungus. Anthracnose is favored by cool, wet periods in spring. The best form of control is to apply fungicides, removed diseased plants from the field, and do not cultivate infested fields.

APHIDS

‘‘‘Aphids:’‘‘ Aphids develop as colonies of small soft-bodied, usually gray-green, insects on leaves causing deformed or curled leaves. Sticky honeydew may be present.

ASTER YELLOWS

‘‘‘Aster Yellows :’‘‘ The first symptoms are blanching and yellowing of the young heart leaves. Center leaves fail to develop normally, often remaining as short, thickened stubs in the plant's center. Pink-to-tan latex deposits collect on the underside of midribs of affected leaves. Younger plants that become infected may turn yellow and be severely stunted. The disease is spread primarily by the aster leafhopper.

BACTERIAL LEAFSPOT OF CAULIFLOWER (PSEUDOMONAS SYRINGAE PV MACULICOLA)

‘‘‘Bacterial Leafspot of Cauliflower (Pseudomonas syringae pv maculicola) ‘‘‘ Bacterial leafspot occurs sporadically, mainly on cauliflower, in coastal valleys and appears as small, faint, water-soaked areas on the undersides of lower leaves. These water-soaked areas develop into brownish-to-purplish gray dry spots that are irregularly shaped. If a plant is severely infected the leaves may become puckered. The disease is believed to be seedborne and spreads during cool, moist weather. There are no control measures available other than rotation and change from sprinkler to furrow irrigation.

BEET WESTERN YELLOWS

‘‘‘Beet Western Yellows, Beet Western Yellows Virus:’‘‘ The virus is transmitted by aphids and the older leaves may become thick and brittle with yellow interveinal areas. In very susceptible cultivars, most of the leaf surface may turn yellow with only the principal leaf veins of the older leaves remaining green. The best form of control is to destroy overwintering weed hosts and use resistant cultivars, if available.

BIG VEIN

‘‘‘Big Vein An Unidentified Virus-like Agent:’‘‘ The veins will appear large and clear if infected by this virus. Vein clearing becomes more intense over time, and leaves pucker and appear ruffled. The disease is favored by cool weather, usually in the spring. It is usually found in poorly drained soils and can be treated by having good drainage, sprinkler irrigate, avoid saturating the soil, and use resistant cultivars where available.

BLACK LEG (PHOMA LINGAM)

‘‘‘Black Leg (Phoma lingam) ‘‘‘ Black leg, a fungal disease, occurs in moist, cool environments and infections are usually limited to the basal part of the stem at or below ground level. Occasionally the disease also produces leaf and stem lesions that contain small, distinctive black fruiting bodies called pycnidia. As the disease progresses, the whole basal part of the stem below the ground may become decayed with a semi-dry, tan colored rot. Water transport is affected, and plants may wilt and die. The best form of control is to use treated seed and fungicides.

BLACK ROT (XANTHOMONAS CAMPESTRIS PV CAMPESTRIS)

‘‘‘Black Rot (Xanthomonas campestris pv campestris) ‘‘‘ Humid rainy conditions are ideal for the development of black rot. Yellow to light brown patches appear at the margins of leaves and later black veins develop within the yellowed areas. Affected areas turn brown and dry out, often leaving a triangular-shaped lesion on the leaf margin with one point of the triangle directed toward the midrib. The best form of control is to avoid sprinkler irrigation and plant tested seed.

BOTRYTIS ROT (BOTRYTIS CINEREA)

‘‘‘Botrytis Rot (Botrytis cinerea) ‘‘‘ This disease is characterized by a firm, watery, brown decay on the underside of the head at the crown. Lower leaves yellow, and eventually the whole head wilts and rots. A dense gray or light brown sporulation appears over the damaged areas when moisture from dew, rain, or irrigation is abundant. If the crop has optimumly matured and the weather is cool and moist, head rot is common. Good drainage, deep plowing, and high beds are recommended for management and control.

BOTTOM ROT (RHIZOCTONIA SOLANI)

‘‘‘Bottom Rot (Rhizoctonia solani) ‘‘‘ Initial symptoms of bottom rot appear after the head begins to form by sunken, brown, dry spots that develop on the bottom of midribs of the lower leaves that touch the ground. A fine, webbed network of white-to-brown mycelium often grows over the lesions. There is currently no control for bottom rot, but is most serious in warm, moist weather.

BROCCOLI BROWN BUD

‘‘‘Brown Bud of Broccoli:’‘‘ As broccoli heads mature, they turn brown if infected with this disorder, although the cause is unknown. Possible calcium nutrition and rapid growth are factors in the discoloration.

CABBAGE MAGGOT

‘‘‘Cabbage maggot (root maggot):’‘‘ Cabbage maggot is characterized by feeding injury (engraving) on the surface of roots, or tunneling through the roots of young plants. These plants fail to grow properly and may wilt and die.

