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Alfalfa Diseases and Deficiencies
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| Verticillium wilt. Early symptoms, including drooping
and curling of leaflets.
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| Verticillium wilt. Advanced symptoms. |
Verticillium wilt causes major reductions in alfalfa stands by the end of the second or third year. This disease is widespread on irrigated fields throughout the interior south of Quesnel, and has been found on the limited acreage in the Lower Fraser Valley.
Identification:
Symptoms first appear on scattered plants as yellow, twisted or drooping leaves on one or more stems. A V-shaped, yellow blotch on the leaflet tip is often an early sign of infection. Eventually leaves on infected stems turn from green to yellow to a bleached tan colour. Infected plants become stunted, and are very susceptible to winter kill. The key plant symptom is that stems remain green and upright even though leaves are wilting or near death.
Disorders with similar symptoms:
Drought: Look for wilting of all plants in a patch rather than individual plants. Stems tend to wilt.Frost: Look for drooping and wilting of the whole plant and bleaching of leaves. Sometimes just the top of the plant is affected.
Boron deficiency: Upper leaves turn bronze to yellow without wilting, and leaves are cupped or curled.
How it Spreads:
Verticillium wilt can be introduced into a field by contaminated farm machinery, contaminated seed, or by air-borne spores from neighbouring, infested fields. Once the disease is present it is easily spread while cutting the crop. Other methods of spread include irrigation run-off from infested fields, movement of contaminated hay, and in manure from animals fed contaminated hay. It can also be spread by spores adhering to insects, such as grasshoppers, aphids and leafcutter bees. It can survive in volunteer alfalfa and many broadleaf weeds during a rotation.Control:
Resistant cultivars - The use of resistant cultivars offers the best method of controlling Verticillium wilt. Refer to the alfalfa disease resistance ratings on-line factsheet or contact your district agriculturist or seed dealer for information on resistant varieties best suited to your area.Sanitation - Resistant cultivars are not completely resistant, therefore it is also important to practice sanitation and crop rotation. Rotate with non-host crops such as cereals for 2 to 3 years. Control weeds and volunteer alfalfa during the rotation. Cut healthy (generally younger) crops before diseased crops. Clean plant debris from your harvesting equipment when moving from diseased to healthy fields. Do not transport infected hay to areas which do not have Verticillium wilt problems. A thiram seed treatment is available for seed known to be infected with Verticillium wilt. However, because the seed treatment does not completely eradicate the fungus, it is recommended that disease-free seed be used. Do not follow an infected crop with sainfoin or soybean as these crops can also be killed by the alfalfa strain of Verticillium.
Bacterial Wilt
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| Bacterial wilt. Stunting, small necrotic leaves, and vascular discolouration in taproot. |
Identification:
Symptoms usually appear in the third or fourth year after planting. A yellowish brown discoloration or ring pattern is visible in the woody cylinder of the tap root. Plants are stunted and produce many yellow, spindly shoots with small, cupped, yellow to brown leaves. Foliar symptoms are most evident on regrowth after cutting. Infected plants are more susceptible to winter kill, particularly in southern areas such as Creston and Okanagan Falls.Diseases with similar symptoms:
Witches Broom: Foliar symptoms are similar, however with witches broom the shoots are finer and more abundant with no browning of leaves, and the tap root remains white.Downy Mildew: This disease also causes yellowing and distortion of leaves, however a grayish cottony fungal growth is also visible on the underside of the leaflets.
Verticillium wilt: Foliar symptoms are similar. Accurate diagnosis usually requires a laboratory test.
How it spreads:
Bacterial wilt can be spread by infected seed or hay, surface water, tillage, and on mowing and harvesting equipment. The stem nematode increases susceptibility to bacterial wilt and also spreads the pathogen.Control:
Resistant cultivars - The use of resistant cultivars is the best control method. Most varieties with Verticillium wilt resistance also have resistance to bacterial wilt. If stem nematode is also a problem, choose a variety that has stem nematode resistance as well.Sanitation - Harvest young or healthy stands before older, infected stands, and cut hay when plants are dry. Practice crop rotation. Thoroughly clean forage harvesters when moving from infested to non-infested fields.
