Alfalfa:Key

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KEY TO ALFALFA AND HAY PESTS AND NUTRITIONAL DISORDERS



Contents

Seed

Seed disorders are limited to disorders caused while the seed is in the ground not to potential disorders inherent in the seed.


Seedling

DARK GREEN

Aluminum toxicity

Plants are stunted and dark green; chlorotic spots may occur on lower leaves; root growth restricted and stubby


GRAY

Aphanomyces root rot

Infected seedlings become chlorotic, roots are gray and water-soaked, but plants remain upright; on older plants nodules decay and lateral roots are destroyed, leaving restricted brown lesions on tap root surface; foliage becomes chlorotic


WHITE

Crown wart

Galls (white when young and gray to brown with age) form at or slightly below the soil surface on young crown buds; when exposed to sunlight, galls may become green; globose resting spores produced in internal tissue


YELLOW

Atrazine

Yellowing and necrosis of seedlings beginning at margins of oldest leaves

Bacterial leaf spot

Favored by high temperatures; seedlings may be severely stunted; leaf lesions are irregular, with yellowish to tan, papery, translucent centers and narrow, water-soaked borders that are most pronounced on lower leaf surface

Bacterial wilt; other bacterial diseases

Plants are stunted with small cupped leaves, yellow margins, many crown shoots; diseased plants scattered throughout stand; xylem tissue in taproot is tan to brown ; discoloration sometimes extends laterally to inside surface of dark

Bromoxynil

Burning of seedling leaves at margins first; small seedlings may die

Downy mildew

Favored by cool, moist conditions; most severe on seedling stands; yellow blotches on upper side of leaves; white to purplish downy growth on the lower surface; systemically infected shoots shortened and chlorotic; terminal leaves twisted

EPTC

Seedlings are stunted due to poor root development; leaves may be malformed


Root

BLACK

Brown root rot

Root rot associated with very cold winters and characterized by brown sunken lesions; brown to black pycnidia occur on surface of lesions or are embedded in the necrotic tissue

Mycoleptodiscus crown and root rot

Black discoloration of lateral and taproots; black sclerotia form in decayed tissue

Root-lesion nematode

Dark brown or black lesions on roots that may fuse to cause overall browning and reduced root growth


BROWN

Aphanomyces root rot

Infected seedlings become chlorotic, roots are gray and water-soaked, but plants remain upright; on older plants nodules decay and lateral roots are destroyed, leaving restricted brown lesions on tap root surface; foliage becomes chlorotic

Corky root rot

Dry, sunken cankers form on tap root and spread until entire root is girdled; the rotted root retains its shape but, due to corky consistency, is lighter in weight than healthy roots

Crown and root rot complexes

Dark brown necrotic tissue in crowns and roots; affected area may range from irregular rotted areas in cortex and xylem to complete disintegration of the root and crown; often difficult to determine causal organism(s)

Cylindrocarpon root rot

Light brown general root or crown rot extending either a short distance below crown or rotting entire root; stromata of causal organism may develop in cracks in the affected cortex of the taproot

Dwarf

Plants are stunted with small, dark green leaves resembling those caused by drought stress; xylem tissue in taproot is dark brown

Fusarium wilt

Favored by warm weather; plants usually have lower leaf yellowing and often a one-sided wilt; longitudinal streaks in xylem tissue of taproots are brick red to dark brown; mycelium may be seen in xylem vessel elements

Nitrogen deficiency; ineffective nodulation

Yellow older leaves; roots poorly nodulated

Phosphorus deficiency

Top growth is stunted, stiff, and erect, with small bluish green leaves; roots light brown with restricted growth

Phymatotrichum root rot

Necrotic lesions on taproots are brown, sunken, and irregular in shape; roots may be entirely rotted; rhizomorphs with cruciate hyphae on surface are highly diagnostic

Root-knot nematode

Stunted plants with excessively branched roots having small galls

Verticillium wilt

Symptoms are more evident at moderate temperatures; leaves become yellow, often in a V-shaped or one-sided pattern, plants wilt, and xylem tissue of taproot becomes brown; stem remains green for a while after all leaves on a shoot are killed


