Alfalfa diseases

Alfalfa is susceptible to a number of diseases that can weaken its foliage, root system, or lifespan. When symptoms appear, the decline in vigor can be rapid, impacting forage quality, yield, and harvest regularity. Identifying the main alfalfa diseases allows for a better understanding of what is happening in the field, distinguishing simple stress from a genuine health problem, and guiding observations and actions more effectively. Agrobiotop, an agrobiology specialist, offers its solutions.

Common leaf spot of alfalfa (Pseudopeziza medicaginis)

This foliar disease, caused by the fungus Pseudopeziza medicaginis , is identified by small, well-defined, circular spots that are brown to black with slightly serrated edges. As the infection progresses, the leaves yellow and then fall off, rapidly compromising the quality of the foliage. In cool, humid conditions, observation with a magnifying glass may reveal small apothecia.

Alfalfa rust (Uromyces striatus)

Rust, caused by Uromyces striatus , is one of the easiest diseases to recognize thanks to its reddish-brown pustules on the leaves and petioles. As it progresses, elongated pustules may also appear on the stems. The spores are easily detached by rubbing, and defoliation often follows quite quickly, giving the canopy a drier and less dense appearance.

Powdery mildew (Erysiphe pisi)

Powdery mildew of alfalfa, usually caused by Erysiphe pisi , is characterized by a white, powdery coating on the aerial parts of the plant. The affected areas eventually turn yellow and then dry out locally. The most telling sign is this clearly visible, powdery coating, very different from the downy growth seen with downy mildew, as here the powdery appearance predominates on the affected surface.

Downy mildew (Peronospora trifoliorum)

Downy mildew, caused by the oomycete Peronospora trifoliorum , produces yellowish-green patches on the upper surface of the leaves, while a purplish-grey down appears mainly on the underside during humid mornings. The leaves may twist, curl, and the stem tips may become stunted.

Summer black stem (Cercospora medicaginis)

Summer black stem is caused by Cercospora medicaginis and often involves both foliar and stem damage. The lesions are brown, irregular, sometimes surrounded by a yellow halo, with defoliation frequently progressing from the base to the top of the plant. When the stems are also affected, the visual diagnosis becomes more suggestive, especially if diffuse yellowing of the lower foliage is also present.

Spring black stem (Phoma medicaginis)

This disease, caused by Phoma medicaginis , manifests as small, dark brown spots on leaves, petioles, and stems, which eventually merge into irregular patches. Affected tissues yellow and then fall off, while the base of the stems may turn black. This combination of dark spots, coalescence, and early involvement of aerial parts makes it a disease to monitor from the first signs of spring.

Stemphylium leaf spot (Stemphylium botryosum)

This disease, attributed to Stemphylium botryosum and sometimes to closely related species, forms brownish leaf spots, occasionally with borders, which vary considerably from one affected area to another. Under high humidity and when disease pressure develops, defoliation can become pronounced.

Anthracnose (Colletotrichum trifolii)

Anthracnose, caused by Colletotrichum trifolii , primarily affects the stems and crown. It causes sunken, oval to diamond-shaped lesions, and the stem tips may bend. When the crown is affected, a dark, bluish-black rot, sometimes organized in a V-shape within the tissues, can be observed.

Sclerotinia (Sclerotinia trifoliorum)

Sclerotinia rot, caused by Sclerotinia trifoliorum and sometimes by closely related species, often begins at the crown or base of the stems as a wet rot. A white, cottony mycelium may form, accompanied by sclerotia, and then the plants wilt and dry out in patches. The contrast between the roots, sometimes still inconspicuous at the beginning, and the rapid collapse of the above-ground parts is often the most noticeable symptom.

Phytophthora root rot (Phytophthora medicaginis)

This disease, caused by the oomycete Phytophthora medicaginis, is typical of poorly drained areas or those prone to waterlogging. A sudden wilting, sometimes leading to plant death, is observed, with light brown to tan lesions on the taproot, often near the point where lateral roots emerge. When cut, internal orange to red streaks may appear, providing further diagnostic clues.