Spring Black Stem
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| Spring black stem. Leaf symptoms. |
Identification:
Small, dark brown or black spots with irregular borders first appear on the lower leaves and may expand until much of the leaflet is covered. Infected leaflets turn yellow and drop prematurely, reducing forage yield and quality. Dark green to black stem lesions may enlarge until most of the lower stem turns black. Infection of flower stalks and pods causes blossom and pod drop, shriveled seed and reduced seed yields. The disease is favored by cool, moist weather and is therefore more damaging in the spring and fall.Control:
When black stem is prevalent, early cutting will reduce leaf loss. Rotate with non-legume crops. Spring burning has been recommended for seed fields, but may cause injury if growth has already begun.Downy Mildew
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| Downy mildew. Pale leaves with grayish fungal growth on undersides. |
Downy mildew is common but seldom causes severe damage. It is favored by cool and wet or humid weather.
Identification:
Leaves turn light green to yellow, and appear twisted and curled. The tops of shoots often become dwarfed. A fine, grayish to violet fungal growth can be seen on the underside of affected leaflets. Downy mildew is most likely to be a problem in the spring.Control:
Varieties with resistance to downy mildew are available. Contact your district agriculturist for information on resistant varieties. If mildew is severe in the spring, do not delay the first cut. Harvesting removes most of the infected tissue and reduces the humidity in the crop canopy. Warmer summer temperatures will reduce the chance of mildew damage to the regrowth. Rotate with non-legume crops.Phytophthora Root Rot
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| Phytophthora root rot. |
Phytophthora root rot is only severe in wet or poorly drained soil. Low soil temperatures and high clay content also increase the risk. Seedling stands can be severely damaged, but losses may also occur in established fields.
Identification:
Seedling damping-off can be caused by Phytophthora when conditions are cool and wet. Symptoms on established plants include yellowing of older leaves, followed by general wilting and plant death. Infected tap roots have yellow-brown patches of rot, often where lateral roots emerge.Control:
Use resistant varieties where the problem is known to exist. Refer to the factsheet "Alfalfa Variety Selection" or contact your district agriculturist for information on resistant varieties. Crop rotation with non-legumes will help to reduce disease incidence. Minimize standing water in irrigated fields, and improve drainage where practical.Anthracnose
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| Anthracnose stem lesion. |
Anthracnose is a fungal disease that occasionally causes damage in high-rainfall areas. Anthracnose on alfalfa is caused by a different fungus than northern anthracnose on clover.
Identification:
Symptoms appear as sunken, oval to diamond shaped lesions on stems. These spots are straw coloured with darker borders. Damage occurs when the spots expand and girdle stems, causing dead shoots in the summer and fall. Anthracnose can also cause a crown rot. Stems broken off at crown level have a bluish-black discoloration at the base.Control:
Most varieties have at least a moderate level of resistance. For information on resistant varieties, contact your district agriculturist. If anthracnose is suspected, clean plant debris from harvesting equipment after cutting infected fields to minimize spread to healthy fields.Sclerotinia Crown and Stem Rot
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| Sclerotinia stem rot. Whitish fungal growth and black sclerotia on lower stems. |
Identification:
Scattered plants turn yellow, then wilt and die. A white, cottony growth of mycelium is visible on the lower stems and crowns during cool, wet weather. Infected stems become soft and mushy at the base. Crown infection leads to plant death. Small, hard, black sclerotia are produced on or inside stem and crown tissue. Sclerotia are fungal resting structures that germinate to produce tiny cup-like mushrooms, which in turn, produce air-borne spores. Snow cover increases the development of Sclerotinia crown and stem rot during the winter. Sclerotinia is less damaging to alfalfa than to clovers.Control:
Plow deeply to bury sclerotia. Use sclerotia-free seed to avoid introducing the disease. Rotate with cereal or grass crops. Where sclerotinia was severe, wait three to four years before replanting forage legumes.Alfalfa Stem Nematode
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| Alfalfa stem nematode. Necrotic crown with infected buds and "white flagging". |
This pest has been identified in Cariboo and southern interior fields in B.C., but it does not occur at damaging levels.
