Alfalfa Disease Management

AD-001 Disease Information Note
Jack Bailey, Extension Plant Pathology Specialist
Lee Campbell, Research Plant Pathologist

Introduction

Plant diseases can affect both the quality and yield in alfalfa. Disease problems can be separated into three major categories based on symptoms and effects: those that affect stand establishment and reduce persistence, those that cause root rot and stem blights, and those that cause leaf spots.

Diseases which cause a reduction in stand and longevity include anthracnose, Fusarium wilt, Phytophthora root rot, Sclerotinia crown and stem rot, bacterial wilt, alfalfa mosaic and damping off. Alfalfa cultivars are available with resistance to anthracnose, Fusarium wilt, Phytophthora root rot and bacterial wilt. Sclerotinia crown and stem rot damage is sporadic and sometimes severe; however, a usable form of resistance has not yet been identified. Cultivars selected for adaptation in the southeast sustain less Sclerotinia crown and stem rot damage than those adapted to other regions.

Diseases which reduce stand persistance, such as stem blights and root rots, are widely distributed throughout North Carolina. These diseases can interrupt the flow of water and minerals causing wilting or leafdrop, and often result in shoot death. The most common stem blights are spring black stem, summer black stem, Rhizoctonia stem and foliar blight, and Sclerotinia blight. In some years southern blight can be serious in the coastal plain. At the present time, few control measures are known for these stem and foliar blight diseases. However, planting cultivars adapted to the southeast and selecting sites conducive to growth will have an overall beneficial effect. The most common root rot is Phytophthora root rot and as noted above, resistant cultivars should be used to control this disease.

Leaf spots interfere with photosynthesis and cause premature defoliation. The most serious in our state are Lepto leaf spot, Stemphylium leaf spot, spring leaf spot and summer leaf spot. Rust occurs during late summer and fall in the coastal plain and piedmont regions and can be severe if harvest is delayed. Rusts can be controlled by planting resistant cultivars, but cultivars resistant to the four leaf spots are not available.

Disease Identification

Alfalfa mosaic (alfalfa mosaic virus): Infected plants have a light green or yellow color which may appear between the leaf veins. Plants may be stunted. Leaves may also be misshapen. This disease will normally cause plants to die after several seasons. Symptoms of alfalfa mosaic are most noticeable on new growth in the spring. However, many infected plants never show recognizable symptoms.

Anthracnose (Colletotrichum trifolii): Symptoms vary from a few irregularly shaped blackened spots to large, sunken, oval to diamond-shaped lesions on stems. Examination with a magnifying glass reveals tiny black hairs within these spots. Stems eventually turn straw colored and die. These dead shoots are often easily seen scattered through the field in summer and fall. This disease can move into the crown of the plant causing a bluish-black discoloration which results in damage to the whole plant.

Bacterial wilt (Corynebacterium insidiosum): Infected plants are usually scattered throughout a stand and are detected by their yellow-green color and stunted growth. Mild symptoms consist of leaf mottling with a slight cupping or upward curling of the leaflets and some reduction in plant height. Severely infected plants are stunted and yellow-green, with many spindly stems and small, distorted leaflets. Diseased plants are usually most evident in the regrowth after clipping. Cross sections of taproots show a yellowish brown discoloration in contrast to the white color of healthy taproots.

Fusarium wilt (Fusarium oxysporum f. sp. medicagines): Wilting shoots are the first evidence of the disease. In early stages, the leaves may wilt during the day and regain turgidity at night. Bleaching of the leaves and stems follows, and a reddish tinge often develops in the leaves. Only one side of a plant may be affected at first, and after several months, the entire plant dies. Dark or reddish brown streaks occur in the roots appearing in cross section as small partial or complete rings.

Fusarium wilt can be distinguished from bacterial wilt by the dark or reddish brown discoloration in the root in contrast to the light or yellowish brown discoloration associated with bacterial wilt. The discolored tissue also is more sharply defined in Fusarium wilt. In Fusarium wilt, the bark usually remains unaffected.

Lepto leafspot (Leptosphaerulina briosiana): This leaf spot primarily affects young leaves, but also attacks other above ground parts. Leaf symptoms vary with the plant's age, stage of growth, and environment. Lesions often start as small black spots and either remain "pepper spots," as on white clover, or enlarge to oval to round "eyespots" 1-3 mm in diameter. The lesions have light brown to tan centers and darker brown borders - often surrounded by a white or slightly yellow ring. When conditions favoring infection and disease development coincide with rapid regrowth, lesions appear as rather large, light tan to almost white areas that merge to kill the entire leaf. High light intensity increases lesion size. Dead leaflets and petioles often remain attached to the stem for a time. In older growth, the young upper leaves become infected and have typical symptoms, but seldom die before harvesting.

