Reports on Plant Diseases |
RPD No. 302 - Root and Crown Troubles
of Alfalfa
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May 1999
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[ Cold or
Winter Injury ] [ Frost Injury ] [ Heaving ]
[ Damage from Low Temperatures ] [ Ice
Sheets ]
[ Dry Root and Crown Rots ] [ Phytophthora
Root Rot ] [ Anthracnose ]
[ Sclerotinia ] [ Fusarium
Wilt ] [ Violet Root Rot ] [ Verticillium
Wilt ]
[ Stagonospora ] [ Rhizoctonia ]
[ Verticillium] [ Nematodes ]
[ Additional Information ] [ Control ]
Root and crown troubles of alfalfa may have several causes. The
chief possibilities are: (1) cold or winter injury; and (2) root
and crown rots caused by a number of common soilborne fungi. These
fungi weaken affected plants and materially reduce the longevity
of the stand, yield, and hay quality. Damage is most severe after
winter injury, summer drought, mechanical injury to the crown,
or root damage by insects. Alfalfa plants in fields with low fertility,
poor drainage, a high water table, and cut or heavily grazed during
the last 5 to 6 weeks of the growing season are usually the first
to be attacked. Nematodes may also be involved in root and crown
troubles.
Cold or Winter Injury
Several distinct types of cold or winter injury may damage alfalfa
stands: frost injury to the growing plant in the spring or early
fall; heaving from alternate freezing and thawing; low-temperature
and desiccation damage to the taproot and crown; and ice-sheet
injury. Winter injuries provide avenues of entrance for pathogenic
fungi and bacteria, making the plant more prone to disease.
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Figure 1. Advanced Fusarium root and crown rot.
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Frost Injury
After alfalfa plants begin to grow in early spring, frost injury
may kill the tops, turning dead leaves a light tan, delaying and
reducing the first harvest. Water in the plants may sometimes remain
unfrozen (supercooled) when temperatures are below the freezing
point, however, if such plants are suddenly jarred, ice crystals
form and the top growth is killed. If an animal walks through an
alfalfa field that is supercooled below the freezing point, every
plant that is disturbed will freeze. Peculiar brown streaks sometimes
seen in alfalfa fields in the spring are the result. The foliage
of alfalfa varieties differs in frost tolerance but this is not
always associated with winter hardiness.
Heaving
Heaving due to alternate freezing and thawing often severely injures
stands of alfalfa by breaking off or mechanically injuring the
taproot and lateral roots. Damage occurs most often on heavy, wet,
poorly drained claypan soils that have a temporary or permanent
water table that is high during the winter.
Damage from Low Temperatures
Cold and desiccation may kill or injure crown buds resulting in
weak shoots. New buds may form on the lower portions of the crown
and as the season progresses, recovery may be nearly normal. Thus,
stands that appear unproductive early in the spring may produce
normal yields later in the season. When more severe, a dark, internal
discoloration develops in the crown and taproot, particularly during
winters with little or no snow cover. This is usually followed
by root and crown rot when affected plants may die. Most damage
occurs when unadapted varieties are grown. In Illinois, stands
of California and Arizona common and other nonhardy alfalfas are
likely to be killed by low temperatures during an average winter;
adapted varieties may also be injured and killed.
Ice Sheets
The most serious winter injury is produced by ice sheets which
prevent normal exchange of gases with increased concentration of
carbon dioxide and depletion of oxygen leading to tissue decline
and death. Less damage usually occurs when stubble protrudes through
the ice, permitting gas exchange. Sleet storms or freezing rain
may cause ice to form over wide areas often resulting in a complete
loss of the stand. Ice sheets can also form in poorly drained areas
where water from rain or melting snow accumulates. The alfalfa
varieties that are most resistant to extreme cold are also the
ones most resistant to ice-sheet damage.
Winter injury is usually complicated and made more severe by the
establishment of parasitic fungi and bacteria within the damaged
plant tissue. Bacterial wilt of alfalfa is often most prevalent
in plants damaged by winter injury.