Aphanomyces root rot (Aphanomyces euteiches)

With Aphanomyces euteiches , the first signs appear very early, at or shortly after emergence. Seedlings are stunted, growth slows, roots turn brown, and fine roots become sparse, sometimes giving the impression of a “pruned” root system. This disease is closely linked to cold, damp soils, where it leads to uneven and weak stands.

Pythium rot (Pythium spp.)

Pythium primarily affects seeds and young roots. Symptoms include damping-off before or after emergence, soft or discolored seeds, and brown lesions that constrict the tissues. The disease is therefore often observed at the planting stage, with poor emergence or seedlings that collapse rapidly after emergence.

Rhizoctonia solani

Rhizoctonia solani causes reddish-brown necrosis at the base of the stem and the hypocotyl, with a well-defined constriction. Outbreaks are generally irregular within the plot, as their appearance varies greatly depending on soil moisture and temperature.

Fusarium wilt (Fusarium oxysporum f. sp. medicaginis)

Fusarium wilt, caused by Fusarium oxysporum f. sp. medicaginis , is observed as a progressive wilting of the plant, accompanied by a pallor of the tissues. Inside the roots or stems, reddish-brown streaks may be observed, a sign of vascular damage.

Verticillium wilt (Verticillium alfalfae)

Verticillium wilt, caused by Verticillium alfalfae , often results in yellowing along the midrib and may remain unilateral on only part of the foliage. The leaflets dry out, and the petiole may remain bare after they fall. In the roots, the xylem turns brown, confirming the vascular nature of the infection.

Bacterial wilt (Clavibacter insidiosus)

Bacterial wilt, caused by Clavibacter insidiosus , significantly alters the plant’s structure. The leaves become smaller, may become mottled, or curl.

Bacterial leaf spot of alfalfa (Xanthomonas euvesicatoria pv. alfalfae)

This bacterial disease begins as small, water-like spots, which then develop into tan to yellow lesions, bordered by a darker rim and sometimes accompanied by a halo. When they merge, they form dry patches that can lead to defoliation.

Bacterial stem blight (Pseudomonas syringae pv. syringae)

Bacterial stem blight makes plants stunted, with short, brittle stems. Watery lesions may be visible at the petiole insertion point, and symptoms often appear after late frosts.

Crown gall (Agrobacterium tumefaciens)

Crown gall, caused by Agrobacterium tumefaciens , results in irregular galls at the base of the stem, often just below the soil line, but also on the branches of the crown. These swellings can be more or less developed depending on how long the infestation has been present.

Brown root rot (Phoma sclerotioides)

Brown root rot, attributed to Phoma sclerotioides , is often noticed at the end of winter. Plants slowly recover in the spring, some die during or after the cold period, and the taproot and secondary roots show dark brown lesions. The context of long winters or prolonged snow cover increases the suspicion of this disease.

Sprout rot (Dickeya spp.)

In alfalfa sprout production, Dickeya fungi cause rapid soft rot. The radicle turns yellowish and translucent, growth ceases, and then a foul-smelling decomposition can quickly spread throughout the entire tray. The speed of development and the associated odor are very telling.

Abiotic problems and deficiencies

• Physiological collapse of the germ (calcium deficiency) : This abiotic disorder is not caused by an infectious agent but by a calcium deficiency. It causes an oily lesion under the cotyledonary hook, followed by constriction of the hypocotyl, which severely weakens the germ. This condition often leads to secondary soft rots, sometimes obscuring the initial problem.
• Boron deficiency : Boron deficiency first affects the young parts of the plant. The internodes shorten, the upper leaves turn yellow or red, and the tips of the leaflets may turn yellow or red.
• Frost stress : Frost causes exposed leaves to brown, accompanied by general discoloration on weaker plants. Young seedlings are the most vulnerable, with a higher risk of mortality after a severe cold spell.

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