Phytophthora root rot (Phytophthora megasperma): Infected plants wilt, and the foliage, particularly the lower leaves, becomes yellow. Regrowth of diseased plants is often slow after cutting. Lesions with diffuse margins on the taproots are yellow to brown and usually start where a lateral root emerges. The yellow discoloration of tissue that extends through the root is a diagnostic feature of the disease. Taproots of numerous surviving plants in the field may be rotted off at various depths. If conditions do not favor the disease, new roots form.

Rhizoctonia stem and foliage blight (Rhizoctonia solani): In the root canker phase, the fungus enters the taproot near the area where lateral roots emerge. The elliptically-shaped, sunken cankers are tan to buff and often are darker and more intense in color in the outer margin than in the center of the lesion. During cool months of the year when the pathogen is inactive, the healed lesions blacken. The plant will die if the lesions encompass the root, but if not, new roots will emerge in the fall and support plant growth for another year. In crown bud rot and crown rot phases, brown lesions first appear on the buds and young shoots below and at ground level. As the infection progresses, buds and shoots die and the fungus grows into the crown, preventing further production of new vegetative buds in the affected tissue. Sunken, whitish to brown lesions also develop near the base of a stem and may girdle and kill it. Often, brown bands develop in the older lighter lesions that indicate expansion of the infected areas. Rhizoctonia stem lesions can be mistaken for anthracnose, but are devoid of the black hairs found in anthracnose.

Seedling loss can also occur and is most severe during high temperature and high soil moisture conditions. In contrast to Pythium spp. that cause damping-off of young seedlings only, Rhizoctonia attacks seedlings at any stage of growth. Roots and lower stems become shrunken and brown.

During prolonged periods of wet hot weather, particularly in humid areas, Rhizoctonia attacks leaves and stems, which collapse quickly into a wet rot. In a few days, if favorable weather conditions continue, the fungus spreads to adjacent plants. A characteristic symptom is dead leaves sticking to neighboring leaves and stems by strands of the fungus. Sometimes large areas of dead plants appear as though killed by scalding water.

Rust (Uromyces striatus): Rust is most prevalent during summer and fall. Reddish brown masses of spores form in small dots that appear on the upper surface of the leaflets, petioles, and stems. Leaves may shrivel and fall prematurely. Stems are attacked severely when harvesting is delayed.

Sclerotinia crown and stem rot (Sclerotinia trifoliorum): Infected leaves and stems become yellow and wilted. A white fluffy mass of fungus grows over the dead plant parts or the soil surface and infecting new nearby plants. When no new tissue is available or environmental conditions are unsuitable for continued growth, the fungus produces small hard black structures called sclerotia on or in the stem and crown.

Southern blight (Sclerotium rolfsii): The fungus produces a white cottony growth on the stem or crown near the soil surface. The plant turns tan color and dies. Small light-brown 'BB'-shaped sclerotia form on the stems and crown and on dead plant material on the ground.

Spring black stem and leaf spot (Phoma medicaginis): All aboveground parts of the plant may become infected, and the disease may extend to the crown and upper root. In early spring, numerous small black to dark brown spots develop on the lower leaves, petioles, and stems. Young shoots are often girdled and killed. Irregularly shaped lesions on leaves increase in size, merge, and become lighter brown; leaves turn yellow and often wither before falling. Lesions on stems and petioles enlarge and may girdle and blacken large areas near the base of the plant. The fungus also causes a crown and root rot. In humid areas, seed pods may discolor and shrivel.

Stemphylium leaf spot (Stemphylium botryorum): Spots on leaves are oval and dark brown with lighter center and are usually surrounded by a pale yellow to tan halo. Older leaves may be concentrically ringed and have a "target" appearance. A single lesion may cause a leaflet to turn yellow and drop off prematurely.

Summer black stem and leaf spot (Cercospora medicaginis): Small brown spots on leaves enlarge to form roughly circular, reddish brown to smoky brown lesions 2-6 mm in diameter. When relative humidity is at or near 100%, the lesions become ashy gray due to spore formation. Heavy infections kill leaflets and cause severe defoliation. As the season progresses, dark brown stem lesions enlarge and merge until most of the stem is discolored. Small stems, and petioles may die, resulting in further defoliation.