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Dry Root and Crown Rots, and Decline
Dry root and Crown Rots, and Decline are caused by a diverse complex
of fungi (primarily species of Fusarium, Rhizoctonia, Pythium,
Phoma medicaginis, Mycoleptodiscus terrestris, Stagonospora meliloti [sexual
stage Leptosphaeria pratensis], and macrophomina phaseoli).
The fungi associated with these rots vary greatly, depending on
the area and season. Determining the specific fungus or fungi causing
crown and root rot is extremely difficult because the same organisms
may be associated with both healthy and diseased plants. In addition,
the fungi isolated from diseased tissue may only be capable of
causing disease under specific conditions or only in specific associations
with other organisms.
Affected plants are often stunted, off-color, and lack vigor,
then commonly wilt, become yellow to straw-colored, turn brown,
and finally die. The stand is thinned, sometimes in circular or
irregular patches; the taproot and lateral roots and crowns are
discolored and rotted; light brown-to-black areas appear on and
within affected roots and crowns (Figure 1). Fungus growth (mycelium)
may be found in or on diseased areas with roots and crowns shredded
leaving a lack of feeder roots and nodulation. Various plant stresses,
including foliar diseases, injury from foliar and root-feeding
insects, frequent or untimely harvests, early frosts, poor fertility,
severe winter weather, low light intensity, low soil pH, poorly
drained soils, or soils with poor structure, increase the rapidity
of this crown and root rot complex. The full effect of dry root
and crown rots that develop during one growing season may not be
evident until the following spring when plants fail to recover.
Secondary fungi and bacteria commonly enter rotted tissue to confuse
the symptoms, especially under conditions of high moisture. The
causal fungi overseason in soil and plant tissues with spread occurring
by the water, all types of equipment, and by sowing infected seed.
This can cause seeds to rot, seedlings to wilt, dry up, and die
(damp-off). Stands may be poor after cool, wet weather in the spring.
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Phytophthora Root Rot
Unlike the dry root and crown rot complex, Phytophthora
root rot, which is caused by the soilborne fungus Phytophthora
megasperma, develops rapidly in fields where there is an excess
of soil moisture caused by inadequate vertical or horizontal drainage
and/or irrigation. The disease can be devastating to seedling stands
in cool, very wet weather, but may also cause a severe loss of
stand in established fields (Figure 2). When conditions are favorable
for infection and disease developmentextended periods of
cool, rainy weather and standing wateraffected plants wilt
and die with little or no stunting. When conditions are less favorablewarm
and dry weather after infectionthe plants are often stunted,
turn yellowish, and die much more slowly ("melting-out").
Regrowth of diseased plants is often slow after cutting. Infected
corn and taproot tissue becomes soft, water-soaked, and turns yellow
to dark brownoften in an area surrounding a lateral root
wit rotted areas later turning black (Figure 3). Taproots of surviving
plants in the field rot off at various depths. When decay is near
the crown, affected plants can be pulled up easily, leaving the
taproot in the soil. The Phytophthora fungus may also cause seedlings
to damp-off before or after emergence with young plants suddenly
turning yellow or reddish, then wither and die. The causal fungus
is believed to survive between crops in soil and plant residues,
primarily as thick-walled resting spores (oospore). These spores
are spread by flowing water, tillage equipment, and any means that
moves infested soil and crop debris. Optimum growth of the Phytophthora fungus
and infection occurs at 76° to 82°F (24° to 27°C);
maximum is about 86°F (30°C).
Anthracnose
Anthracnose, caused by the fungus Colletotrichum trifolii,
appears following prolonged periods of hot and wet summer weather
in the southern part of Illinois. Scattered, dead, pearly- white
to straw-colored stems with the shoot tips commonly curved downward
into "shepherd's crooks" occur in late summer. Symptoms
vary from a few irregularly shaped blackened areas on resistant
stems to large, sunken, tan-to-dark brown, oval to diamond-shaped
lesions, ½ to 1 ½ inches long with a dark brown border,
on the lower stems of susceptible plants. These straw-colored lesions
are soon sprinkled with black specks (acervuli) that are easily
seen with a hand lens or reading glass. The lesions enlarge, merge,
girdle, and kill one to several stems on a plant. Invasion of the
crown results in a bluish-black crown rot (Figure 4). This symptom
is often observed when killed stems are broken off at the crown.
