Diseases of Alfalfa - Fungal

What to look for?
Common in the southern States but relatively rare in Canada.

Photo: Howard

Picture description Wilted shoot.

Photo: Hunter

Picture description Lesions may girdle the stems resulting in "shepherd's crook" shoots. Blue-black lesions may develop in the crowns.

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Management strategy
Resistant cultivars are available.
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Black Stem
Ascochyta imperfecta(Phoma medicaginis)

What to look for?
A common disease favored by cool wet weather.

Photo: Richard

Picture description Dark purple lesions develop in lower leaves and at the base of young stems.

Photo: Howard

Picture description Affected leaves drop off and young stems may be killed.

Management strategy
The fungus persists in crop residue and can be spread via seed. Use certified, fungicide treated seed and cut early to reduce quality loss from dropped leaves.

Brown Root Rot
Plenodomus meliloti

What to look for?
This disease can be severe under prolonged deep snow cover that persists into the spring months.

Photo: Evans

Picture description Black (brown) lesions on the tap roots

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Management strategy
See diseases of sweet clover - brown root rot.
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Cercospora Leaf Spot / Summer Black Stem of Clovers
Cercospora zebrina

What to look for?
A leaf spotting disease that occasionally shows up in some alfalfa crops. This is primarily a disease of clovers. Red clovers are the most susceptible crop whereby stems and leaves are affected and appear blackish.

Photo: Basu

Picture description Black spots on alfalfa leaves.

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Management strategy
There are many leaf spotting diseases of alfalfa some are common and other are of sporadic occurrence. With some of the less common leaf spotting diseases, appropriate information or elucidating the proper identification is lacking.

Common Leaf Spot
Pseudopeziza medicaginis

What to look for?
A very common disease that in cool wet seasons can result in major defoliation of the alfalfa stand.

Photo: Richard

Picture description Very distinct black spots.

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Management strategy
No resistant cultivars are available and early cutting may be necessary to avoid excessive leaf drop.
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Crown Bud Rot
Unknown

What to look for?
Likely caused by Coprinus psychromorbidus under prolonged snow cover.

Photo: Evans

Picture description Significant crown rot

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Management strategy
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Crown and Root Rot
Rhizoctonia solani

What to look for?
Other fungi such as Phoma and Fusarium spp. may be involved in this disease syndrome. Frost, poor drainage, low fertility or grazing are associated with diseased crowns. Such poor management can result in a rapid stand decline.

Photo: Howard

Picture description Severely damaged alfalfa crowns.

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Management strategy
Improve drainage, fertility and appropriate cutting to control this problem.

Crown Wart
Physoderma alfalfae

What to look for?
An unusual and fairly rare disease of alfalfa in Canada.

Photo: Howard

Picture description Warts on the alfalfa roots.

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Management strategy

Downy Mildew
Peronospora trifoliorum

What to look for?
Widespread in cool, wet springs. Usually only the first cut may be significantly damaged. Upper leaves are light green to yellow and leaf undersides are gray.

Photo: Howard

Picture description Mosaic appearance and grayish felty undersides of infected leaves.

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Management strategy
Use resistant cultivars and cut stands early to reduce foliage loss.

Gray Leaf Spot of Clovers
Leptosphaeria pratensis

What to look for?
A common disease of all clovers but generally of minor importance in Canada. Not a common disease on alfalfa but can be confused with pepper spot or yellow leaf blotch (Leptostrochila mediaginis).

Photo: Richard

Picture description Diffuse yellowing and pepper spots.

Photo: Richard

Picture description Yellowish lesions on younger leaves.

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Management strategy
In all instances cut the crop early to avoid excessive leaf drop.

Pepper Spot or Lepto Leaf Spot
Leptosphaerulina briosiana (L. trifolii)

What to look for?
Similar damage to common leaf spot and again favoured by wet conditions. Small black pepper spots on the leaves. Spots enlarge to form tan lesions and a chlorotic halo.

Photo: Basu

Picture description Distinct pepper spots on the leaves.

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Management strategy
Early cutting is recommended to reduce leaf loss.

Phytophthora Root Rot
Phytopthora sp.

What to look for?
A disease that is important in eastern Canada and sporadic elsewhere. Seedlings may be severely affected and mature plants damaged or killed under wet or poorly drained soil conditions.

Photo: Richard

Picture description Rotted alfalfa crown.

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Management strategy
Resistant cultivars are available.

Rust
Uromyces striatus; U. trifolii

What to look for?
Rust shows up on clovers particularly late in the season but the disease is of minor importance in alfalfa.

Photo: Howard

Picture description Conspicuous red rust lesions.

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Management strategy

Sclerotinia Stem Rot
Sclerotinia trifoliorum; S. sclerotiorum

What to look for?
This disease also occurs on alfalfa but is usually only severe on clovers. Clovers of all ages are susceptible particularly red and crimson clovers, especially after a prolonged snow cover. In some seasons S. sclerotiorum can be a significant problem in alfalfa seed crops. Some damage can occur to the alfalfa flowers preventing any seed set. In the Peace region of Alberta significant losses have occurred to the seed yield. Botrytis cinerea may also be involved in this blossom blight seed pod rotting complex.

Photo: Platford

Picture description Rotting alfalfa crown along with black sclerotes.

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Management strategy
Foliar fungicide may control this disease.

Stemphylium Leaf Spot
Stemphylium botryosum, S. loti and S. sarciniforme

What to look for?
These diseases can cause considerable yield and quality loss in trefoils and clovers.

Photo: Howard

Picture description Irregular yellow leaf spots

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Management strategy
Rotation is the only control procedure.

Verticillium Wilt
Verticillium albo-atrum

What to look for?
A very destructive disease of alfalfa introduced from Europe some 30 or 50 years ago. In irrigated crops, this disease can reduce yields by 50% or more in a year or so. The disease is seed- and residue-borne. The fungus infects alfalfa systemically.

Photo: Huang

Picture description Wilted plant - not all shoots are wilted on the same plant.

Photo: Huang

Picture description Individual wilted leaflets.

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Management strategy
Use fungicide treated seed to avoid introducing the disease. On land when the disease is established use resistant varieties.

Winter Crown Rot/Cottony Snow Mold
Coprinus psychromorbidus, Plenodomus meliloti and Fusarium nivale

What to look for?
All these fungal diseases can be active under prolonged snow cover in late springs typically occurring in Alberta's Peace Region.

Photo: Davidson	Picture description Severe crown damage	Photo: Davidson	Picture description Full infected alfalfa crown
. Photo: Davidson	Picture description Dead alfalfa crown

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Management strategy
Some alfalfa cultivars are more resistant than others.

Yellow Leaf Blotch
Leptotrochila medicaginis

What to look for?
Yellow leaf blotch is common in Alberta particularly as you progress north. Prominent yellow angular lesions develop on the leaves. Disease is spread from over-wintered ascospores which can be released throughout the growing season.

Photo: Howard

Picture description Yellow angular lesions on the leaves

Photo: Evans

Picture description Angular lesions

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Management strategy
Cut early as with all foliar diseases to avoid excessive leaf drop.

Photographs and information assembled and prepared for ARD by Dr. Ieaun R. Evans Agri-Trend Agrology Ltd.

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