CATERPILLARS

‘‘‘Caterpillars: cabbage looper, imported cabbage-worm, armyworms:’‘‘ Plants infected with caterpillars have irregular holes in the leaves and small or seedling plants may be destroyed.

CAULIFLOWER RICEYNESS

‘‘‘Riceyness of Cauliflower:’‘‘The curd appears uneven and fuzzy but only occurs when temperatures are warm during development. Some cultivars are more resistant than others, but there is currently no prevention method.

CLUBROOT (PHASMODIOPHORA BRASSICAE)

‘‘‘Clubroot (Plasmodiophora brassicae) ‘‘‘ Although there is no economical way to eliminate clubroot, rotation can significantly reduce the disease along with liming in alkaline soils. The first above ground symptom is the slight wilting or flagging of leaves during the day and recovering at night. Roots of most crucifers infected at a single site are spindle shaped, but multiple infections of the same root cause extreme swelling and distortion. It is spread by animals, water, transplanting, or movement of infested soil.

COLD INJURY

‘‘‘Cold Injury:’‘‘ Silvery leaves and a ruptured epidermis could have been the result from a sudden drop in temperature. Little economic loss is suffered because of the limited damage to the outer leaves.

CORKY ROOT

‘‘‘Corky Root An Unidentified Bacterium:’‘‘ Corky root is believed to be caused by a soilborne bacterium. Unlike most diseases, symptoms most commonly occur on the roots causing them to darken or turn faint green. The root's surface becomes cracked, rough, or pitted. Severely infected plants exhibit stunted and uneven growth, yellowing of lower leaves, wilting at midday, and poor head formation. The best forms of control include avoid planting in soil compacted and waterlogged areas and rotating the crop.

CYST NEMATODE

‘‘‘Cyst nematode:’‘‘ Plants with cyst nematode are stunted, wilted, have yellowish leaves and small glistening white specks on the roots.

DAMPING OFF (PYTHIUM SPP AND RHIZOCTONIA SOLANI)

‘‘‘Damping-off of Lettuce or Cole Crops (Pythium spp. and Rhizoctonia solani) ‘‘‘ The stem becomes dark and shriveled due to damping-off and is most common to occur either after seedlings germinate and before or after emergence in cool wet conditions. Pythium may invade roots as well as the stem, turning them brown. Rhizoctonia may invade the cortical tissue and girdle young stems. Fields which contain high green organic matter, compacted soil, or poor drainage have been reported to commonly have damping-off. The use of a fungicide seed dressing can prevent infection and protect the seedlings in the most susceptible stage. Also, preparation of good seedbeds and good water management will help reduce losses from damping-off.

DIAMONDBACK MOTH CATERPILLAR

‘‘‘Diamondback moth caterpillar:’‘‘ This caterpillar causes small holes in the leaves and weaves cocoons about 1/3 inch long on the leaves. The best form of control is to apply an effective insecticide.

DOWNY MILDEW (PERONOSPORA PARASITICA)

‘‘‘Downy Mildew of Cole Crops (Peronospora parasitica) ‘‘‘ The most common symptom of downy mildew is grayish white, fluffy growth on the underside of the leaves and some florets may develop black streaks or dark purple spots during cool, moist weather. Irregular, yellow to brown spots develop on both leaf surfaces and may spread to the stems and flower parts. Repeated chemical applications is the best way to control the disease as well as the use of resistant cultivars where they are available.

FUSARIUM WILT OR YELLOWS OF CABBAGE (FUSARIUM OXYSPORUM PV CONGLUTINANS)

‘‘‘Fusarium Wilt or Yellows of Cabbage (Fusarium oxysporum pv conglutinans) ‘‘‘ The yellowing of leaves are the first symptom of fusarium wilt which eventually results in the plant wilting and dying. Soon after yellowing appears, the water conducting tissue becomes reddish brown, usually first on one side of the stem. The disease is most common in the summer months. Fumigation and choosing resistant cultivars help control fusarium wilt in plants and in soil.

HARLEQUIN BUG

‘‘‘Harlequin bug:’‘‘ Harlequin bug is characterized by an attractive shield-shaped insect usually black with bright red, yellow or orange markings. Plants become wilted and have distorted leaves that turn brown. The best form of control is to handpick the bugs and their egg masses off of infected plants.

HERBICIDE INJURY

‘‘‘Herbicide Injury:’‘‘ Herbicide use can cause yellowing or spotting of leaves which is often confused with nutrient deficiency symptoms. In order to identify an herbicide injury, a lab test must be done but does not guarantee that the herbicide caused the injury. Use caution and follow directions when applying herbicides.