Other symptoms of anthracnose include blackening and killing of
petioles.
The anthracnose fungus overwinters as mycelium in
living stem-crown tissue and crop refuse and on the surfaces of
protected harvesting equipment. This is an important means of spread
from an old field to a new planting. The Colletotrichum fungus
is also commonly disseminated by waterborne microscopic spores
(conidia) during warm humid weather. Spore masses form on stem
lesions throughout the growing season supplying a ready source
of secondary infections. The fungus can grow down infected stems
into the crown and cause decay and predisposing the plant to winter
injury; or it may kill the plant directly. Stands of susceptible
alfalfa varieties can be reduced significantly in two or more years.
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Figure 2. Pattern in a diseased alfalfa field, plants killed-out
in the low spots by Phytophthora root rot.
Figure 3. Diseased tissue soft, water-soaked, turning dark;
caused by Phythophthora root rot. (Courtesy Dr. F.I. Frosheiser).
Figure 4. Anthracnose crown rot; infected plant to right
(Courtesy Dr. A.S. Williams).
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Figure 5. Sclerotinia crown and stem rot. Note cottony mold,
black round bodies (sclerotia) embedded (British Ministry of Ag.).
Figure 6. Sclerotia and apothecia of Sclerotinia sclerotiorum (Courtesy
Dr. K.T. Leath).
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Sclerotinia Crown and
Stem Rot
Sclerotinia crown and stem rot, caused by the fungus Sclerotinia
(Whetzelinia) sclerotiorum or trifoliorum, is evident
by the small-to-large patches of dead and dying plants in the
early spring. Plants of all ages are susceptible. Infected leaves
and stems become yellow, wilt, and collapse from a light brown,
mushy rot of the stem bases and crown. A cottony mold grows over
the dead plant parts and adjacent soil surface. Round-to-irregular
black bodies (sclerotia), up to 8 to 10 mm in diameter, form
in the cottony mold and in or on the dead stem and crown tissue
(Figure 5). The sclerotia fall to the soil and germinate in cool,
moist weather to produce one to several small, light brown, cup-shaped
structures called apothecia (Figure 6). Large numbers of microscopic
ascospores are shot out of the apothecia and spread to nearby
plants by air currents. Infections occur during cool, moist periods.
The fungus is also spread by moving infested soil and diseased
crop refuse. Sclerotinia crown and stem rot is favored by a prolonged
snow cover, cool and wet spring and autumn weather, and rank
foliar growth. The fungus grows between 28° and 82°F
(-2° and 27°C) with an optimum around 60°F (15° to
16°C).
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Fusarium Wilt
Although usually caused by the soilborne fungus Fusarium oxysporum f.
sp. medicaginis, Fusarium wilt may also be incited by F. o.
f. sp. vasinfectum races 1 and 2, and F. o. f. sp. cassia.
Scattered plants in a field wilt partially or completely, gradually
turn a bleached yellow, then wither and die. A dark or reddish
brown discoloration is seen in the stele, between the bark and
the wood, when the taproot is cut through (Figure 7). A considerable
loss of stand may occur over several years. The Fusarium fungi
can live almost indefinitely in soil and plant debris as chlamydospores
and mycelium. The fungi are spread by soil water, equipment, and
infected hay. Fusarium wilt is favored by relatively high soil
temperatures, moist weather, and a lack of crop rotation. The only
practical control is to grow resistant alfalfa varieties. A number
of cultivars contain a moderate to a high percentage of resistant
plants.
Stagonospora Leaf Spot and Root Rot
Stagonospora leaf spot infections, caused by Stagonospora meliloti
(sexual stage Leptosphaeria pratensis), sometimes invade the crown
and taproot. When large stems, crowns, and taproots are cut open,
small pockets of an orange-red material give the solid and dry
diseased tissue an irregularly speckled appearance (Figure 8).
