Major Alfalfa Diseases

Map areas shaded blue indicate area of disease distribution

Phytophthora Root Rot  (Phytophthora megasperma f. sp. medicaginis)

Optimal disease conditions - Occurs most often in soils with poor drainage, or where water stands for an extended amount of time.

Symptoms - Stunting and/or plant death in low areas of field where water stands. Damaged plants may have taproot girdled at same depth as water table in soil.

Control - Resistant varieties; cultural practices that promote better drainage i.e. deep plowing, laser leveling, planting on beds and tiling.

Bacterial wilt  (Clavibacter michiganense subsp insidiosum)

Optimal disease conditions - Can occur in most soil types but damage can be more severe in the presence of nematodes or root feeding insects that create sites for entry into root system. More common in cold climates.

Symptoms - Stunting of plants; Yellowish to brown discoloration inside the root that becomes more severe with age of stand.

Control - Resistant varieties.

Fusarium wilt  (Fusarium oxysporum f.sp. medicaginis)

Optimal disease conditions - Can occur in most soil types but damage can be more severe in the presence of nematodes or root feeding insects that create sites for entry into root system. More common in warm climates.

Symptoms - Stunting of plants; Red to reddish brown discoloration inside the root that becomes more severe with age of stand.

Control - Resistant varieties; Root knot nematode resistance may also be desirable to complement Fusarium wilt resistance. This reduces exposure of the plant to the pathogen by nematode feeding on the roots.

Crown Rot Complex  (Complex of various pathogens: Fusarium, Pythium, Rhizoctonia, Phoma & Stagonospora)

Optimal disease conditions - Can occur in most soil types but damage can be more severe in the presence of nematodes or root feeding insects that create sites for entry into root system. More common in warm climates.

Symptoms - Stunting of plants; Red to reddish brown discoloration inside the root that becomes more severe with age of stand.

Control - Resistant varieties; Root knot nematode resistance may also be desirable to complement Fusarium wilt resistance. This reduces exposure of the plant to the pathogen by nematode feeding on the roots.

Anthracnose  (Colletotrichum trifolii)

Optimal disease conditions - Occurs most often in spring or fall and spreads rapidly under warm wet conditions from spores produced on lower stems of infected plants.

Symptoms - Early stages may appear as individual straw colored stems on plants that display a curved top "Shepherds Crook". Diamond shaped lesions will occur on lower part of the stem. Advanced stages will be seen in the crown tissue as a dark black or coal color. Plant death usually occurs at this stage.

Control - Resistant varieties; Avoid spreading spores from plant debris on harvest equipment to uninfected fields.

Rhizoctonia  (Rhizoctonia solani)

Optimal disease conditions - Wet humid conditions; Root damage generally occurs in warm soils or those conditions that favor high-temperature flooding injury (scald)

Symptoms - Seedling damage may appear as damping off; Root damage in established fields occurs as elliptical shaped lesions on the taproot at the point where the lateral roots emerge.

Control - Some varietal differences may occur however no clear-cut control is available.

Stagonospora root rot   (Stagonospora meliloti)

Optimal disease conditions - Spores are produced on lower stems and leaves and are spread by irrigation water or rainwater to other plants. Root infection develops from stem and crown infections.

Symptoms - Evidence of the pathogen may be seen in cross sections of taproots or large stems as pockets of red-orange specks in the tissue. This pathogen is considered by some pathologists as one of the major causes of stand decline in California. It has also been identified as one of the causal agents of crown rot. Major effect of pathogen is seen in second and third years of stand.

Control - Resistant varieties are not known. No effective cultural control measures are known at this time.

Aphanomyces Root Rot   (Aphanomyces euteiches)

Optimal disease conditions - Occurs most often in soils with poor drainage where water stands for an extended amount of time, or when there is a extended wet period at planting.

Symptoms - Damping-off of seedlings may be yellow-red to brown. Surviving plants may be stunted and feeder roots of older plants may turn brown.

Control - Resistant varieties and/or fungal seed treatment at planting. Cultural practices that promote better drainage i.e. deep plowing, laser leveling, planting on beds and tiling.

Verticillium wilt  (Verticillium albo-atrum)

Optimal disease conditions - Thought to occur only in cooler northern climates until it was identified in the late 1980’s in parts of S.California. The pathogen can be spread by dry or fresh plant material on harvest equipment. Cutter bar blades of mowing equipment are extremely effective in spreading the pathogen spores.

Symptoms - Stunting of plants; Yellow ‘V’ shape at the tip of leaves. Leaves may curl along midrib and turn a pinkish color. Stems will remain green after leaves die.

Control - Resistant varieties; Clean farm equipment between fields, and mow younger fields before older to prevent spore transfer on mower blades. Cutter bar sanitation with 10% bleach may be useful.

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Bob O'Donnell - America's Alfalfa