HOLLOW STEM

‘‘‘Hollow Stem of Cole Crops:’‘‘ The head or stem must be cut open to see the affected interior pith tissue collapsed, cracked, or often hollow. Wide spacing, high nitrogen, and high temperatures contribute to the disorder. Maintaining nutritional requirements at recommended levels and avoiding rapid growth assist in the prevention of hollow stem.

INSECTICIDE INJURY

‘‘‘Insecticide Injury:’‘‘ Young plants are the most susceptible showing signs of twisted and thickened leaves, deformed shoot apex, and distorted growth. Limit insecticide treatments to avoid yield reduction or loss.

LETTUCE DOWNY MILDEW (BREMIA LACTUCAE)

‘‘‘Downy Mildew of Lettuce (Bremia lactucae) ‘‘‘ Downy mildew is a common lettuce fungus, especially during the cool, moist weather often prevalent in early spring and late fall or in summer in the coastal regions. The fungus attacks older leaves first, causing light green or yellow lesions on the upper surface of the leaves and are later covered with white spores on the underside. Affected portions on the leaf eventually turn brown, and with numerous lesions the leaves may die. The best forms of control include using resistant cultivars and fungicide applications.

LETTUCE INFECTIOUS YELLOWS

‘‘‘Lettuce Infectious Yellows Virus:’‘‘ A yellowing around the margin of the old leaves is the first sign, later resulting in the outermost edge turning brown. Soon the whole plant takes on a yellowish cast that remains most intense around the outer edges of leaves. Destroying weeds and crop residues are effective ways of management and control.

LETTUCE MOSAIC

‘‘‘Lettuce Mosaic Virus:’‘‘ Leaves of young infected plants are slightly rolled along the axis. Light green to yellow mottling on the leaves is an additional symptom of this virus. Half-grown to mature plants may be severely stunted and discolored and margins of older leaves roll downward. Lettuce mosaic virus is seedborne and is usually transmitted by aphids. Disking of fields and elimination of weed hosts reduces the impact of the virus on lettuce.

LETTUCE POWDERY MILDEW (ERYSIPHI CICHORACEARUM)

‘‘‘Powdery Mildew of Lettuce (Erysiphe cichoracearum) ‘‘‘ This disease is favored by warm dry conditions and rarely reduces yield and quality and has little economic importance. The white sporulation of powdery mildew covers much of both the lower and upper sides of the leaf and occasionally small, black, fruiting bodies will also develop in the powdery mildew mycelium. Areas affected with powdery mildew enlarge in a circular pattern, spreading a white, dusty growth over the whole leaf. The leaves become brown and dried. Because of the rare economical importance, no treatment is required.

MOLYBDENUM

‘‘‘Molybdenum:’‘‘ Molybdenum deficiency symptoms include brittle, curled up leaves with a very thin blade occurring on the midrib.

MOSAIC OR VIRUS DISEASES

‘‘‘Mosaic or Virus Diseases:’‘‘ Aphids spread this disease which cause drying, yellowing leaves, and occasionally vein clearing. Plants may be stunted if infection occurs early. The best form of control is to plant away from infected areas.

NITROGEN

‘‘‘Nitrogen:’‘‘ Yellowing is common among nitrogen deficient plants and is seen in cool and waterlogged conditions.

NUTRIENT DEFICIENCIES

‘‘‘Nutrient Deficiencies:’‘‘ Discoloration and abnormal growth is common for most nutrient deficiencies. Root damage, excessive water, water stress, too high or too low temperatures or soil pH, or other nutrient imbalances may contribute to a plant's inability to obtain adequate amounts of one nutrient. Symptoms often include slow growth, yellowing, change in leaf color, or spotting of leaves. To avoid deficiencies, sample soil regularly and test the plant tissue for deficiency.

OVERMATURE

‘‘‘Overmature, hot or dry weather, or excess nitrogen fertilizer:’‘‘ Plants that are overmature may have leaves that grow through the head or heads that look cracked instead of smooth. The best forms of control include growing varieties adapted to your area, plant so that that crop develops in mild or cool weather, fertilize properly and do not let the soil dry out.

PESTICIDES

‘‘‘Pesticides:’‘‘ Fungicides and bactericides can prevent or reduce damage from some diseases, especially those that occur aboveground. Protective pesticides applied before the disease occurs are generally more effective than applications made after it has become established.

PHOSPHORUS

‘‘‘Phosphorus:’‘‘ When cool temperatures are prominent, phosphorus deficiency becomes visual. The leaves turn slightly purple, the leaves become spindly, and the plants become stunted.