The fungus progresses slowly in root tissue, but often fine, bright
red streaks occur in the stele (xylem) well beyond the decayed
areas in the crown and taproot. The exterior of an infected root
has a rough texture. Eventually, the root decays and the plant
dies. The Stagonospora fungus overseasons as mycelium in crop debris,
and is believed to be spread by air- and water-borne spores (conidia
and ascospores). The root rot phase appears to develop from stem
and crown infections and proceeds slowly for two or three years.
The disease is most severe during prolonged periods of warm, moist
weather and where alfalfa or sweetclover are not rotated with corn,
soybeans, small grains, sorghum, or forage grasses.
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Figure 7. Fusarium wilt (Courtesy Dr. F.I. Frosheiser).
Figure 8. Stagnospora crown and root rot.
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Figure 9. Rhizoctonia stem blight and canker (Dr. D.C. Erwin).
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Rhizoctonia Stem Blight, Root Canker,
Crown and Bud Rot
These diseases are caused by the very common soilborne fungus
Rhizoctonia solani and occur throughout the world. The foliage
on affected plants wilts and blights during prolonged periods of
hot, wet, and humid weather from dark, somewhat sunken, girdling
cankers near the base of stems, at the crown and the taproot near
where lateral roots emerge (Figure 9). Brown bands often develop
in the older, light-colored lesions. Coarse brown mycelium may
be evident on the surface of diseased tissue. Rhizoctonia solani
survives in soil and crop debris as mycelium and small (less than
0.5 mm in diameter), roundish-to-irregular, dark brown-to-black
bodies (bulbils; sclerotia-like structures). The fungus is spread
primarily by equipment and infected hay. Optimum growth of the
fungus occurs at 77° to 87°F (25° to 30°C) with
a maximum around 95°F (35°C).
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Violet Root Rot
Violet root rot, caused by the soilborne fungus Rhizoctonia crocorum
(R. Violaceae), is a widely distributed but generally minor root
rot of alfalfa that is most common after midsummer. The foliage
of affected plants turns straw-yellow then brown in enlarging,
circular to irregular patches. All or nearly all the plants within
a patch die (Figure 10). Affected plants at the margin of a patch
are conspicuously brown when compared to surrounding green healthy
plants.
Diseased roots and crowns are covered with a felt-like bright
violet to cinnamon mantle of mycelium that often extends 8 inches
(20 cm) or more below the soil line (Figure 11). As the root decays,
its bark loosens while the central cylinder becomes soft and shredded.
Speck-sized black bulbils are commonly present on dying roots.
The Rhizoctonia fungus may persist in the soil and plant
debris for 3 to 20 years. The optimum temperature for growth of
the causal fungus is about 77°F (25°C).
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Figure 10. Circular patch of alfalfa affected by violet
root rot (U. of Wis. Photo).
Figure 11. Alfalfa stems and roots covered with felt-like
mantle of mycelium of violet root rot fungus (U. of Wis. Photo).
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Verticillium Wilt
Verticillium wilt, caused by the common soilborne fungus Verticillium
albo-atrum, is a disease of cool temperate regions throughout
the world. It was first reported in the United States in 1977
and in northern Illinois in 1984. The upper leaves of affected
plants temporarily droop and wilt on warm summer days. Later,
the lower leaves and shoots wilt, become pale, then yellow, and
finally bleached and withered (Figure 12). The stems may remain
green after the leaves turn yellow and new shoots can develop
from the crown but they rapidly wilt and die. Diseased plants
are commonly stunted with yellowed and withered shoots and twisted
leaves. Fungus growth (conidiophores bearing conidia) over the
base of infected stems in humid weather gives them a gray appearance.
When the roots of diseased plants are sliced through, a light
to dark brown discoloration is evident internally in the vascular
(xylem) tissue.
The Verticillium fungus overwinters as dark, thick-walled
mycelium in plant debris, in infected plants, and possibly in weed
hosts. Microscopic spores (conidia) are produced during cool wet
weather and are spread primarily by wind currents and windblown
debris. The fungus is also transported on trash with seed and can
persist directly on alfalfa seed for only short periods of time.
Secondary spread during the growing season is mainly by mowing
and windblown conidia and infested debris. Optimum growth of the Verticillium fungus
occurs at 68 to 73 F (20 to 22 C) with a maximum at 87 F (30 C).
