Alfalfa Diseases and Insects

 Knowledge Nuggets | Fact Sheets | Research Papers
Knowledge Nuggets

A brief description of the disease, where it is mainly a problem, and control measures are listed below.
  • Anthracnose – mainly a disease of southern and eastern United States. It rarely occurs in Canada.
  • Aphanomyces root rot (ARR) – mainly a disease of eastern and western United States, in wet soils. Symptoms similar to nitrogen deficiency.
  • Bacterial wilt (BW) – in the south and central prairies, especially irrigated areas. Cause serious stand losses in 3-5 yr stands. Plants start out yellow-green and advance to stunted with spindly stems. Almost all cultivars available today are resistant.
  • Blossom blight (BB) - common in cool, wet conditions. Can reduce seed yield by attacking flowers, causing them to shrivel and die.
  • Brown root rot (BRR) - affected plants become yellow and growth is stunted. Will end up with irregular patches of affected plants in a field.
  • Common leaf spot (CLS) – common in all areas where alfalfa is grown. Occurs in 1st and 2nd cuttings and in fall regrowth. Causes reduced yield and lower forage quality.
  • Crown bud rot (CBR) – common in irrigated or moist soils. Progressive decline in stand and yield. Control – delay harvest until the end of the growing season to allow plants time to replenish root reserves, leave 25 cm on plant after the last harvest.
  • Damping-off/seedling blight (DO) - occurs in the prairies in areas with crown rot. Severe in fields where germination is slowed by low seed vigour, cold wet soil, poor seed to soil contact, etc.
  • Downy mildew (DM) - common in the prairies during cool, humid weather.
  • Fusarium wilt (FW) - in the south and central prairies. Causes gradual stand thinning. Symptoms are similar to bacterial wilt, but plants are not stunted. Control – control pea aphids and potato leafhoppers and seed resistant alfalfa varieties.
  • Lepto spot (LS) - favoured by wet conditions and moderate temperatures. Attacks young regrowth and yield and quality are reduced. There are no resistant cultivars available. Best to cut early to avoid leaf loss.
  • Phytopthera root rot (PRR) – a problem in the south and central prairies, in wet or slowly drained soils. Plants have reduced root mass and growth rate, and can die rapidly. Control – crop rotation is not effective, avoid untimely cutting (plants stressed, before a heavy rain), control leaf-feeding insects, use tillage and land-leveling and seeding resistant alfalfa varieties.
  • Root lesion nematodes – moderate problem in eastern Canada and along the southwest coast of B.C.
  • Sclerotinia – occasionally occurs in alfalfa.
  • Spring black stem - occurs almost everywhere alfalfa is produced. Favoured by cool, wet conditions. Causes leaf drop, stem girdling and infection of the crown and roots resulting in death. Reduces yield and quality.
  • Summer black stem – mainly a disease of the eastern and western United States.
  • Verticillium Wilt – one of the most destructive diseases in North America on alfalfa. Found in almost all areas alfalfa is produced. Upper leaves start to wilt and yellow, often leaving a green stem. Stem turns grey and then black as fungal spores grow. Crown infection results in death. Yield can be reduced up to 50%. Control – harvest non-infected fields first.
  • Winter crown rot or snow mold (WCR) – dark brown rot of crown. Occasionally an issue in central and northern areas when snow accumulates before soil temperatures drop. Control – plant winter hardy cultivars
  • Yellow leaf blotch (YLB) - most destructive in northern Alberta. Yellow blotches form on the leaves and older lesions will appear brownish-orange. Causes defoliation, yield loss, and a reduction in feed quality.

There are five common control methods that can be used to keep these diseases in check as listed below.
  • Resistant Varieties – CBR, ARR, BW, CLS, DO, DM, FW, PRR, SBS, VW
  • Crop Rotation – BRR, CBR, DO, DM, VW, YLB
  • Maintain Stand Vigor – WCR, CBR
  • Maintain Good Fertility – CBR, FW, PRR
  • Harvest Early – CLS, SBS, YLB
  • Although spring frost injury is not considered a disease, its effects can be similar to the productivity and vigor of alfalfa plants.

Fact Sheets

Alfalfa Analyst - available in PDF format only

Alfalfa Anthracnose - available in PDF format only

Alfalfa Crown and Root Rots and Stand Longevity - available in PDF format only

Alfalfa Disease Resistance

Alfalfa Looper

Alfalfa Weevil

Aphanomyces root rot

Biology and Management of Phytophthora Root Rot of Alfalfa

Crown and Root Diseases of Alfalfa

Disease and Deficiencies in Forages MB

Diseases of Field Crops: Alfalfa Diseases

Frost Injury to Alfalfa - available in PDF format only

Lepto leaf spot

Monitoring Alfalfa Weevil - Saskatchewan Agriculture - Web video


Root and Crown Troubles of Alfalfa

Spring Black Stem of Alfalfa

Verticillium Wilt on Alfalfa - UC Davis

Research Papers

A novel source of resistance to verticillium wilt in alfalfa - available in PDF format only

Assessing resistance to spring black stem and leaf spot of alfalfa caused by Phoma spp. - available in PDF format only
  For more information about the content of this document, contact Grant Lastiwka.
This information published to the web on October 24, 2003.
Last Reviewed/Revised on February 21, 2017.

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