PHYTOPHTHORA STEM AND ROOT ROT (PHYTOPHTHORA SPP)

‘‘‘Phytophthora Stem and Root Rot (Phytophthora spp.) ‘‘‘ The reddening of purpling of older leaves is the first visual sign of the disease. The invaded cortical tissue near the soil line develops cankers and the surrounding tissue becomes tan with a dark border. Eventually discoloration in the stem root and the plant wilts and dies. The best forms of control include using resistant cultivars, where available and plant in properly drained soil.

RINGSPOT (MYCOSPHAERELLA BRASSICIOLA)

‘‘‘Ringspot (Mycosphaerella brassiciola) ‘‘‘ Cool moist weather favors the development of ringspot on brussels sprouts. Small tan spots rapidly enlarge and turn gray even though it does not damage the plant directly. Applying protective fungicides and plowing under remaining plant residue at the end of harvest will reduce the inoculum level in the soil.

SALT INJURY

‘‘‘Salt Injury:’‘‘ A high salt content can prevent seed germination, cause orange-colored and rough appearing roots, stunted growth, dark green foliage, and marginal yellowing or burning.

SCLEROTINIA DROP (SCLEROTINIA SCLEROTIORUM)

‘‘‘Sclerotinia Drop (Sclerotinia sclerotiorum) ‘‘‘ The lower leaves wilt first, and hard, black sclerotia on white, cottony mycelium develops on the underside of the lower leaves. Eventually, the whole head wilts, yellows, and drops due to sclerotinia drop disease. Cool moist weather enhances the spread and development of the infection. Management methods for Sclerotinia minor include deep plowing, keeping the soil bed surface dry, removing infected crop residues, and applying fungicides as soon after thinning as possible. Management for Sclerotinia sclerotiorum includes keeping bed surfaces dry, removing weedy areas around the fields, and applying fungicides, although they may differ from those used for Sclerotinia minor.

SUNBURN

‘‘‘Sunburn:’‘‘ Sunburned plants have yellow or brown heads on cauliflower instead of white heads. To control sunburn, , tie the outer leaves around the head with twine as soon as white heads have a 3 inch diameter and harvest 4 to 7 days later.

TIPBURN

‘‘‘Tipburn:’‘‘ Although symptoms are not visual from the outside of plant, the edges of leaves on the inside are brown. Necrosis may occur only along the margin or along the whole edge of the leaf. The best forms of control include applying nitrogen in nitrate (NO3 -) forms rather than ammonium (NH4+) to fertilize, avoid water stress, and have good irrigation practices.

VARNISH SPOT (PSEUDOMONAS CICHORII)

‘‘‘Varnish Spot (Pseudomonas cichorii) ‘‘‘ Varnish spot can only be detected by removing the outside leaves exposing the shiny, dark brown, firm, necrotic spots. The bacterium requires specific moisture and maturity conditions to infect. It is believed to survive in weed or crop hosts, roots or plant debris in the soil, or in reservoir water used to sprinkle irrigate. After thinning the crop, avoiding sprinkler irrigation can prevent varnish spot.

VERTICILLIUM WILT (VERTICILLLIUM DAHLIAE)

‘‘‘Verticillium Wilt (Verticillium dahliae) ‘‘‘ Verticillium wilt does not kill plants but can significantly reduce and limit yields. Typical symptoms on cole crops are stunting and irregular patches of yellow developing between major veins of older leaves. These symptoms are not obvious and may be overlooked. If you cut an infected stem in a diagonal cross section, you will see discontinuous streaks of dark brown discoloration in the woody, water-conducting vessels. There are no control measures available.

WATERING

‘‘‘Watering:’‘‘ Sudden, heavy watering after prolonged dry period results in too fast growth, and heads crack. Do not allow the soil to get too dry and if it does, apply water slowly at first.

WHITE MOLD (SCLEROTINIA SCLEROTIORUM)

‘‘‘White Mold (Sclerotinia sclerotiorum) ‘‘‘ During cool, wet weather, stalks, heads, leaves, or flowers of infected plants may be covered with a white cottony growth. The plant tissue beneath the white mycelium usually turns soft and watery and hard, black resting bodies-sclerotia are produced in or on diseased tissue. Chemicals and deep plowing can assist in fighting the disease.

WHITE SPOT (PSEUDOCERCOSPORELLA CAPSELLAE)

‘‘‘White Spot (Pseudocercosporella capsellae) ‘‘‘ In wet conditions a "fuzz" is present on the underside of the leaf, but usually white to light tan spots are scattered over the leaf surface. Currently there is no control method is recommended.

WIND INJURY

‘‘‘Wind Injury:’‘‘ Wind injury is caused by strong winds carrying abrasive sand and injuring plants of all ages. Wounded areas will often heal over on cole crops, leaving thickened, wartlike, discolored spots that would be mistaken for pathogen injury. The injured areas can be prevented if the field is sprinkled as soon as the storm begins.