DARK GREEN

Aluminum toxicity

Plants are stunted and dark green; chlorotic spots may occur on lower leaves; root growth restricted and stubby


ORANGE

Stagonospora root and crown rot

Cortex and xylem tissue have distinctive orange to brick red flecks; bark of infected region may be rough or fissured longitudinally


PURPLE

Cylindrocladium root and crown rot

Shoots are killed following dark brown decay at crown; roots become purplish black; orange-red perithecia may form at base of plant


RED

Acrocalymma root and crown rot

Cortical and woody tissues of crown and upper taproot are flecked with reddish colored tissue; bark of affected plants is often fissured longitudinally; similar to Stagonospora root rot but with different morphology and no leaf spots


TAN

Bacterial wilt; other bacterial diseases

Plants are stunted with small cupped leaves, yellow margins, many crown shoots; diseased plants scattered throughout stand; xylem tissue in taproot is tan to brown ; discoloration sometimes extends laterally to inside surface of dark


VIOLET

Violet root rot

Plants in low spots in field die; patches are circular to irregular and enlarge as disease progresses; violet mycelium eventually forms a violet felt on taproots; bark loosens and slips from the central cylinder


YELLOW

Phytophthora root rot

Occurs in saturated soils; affected area on taproot is brown to yellow, eventually turns black, and has diffuse and irregular margins; rotted tissue is usually firm; taproot may rot off completely; oospores form in rotted tissue; new roots may emerge


YELLOW/BLACK

Rhizoctonia root canker

Brown to yellow elliptical cankers with entire margins form on the taproot at point of emergence of the lateral roots; cankers become black and inactive during winter; disease is important only in irrigated high-temperature desert areas


No Distinct Discoloration

Calcium deficiency

Root growth is severely impaired, and rotting may occur; petioles collapse and undersides of leaflets become reddish purple

EPTC

Seedlings are stunted due to poor root development; leaves may be malformed


Crown

BLACK

Anthracnose

Dark, bluish black necrotic area, sometimes V-shaped, extending into the crown from diseased stems

Mycoleptodiscus crown and root rot

Black discoloration of lateral and taproots; black sclerotia form in decayed tissue


BROWN

Crown and root rot complexes

Dark brown necrotic tissue in crowns and roots; affected area may range from irregular rotted areas in cortex and xylem to complete disintegration of the root and crown; often difficult to determine causal organism(s)

Cylindrocarpon root rot

Light brown general root or crown rot extending either a short distance below crown or rotting entire root; stromata of causal organism may develop in cracks in the affected cortex of the taproot

Winter crown rot

Patches of dead plants with dark brown rotted tissue on crown or on crown buds; white mycelium of the causal fungus may be seen on soil surface


GRAY/GREEN

Crown wart

Galls (white when young and gray to brown with age) form at or slightly below the soil surface on young crown buds; when exposed to sunlight, galls may become green; globose resting spores produced in internal tissue


GREEN

Witches'-broom

Short, spindly shoots are produced profusely on crowns; leaves are puckered, small, and marginally to generally chlorotic


ORANGE

Stagonospora root and crown rot

Cortex and xylem tissue have distinctive orange to brick red flecks; bark of infected region may be rough or fissured longitudinally


RED

Acrocalymma root and crown rot

Cortical and woody tissues of crown and upper taproot are flecked with reddish colored tissue; bark of affected plants is often fissured longitudinally; similar to Stagonospora root rot but with different morphology and no leaf spots

Cylindrocladium root and crown rot

Shoots are killed following dark brown decay at crown; roots become purplish black; orange-red perithecia may form at base of plant


WHITE

Winter crown rot

Patches of dead plants with dark brown rotted tissue on crown or on crown buds; white mycelium of the causal fungus may be seen on soil surface


WHITE/BLACK

Sclerotinia crown and stem rot

Favored by cool, humid conditions; leaves and stems become flaccid and collapse; white fluffy mycelium grown over dead parts and produces hard black sclerotia on and in infected tissue


