A closer look at leaf diseases of alfalfa

This third of three articles on alfalfa diseases includes common leaf diseases in Michigan and other problems sometimes confused with alfalfa leaf disease.

Many diseases can impact alfalfa leaves in Michigan, resulting in interference with photosynthesis and early defoliation. Yield and quality suffer when disease problems become serious. Along with disease pathogens, other factors can damage alfalfa leaves. These include insect injury, herbicide injury and nutrient deficiency. Familiarity with symptoms of more common alfalfa leaf diseases and other problems can help producers identify and manage their crop.

Leaf diseases of alfalfa tend to become more severe after flowering begins, so harvesting at no later than 10 percent flower, or earlier, is a good cultural practice to minimize leaf disease losses. Seeding alfalfa with a companion, or “nurse,” crop can also increase the incidence of leaf and stem diseases. Alfalfa varieties are screened for root and crown disease resistance by seed companies and universities. However, leaf and stem disease resistance ratings are not widely available, except for anthracnose.

Common leaf spot

Caused by the fungus Psuedopesziza medicaginis, this disease results in both early- and late-season defoliation. Cool and moist (or very humid) weather or alfalfa under a heavy companion crop can increase severity. Low fertility and acidic soil seems to make this disease worse. Symptoms include small, circular spots, brown or black in color, developing first on lower or inner leaves. Tiny, cup-shaped fungal fruiting structures appear in the center of fully developed spots on the upper leaf surface. Premature defoliation results as the number of leaf spots increases. The disease overwinters in un-decomposed leaves on the soil surface.

Downy mildew

Caused by the fungus-like organism Peronospora trifoliorum, this disease seldom causes plant death in alfalfa stands, but affects yield and quality. It appears mainly in spring and fall during cool, moist or very humid weather. Young leaflets on rapidly growing plants have light green blotches and may twist, or curl, downward if the disease is severe enough. The underside of leaflets may have a pale, violet, downy growth. Symptoms disappear during warm, dry weather, but can come back in the fall. The fungus overwinters in crop residue and can be seed borne.

Stemphylium leafspot

Caused by the fungus Stemphylium botryosum, this disease is also called “target spot.” It is most common in lush stands following warm, wet weather and when harvest is delayed. The most noticeable symptom is formation of oval-to-elongate brown lesions with lighter centers, enlarging to form concentric light and dark brown zones. A single, large lesion can cause a leaflet to yellow and drop. Older lesions can be covered with a soot-like mold. The disease overwinters on old, infected stems, or on seed.

Lepto leaf spot

Caused by the fungus Leptosphaerulina briosiana, this disease is also called “pepper spot” and is most common in cool, wet weather on young leaves and petioles of recently cut stands. It produces small, reddish-brown to black spots, enlarging to form oval-round tan spots with a darker border. These spots can merge and kill the entire leaf. Dead leaves often cling to the stem for some time.

Other alfalfa leaf diseases include:

Non-disease alfalfa leaf problems include:

  • Insect injury: Potato leafhoppers cause “hopperburn,” a v-shaped yellowing of the leaf.
  • Herbicide injury: Triazine carry-over in new seedings was common before the prevalence of glyphosate-resistant corn hybrids and Dicamba/2,4-D injuries can occur.
  • Nutrient deficiencies: Boron – top leaves are bunched and yellow-reddish in color. Can be mistaken for hopperburn. Phosphorus – causes yellowing of leaves and is most common in low or high pH soils. Sulfur – upper leaves turn pale yellow, most likely on sandy, low organic matter soils. Magnesium – most likely on acid, sandy soils. Area between leaf veins turns yellow-white with plant stunting. Symptoms move from older to newer plant growth.

See the first article of this series, “Be familiar with root rot diseases of alfalfa,” for more information on certain root rot diseases, and the second article, “Gain knowledge on stem diseases of alfalfa,” for more information on certain stem diseases.

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