Control

Disease control in alfalfa is accomplished by using resistant cultivars and by following recommended agronomic practices which assure a vigorously growing crop. Resistant plants (Table 2) will be less diseased than susceptible plants, however, these plants are not immune and may sustain damage and yield loss. Control of rank growth by cutting or grazing will help lower the humidity in the foliage, thus reducing available moisture for pathogen growth and infection. Areas which become heavily diseased with Phytophthora root rot, Sclerotinia crown and stem rot, bacterial wilt, southern blight, or Rhizoctonia stem and foliage blight may need to be taken out of alfalfa production for two to three years or more. When possible, the field should be rotated with corn or sorghum to help prevent the build up of these pests. Deep tillage should be used to aid in decomposing pest infested residues. Fields with a history of Sclerotinia blight may do better if planted in the spring (avoid fall seeding into sod). Poorly drained soils can lead to soil-borne disease problems such as Phytophthora root rot and should be either improved by tillage or other means.

Field Monitoring

Disease control in alfalfa requires that problems be anticipated. Fields should be monitored for diseases in order to determine the best time for harvest and to plan future rotations. For example, diseases are more severe during wet weather. Therefore, if significant leaf spotting is observed and showers are in the forecast, a grower could anticipate rapid defoliation and reduction in hay quality. Dry weather would slow disease progression and reduce the hazard of disease loss.

Fungi which cause root and stem diseases form spores which infest the soil. Future rotations, tillage practices, and cultivar selection should be based on the severity and distribution of these soil-borne pests.

Fields should be scouted weekly during active growth and records kept on insects as well as diseases. As you walk, record the percentage of the leaflets which have leafspots. Look for thinned areas which may have root and stem blights. Record the percentage stand reduction due to this problem. Field mapping is a quick and effective way to maintain these records. For example, draw a picture of each field, circle problem areas, and identify the cause and severity. If you have trouble identifying the problem contact your county agent. Keep these records handy for future reference. An adequate number of stops (observations) to make in any field will depend upon the field size (Table 1).

Table 1. Number of stops to make while scouting in relation to field size.

Field Size
Number of Stops
10 or less
3
11-20
6
21-30
9
31-50
12
51 or more
split into 2 fields

Take an average of the numbers recorded and use these to determine if diseases are getting better, worse, or staying the same.

References

Graham, J. H., D. L. Stuteville, F. I. Frosheiser, and D. C. Erwin. 1979. A compendium of alfalfa diseases. The American Phytopatholgical Society, 3340 Pilot Knob Road, St. Paul, MN 55121.

Flint, M. L., and J. K. Clark. 1985. Integrated pest management for alfalfa hay. Pub. # 3312 (formerly 4104). ANR Publications, Univ. Calif., 6701 San Pablo Ave., Oakland, CA, 94608-1239.

Table 2. Disease resistance in selected alfalfa varieties.

 
Disease
Variety
BW
VW
FW
AN
PRR
120
HR
--
R
LR
R
DK 125
HR
R
R
HR
R
5331
HR
LR
HR
HR
R
5432
HR
R
HR
--
MR
Anstar
R
--
MR
R
--
Apollo
R
--
R
LR
R
Apollo II
R
MR
R
MR
HR
Apollo Supreme
HR
R
HR
HR
R
Arc
LR
--
MR
HR
--
Cimarron
HR
LR
HR
R
MR
Classic
R
--
R
LR
LR
Commandor
R
MR
R
HR
S
DK 135
R
MR
R
MR
MR
Fortress
R
R
R
R
HR
Hi-phy
HR
--
HR
--
MR
WL 316
MR
R
R
HR
MR
Shenandoah
HR
--
HR
HR
HR
WL 320
R
MR
HR
MR
R
S. Special
R
LR
HR
MR
MR
Florida 77
--
--
HR
MR
--


Abbreviations for resistance levels: S=susceptible; LR=low resistance; MR=moderate resistance; R=resistant; HR=high resistance; blank spaces (--) indicates variety is susceptible or has not been adequately tested.
Abbreviations: BW=bacterial wilt; VW=verticillium wilt; FW=fusarium wilt; AN=anthracnose; PRR=Phytopthora root rot.

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