YELLOW

Bacterial wilt; other bacterial diseases

Plants are stunted with small cupped leaves, yellow margins, many crown shoots; diseased plants scattered throughout stand; xylem tissue in taproot is tan to brown ; discoloration sometimes extends laterally to inside surface of dark


No Distinct Discoloration

Crown gall

Galls on crown branches are convoluted and irregular in shape; absence of globose resting spores in galled tissue differentiates the disease from crown ward; disease is rare



Stem

BLACK

Bacterial stem blight

Usually restricted to frost-damaged spring growth; rare in occurrence; stem lesions begin at point of leaf attachment and extend downward; water-soaked lesions become amber-colored and shiny, then blacken with age; bacteria in ooze from water-soaked areas

Spring black stem and leaf spot

Favored by low temperatures; small, irregular, black to brown spots enlarge and coalesce on leaves; infected leaves turn yellow, wither, and fall; stems blacken; pycnidia with conidia on overwintered stems


BROWN

Anthracnose

Favored by warm, humid conditions; stem lesions are sunken, whitish to tan, oval to diamond-shaped with brown borders mostly near base of plant and may girdle stems; black specks (acervuli) develop in lesions

Rhizoctonia stem blight

Favored by hot, humid conditions; bleached, elongated lesions with brown bands that girdle stems at base of plant; coarse brown mycelium usually on lesion

Sclerotium blight

Favored by hot, moist conditions; plants killed are devoid of green color; small, brown, spherical sclerotia resembling mustard seeds form on decayed tissue


GREEN

Sulfur deficiency

Plants are pale green, including veins on younger leaves; plants are stunted

Verticillium wilt

Symptoms are more evident at moderate temperatures; leaves become yellow, often in a V-shaped or one-sided pattern, plants wilt, and xylem tissue of taproot becomes brown; stem remains green for a while after all leaves on a shoot are killed


RED

Rust

Most prevalent late in the growing season or earlier in fields harvested for seed or if forage harvest is delayed; circular reddish brown pustules protruding from leaves and stems; the rusty spots (spores) rub off easily

Summer black stem and leaf spot

Favored by hot, humid conditions; leaf lesions are roughly circular, reddish brown, and up to 6 mm in diameter; conidia give lesions a silver appearance; brown stem lesions enlarge and coalesce


WHITE

Anthracnose

Favored by warm, humid conditions; stem lesions are sunken, whitish to tan, oval to diamond-shaped with brown borders mostly near base of plant and may girdle stems; black specks (acervuli) develop in lesions

Copper deficiency

Young leaves wilt and may bend back against lower side of petiole and develop necrotic gray to white spots on margins or on the basal half of leaflets; growing point dies


WHITE/BLACK

Sclerotinia crown and stem rot

Favored by cool, humid conditions; leaves and stems become flaccid and collapse; white fluffy mycelium grown over dead parts and produces hard black sclerotia on and in infected tissue


YELLOW

Bacterial leaf spot

Stem lesions are greasy and water-soaked, become grown, and usually coalesce; bacteria in ooze from water-soaked areas

Boron deficiency

Plants are dwarfed due to shortening of internodes; margins of youngest fully developed leaves become yellow, then purplish


No Distinct Discoloration

Downy mildew

Favored by cool, moist conditions; most severe on seedling stands; yellow blotches on upper side of leaves; white to purplish downy growth on the lower surface; systemically infected shoots shortened and chlorotic; terminal leaves twisted

Stem nematode

In cool, humid weather, leaves curled and distorted or white; stem internodes are greatly shortened, swollen, and engorged with nematodes


BLACK

Stagonospora leaf spot

Favored by cool to moderate temperatures of early spring and late fall; bleached, roughly circular leaf spots with dark specks (pycnidia)



Leaf

BLUE/GREEN

Phosphorus deficiency

Top growth is stunted, stiff, and erect, with small bluish green leaves; roots light brown with restricted growth