Identification:
The nematode penetrates the developing buds which causes infected stems to enlarge and become discoloured. Growing stems are shortened between leaf nodes. Severe infection under moderate temperatures and high humidity results in blackened stems up to 30 cm above the ground. The number of stems per crown are reduced and eventually the plant dies. Some infested shoots may have white, slightly smaller leaves. This condition is called white flagging.How it spreads:
The stem nematode primarily overwinters in alfalfa crown tissue and young stems covered by snow. It can be spread by infested seed, harvesting equipment and irrigation water.Control:
Use resistant varieties where the problem is known to occur. A two or three year crop rotation with non-legume crops such as cereals usually reduces the nematode population. However, the field can quickly be recontaminated by nematodes moving with machinery, animals or waste water. Control alfalfa volunteers during the rotation. Limit the life of an infested stand to 4 years. Clean equipment before moving from infested to clean fields.Spring Frost
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| Frost damage. |
New growth in the spring is often subject to frost damage, particularly after a period of mild weather. Damage may occur to whole plants, or only to the top leaves. Frozen tissue appears water-soaked. Tolerance of leaves to frost varies among varieties and individual plants, and is not always related to winter hardiness.
Heaving
Identification:
Heaving is most common on heavy, wet soils. Alternate freezing and thawing in early spring cause the soil to expand and exert upward pressure on the plant crown. The tap root will often break 5 to 10 cm below ground level.Control:
Poorly established or late-seeded crops are most susceptible to heaving injury. Plant early so that plants are well established by fall. Retain tall stubble to encourage good snow retention.Boron deficiency
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| Boron deficiency. Yellow and reddish leaf discolouration. |
Identification:
Symptoms of boron deficiency include yellowing or bronzing of leaves, followed by reddish discolouration along the leaflet margins and undersides of the youngest fully developed leaves. Eventually the upper leaf surface also turns red or reddish yellow. The lower leaves remain green. Plant tops become bunched due to shortened internodes and the growing point may die. Severe boron deficiency prevents plants from flowering or setting seed. Symptoms are more severe under dry conditions. Alfalfa which winterkills the first season is an indication of boron deficiency. The use of boron on borderline acid soils (pH over 5.9) will greatly improve alfalfa survival.Control:
Alfalfa will generally show deficiency symptoms if a soil test indicates levels below 0.75 ppm boron or if tissue tests of the top 7.5 cm of alfalfa measure below 20 ppm boron.Application of 3 to 4 kg/ha (2.7 to 3.6 lb./acre) of actual boron once every three years is generally recommended. Boron may be applied in custom blends or as a broadcast application of Borate 40 or Borate 68. Boron is best applied to new or established alfalfa fields in early fall. Boron should never be applied when seeding alfalfa with a cereal nurse crop, because cereals are susceptible to injury when boron is in close contact with the germinating seed. Peas, beans and sometimes cereals can be injured by residual boron left from a recent application to alfalfa.
Potassium deficiency
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| Potassium deficiency. Pattern of white leaf spots.
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| Phosphorus (left) and potassium (right) deficiency.
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| Sulphur response at Hixon, B.C.
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| Response to lime on acid soil. |
Identification:
Symptoms first appear on the lower leaves as small, white spots along the margins of leaflets. The areas between the spots eventually turn yellow and die. The margins of older leaves may also turn a pinkish cinnamon colour before turning brown.Control:
Fertilize according to soil test results to improve yields. Winter survival may also be improved by the addition of potassium.Phosphorus deficiency
Identification:
Phosphorus deficiency is difficult to recognize. Plants are stunted, and may appear bluish-green in colour. Under some soil conditions plants appear yellowish, similar to nitrogen or sulphur deficiency symptoms.Control:
Fertilize according to soil test results to improve yields.Sulphur deficiency
Identification:
Leaves are pale green or yellowish. The yellowing may affect the new growth or the whole plant, while nitrogen deficiency tends to affect the older leaves first. Sulphur deficiency reduces nitrogen fixation and protein production.Control:
Apply 20 to 33 kg/ha (18 - 29 lbs/acre) of sulphur when soil test levels fall below 20 ppm.Low pH
Identification:
Alfalfa will grow poorly on soils when the pH is less than 5.9. Plants are light green, stunted and stands become thin. When alfalfa is grown on moderately-acid soil, yields and nitrogen content will be reduced.Control:
Apply lime according to soil test results. Applications should be made one to two years before planting alfalfa to allow for thorough incorporation. If lime cannot be applied economically, grow acid-tolerant legumes such as red or alsike clover.
Prepared by G.D. Jesperson.
Plant Pathologist
Crop Protection Program
Kelowna, B.C.
Acknowledgments:
Photographs illustrating anthracnose, Phytophthora root rot and black stem
courtesy of the Western Committee on Plant Diseases. Phosphorus and potassium
deficiency photographs are from the Potash and Phosphate Institute. Sulphur
response photograph courtesy of Al Watkins.
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