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Figure 12. Verticilium wilt: (Left) withered, twisted and
yellowed leaves; (Right) close-up of leaf showing typical V-shaped
chlorotic areas (Dr. K.T. Leath).
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Figure 13. Lesion nematodes: (left) poor stand discolored,
stunted alfalfa; (right) two nematodes inside alfalfa root (J.A.
Thies, USDA-ARS, MN).
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Nematodes
Dagger (Xiphinema americanum), lesion (Pratylenchus
penetrans), and northern root-knot (Meloidogyne hapla)
nematodes are the most common root-infecting types in Illinois.
All three nematodes reduce plant vigor, cause yellowing and stunting
in round to irregular patches throughout a field and may predispose
plants to other root and crown problems (Figure 13). Dagger nematodes
reduce the number of fine-feeder roots and the overall root mass.
Lesion nematodes cause oblong, yellow to dark brown spots or
lesions on the roots, ranging from pinpoint size to several millimeters
in length. Root-knot nematodes produce small root galls visible
only when the roots are closely examined. The nematodes may kill
young plants and increase the incidence of bacterial and Fusarium
wilts in older resistant as well as susceptible varieties. Dagger
and lesion nematodes can only be diagnosed by submitting soil
and root samples for analysis to a trained nematologist. Take
soil and root samples to a depth of 10 inches from the base of
the plants at the margin of the affected area, place them in
a sturdy plastic bag, and keep them cool until they are submitted.
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Additional Information
Several of the same fungi that cause alfalfa roots and crowns to rot
also induce seed decay and seedling blights. It is sometimes difficult
to clearly separate stem- and crown-rotting problems.
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1. Grow only well-adapted, high-yielding, winter-hardy varieties
suggested for growing in your area. Varieties differ in their resistance
to bacterial wilt, Phytophthora root rot, Anthracnose, Verticillium
wilt, and winter hardiness.
2. Maintain vigorous growth by maintaining a high, well-balanced
fertility level, especially of phosphorus and potassium, based on a
soil test.
3. Sow certified, thoroughly cleaned, disease-free seed grown in
arid regions. Several leaf and stem diseases plus the organisms
causing seed decay, seedling blights, and damping-off are spread by
planting contaminated seed. Many of the fungi that cause dry root and
crown rots are also carried in or on alfalfa seed.
4. Plant in warm, well-drained soil in a well-prepared seedbed that
is slightly acid to neutral (pH 6.5 to 7.0). Avoid low areas that are
subject to flooding and/or excessive irrigation. Deep tillage to
break up compacted layers or plow soles and to improve internal percolation
of water will help to eliminate conditions favoring infection of several
root-rotting fungi.
5. Control insects following suggestions of University of Illinois
Extension entomologists. Insects commonly provide wounds by which
wilt, crown-rotting, and root-rotting fungi and bacteria gain entry.
6. Maintain a proper cutting schedule. Avoid rank growth and
retain hay quality by harvesting on time and not applying excessive amounts
of nitrogen. Avoid overgrazing or overcutting. Do not cut or graze alfalfa
during the last 5 or 6 weeks of the growing season (about September 1
in northern Illinois to October 15 in southern Illinois). A top growth
of 8 to 12 inches is needed to build reserves before winter. Fields on
well-drained soils may be cut or pastured after the growing season has
ended.
7. Thoroughly clean all harvesting equipment of debris before the
first harvest in the spring and again periodically during the growing
season when going from fields with infected plants to other plantings.
8. Where root and crown rots have limited alfalfa production, a three-year
or longer rotation with corn, sorghum, small grains, or forage grasses
is desirable. Exclude other forage legumes in the rotationsuch
as sweetclover, clovers, birdsfoot trefoil, lespedezas, and crown vetch.
9. Avoid mechanical injuries to the stems and crowns as much as possible.
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For further information concerning diseases of crucifers
and other vegetables, contact Mohammad Babadoost, Extension Specialist
in Fruit and Vegetable Diseases, Department of Crop Sciences, University
of Illinois at Urbana-Champaign.
University of Illinois Extension provides
equal opportunities in programs and employment.
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