BROWN

Common leaf spot

Lesions are brown, 1-3 mm in diameter, and rarely coalesce; a raised disc (apothecium) to 1 mm in diameter develops in center of mature lesions on upper leaf surface; leaves turn yellow and drop

Hydrogen fluoride

Marginal necrosis of leaves

Leptosphaerulina leaf spot

Young regrowth burned (large bleached areas) during or shortly after cool, rainy periods; on older leaves, 1-3 mm diameter brown eyespots with lighter centers surrounded by chlorotic halos; ascocarps mostly on decayed leaves

Manganese deficiency

Reduced growth and severe interveinal chlorosis of the youngest leaves, followed by light brown necrotic areas on upper leaf surface

Manganese toxicity

Older leaves become chlorotic at leaf margins, which cup downward and develop small necrotic spots

Metribuzin; hexazinone; terbacil

Plants in established stands are stunted; leaves chlorotic and/or necrotic

Sclerotium blight

Favored by hot, moist conditions; plants killed are devoid of green color; small, brown, spherical sclerotia resembling mustard seeds form on decayed tissue

Spring black stem and leaf spot

Favored by low temperatures; small, irregular, black to brown spots enlarge and coalesce on leaves; infected leaves turn yellow, wither, and fall; stems blacken; pycnidia with conidia on overwintered stems

Stemphylium leaf spot

Favored by wet periods during summer and fall; lesions are oval, slightly sunken, and dark brown, with lighter centers and chlorotic halos; dark conidia develop profusely in older lesions

Sulfur dioxide

Necrotic margins and interveinal leaf areas on older leaves first; chlorosis also common

Zinc deficiency

Youngest leaves remain small and curl upward, then brown spots develop on upper surface around margins of the top (but not youngest) leaves


GRAY

Copper deficiency

Young leaves wilt and may bend back against lower side of petiole and develop necrotic gray to white spots on margins or on the basal half of leaflets; growing point dies


GREEN

Dwarf

Plants are stunted with small, dark green leaves resembling those caused by drought stress; xylem tissue in taproot is dark brown


GREEN (PALE)

Molybdenum deficiency

Plants are poorly nodulated, pale green, and stunted; lower leaflets show interveinal white notching and then whitening of tips and centers; they fall prematurely


PURPLE

Boron deficiency

Plants are dwarfed due to shortening of internodes; margins of youngest fully developed leaves become yellow, then purplish


RED

Calcium deficiency

Root growth is severely impaired, and rotting may occur; petioles collapse and undersides of leaflets become reddish purple

Magnesium deficiency

Interveinal chlorosis of lower leaves first; leaf margins initially remain green but later become chlorotic or red and finally die

Rust

Most prevalent late in the growing season or earlier in fields harvested for seed or if forage harvest is delayed; circular reddish brown pustules protruding from leaves and stems; the rusty spots (spores) rub off easily


SILVER

Summer black stem and leaf spot

Favored by hot, humid conditions; leaf lesions are roughly circular, reddish brown, and up to 6 mm in diameter; conidia give lesions a silver appearance; brown stem lesions enlarge and coalesce


WHITE

Molybdenum deficiency

Plants are poorly nodulated, pale green, and stunted; lower leaflets show interveinal white notching and then whitening of tips and centers; they fall prematurely

Potassium deficiency

Small white spots appear at margins on lower leaves first

Stem nematode

In cool, humid weather, leaves curled and distorted or white; stem internodes are greatly shortened, swollen, and engorged with nematodes


WHITE/BLACK

Sclerotinia crown and stem rot

Favored by cool, humid conditions; leaves and stems become flaccid and collapse; white fluffy mycelium grown over dead parts and produces hard black sclerotia on and in infected tissue


WHITE/PURPLE

Downy mildew

Favored by cool, moist conditions; most severe on seedling stands; yellow blotches on upper side of leaves; white to purplish downy growth on the lower surface; systemically infected shoots shortened and chlorotic; terminal leaves twisted


YELLOW

Alfalfa mosaic

During cool spring and autumn conditions, light green or yellow leaf mottle and leaf distortion

Aluminum toxicity

Plants are stunted and dark green; chlorotic spots may occur on lower leaves; root growth restricted and stubby

Aphanomyces root rot

Infected seedlings become chlorotic, roots are gray and water-soaked, but plants remain upright; on older plants nodules decay and lateral roots are destroyed, leaving restricted brown lesions on tap root surface; foliage becomes chlorotic

Atrazine

Yellowing and necrosis of seedlings beginning at margins of oldest leaves

Bacterial leaf spot

Favored by high temperatures; seedlings may be severely stunted; leaf lesions are irregular, with yellowish to tan, papery, translucent centers and narrow, water-soaked borders that are most pronounced on lower leaf surface

Bacterial wilt; other bacterial diseases

Plants are stunted with small cupped leaves, yellow margins, many crown shoots; diseased plants scattered throughout stand; xylem tissue in taproot is tan to brown ; discoloration sometimes extends laterally to inside surface of dark

Bromoxynil

Burning of seedling leaves at margins first; small seedlings may die

Common leaf spot

Lesions are brown, 1-3 mm in diameter, and rarely coalesce; a raised disc (apothecium) to 1 mm in diameter develops in center of mature lesions on upper leaf surface; leaves turn yellow and drop

Fusarium wilt

Favored by warm weather; plants usually have lower leaf yellowing and often a one-sided wilt; longitudinal streaks in xylem tissue of taproots are brick red to dark brown; mycelium may be seen in xylem vessel elements

Iron deficiency

Yellowish green interveinal chlorosis, first in youngest leaves; leaves turn yellow, then white; youngest leaves may die

Lucerne transient streak

Leaf vein chlorosis, chlorotic vein banding, and leaf distortion in spring growth

Magnesium deficiency

Interveinal chlorosis of lower leaves first; leaf margins initially remain green but later become chlorotic or red and finally die

Manganese deficiency

Reduced growth and severe interveinal chlorosis of the youngest leaves, followed by light brown necrotic areas on upper leaf surface

Manganese toxicity

Older leaves become chlorotic at leaf margins, which cup downward and develop small necrotic spots

Metribuzin; hexazinone; terbacil

Plants in established stands are stunted; leaves chlorotic and/or necrotic

Nitrogen deficiency; ineffective nodulation

Yellow older leaves; roots poorly nodulated

Red clover vein mosaic

Chlorosis at leaf edges and yellow vein mosaic that becomes necrotic

Spring black stem and leaf spot

Favored by low temperatures; small, irregular, black to brown spots enlarge and coalesce on leaves; infected leaves turn yellow, wither, and fall; stems blacken; pycnidia with conidia on overwintered stems

Sulfur dioxide

Necrotic margins and interveinal leaf areas on older leaves first; chlorosis also common

Verticillium wilt

Symptoms are more evident at moderate temperatures; leaves become yellow, often in a V-shaped or one-sided pattern, plants wilt, and xylem tissue of taproot becomes brown; stem remains green for a while after all leaves on a shoot are killed

Witches'-broom

Short, spindly shoots are produced profusely on crowns; leaves are puckered, small, and marginally to generally chlorotic


YELLOW/ORANGE

Yellow leaf blotch

Most prevalent in late spring and early summer; lesions elongate; chlorotic to yellowish-orange blotches parallel to leaf veins; numerous dark pinpoint dots (pycnidia) develop in the blotched areas


No Distinct Discoloration

2,4-D; MCPA

Twisted shoot terminals

Alfalfa enation

Erect, bushy plants with crinkled leaves and enations on underside of leaflet veins; flowers are deformed

EPTC

Seedlings are stunted due to poor root development; leaves may be malformed

Ozone

Chlorosis with small necrotic areas between veins of older leaves

Flower

No Distinct Discoloration

Alfalfa enation

Erect, bushy plants with crinkled leaves and enations on underside of leaflet veins; flowers are deformed

Calcium deficiency

Root growth is severely impaired, and rotting may occur; petioles collapse and undersides of leaflets become reddish purple