Disease and Nutrient Deficiencies in Forage Crops
Overview
Plant diseases and nutrient deficiencies significantly impact forage legume and grass production. Early recognition and integrated management are essential to minimize losses.
  • Disease Control: Resistant varieties, crop rotation, and fungicides (where applicable).
  • Deficiency Correction: Apply the missing nutrient based on soil and tissue tests.
 
Common Forage Legume Diseases
Alfalfa
1.     Black Stem (Phoma medicaginis) fungus:
Symptoms
o   Small black to dark brown spots on lower leaves, petioles, and stems in early spring.
o   Spots expand into large black areas; severe infections cause yellowing and premature leaf drop.
o   Large lesions girdle stems, leading to wilting above the damaged area.
o   Infection can extend to roots and crown, causing crown rot and reduced plant vigor.
Favorable Conditions
o   Cool, moist weather in spring and fall.
o   Fungus overwinters on diseased stubble and can be seed-borne.
o   Cannot survive in soil for more than two years without alfalfa.
Control Measures
o   Early cutting before extensive yellowing reduces yield.
o   Crop rotation: Plant non-legume crops for several years before re-establishing alfalfa.
 
2.     Common Leaf Spot (Pseudopeziza trifolii) fungus:
Symptoms
o   Brown-black spots with raised discs; causes leaf drop.
Favorable Conditions
o   The fungus overwinters on plant debris.
o   In spring, spores are discharged and carried by wind, enabling infection of new plants.
o   Infection can occur throughout summer under moist conditions, making wet weather a high-risk period for disease development.
Control Measures
o   Resistant varieties (e.g., Rambler), early cutting.
 
3.     Downy Mildew:
Symptoms
o   Light green blotches on upper leaf surfaces.
o   Pale violet downy fungal growth on lower leaf surfaces.
o   Shoots appear dwarfed and bunched.
o   Leaves twisted or rolled downward.
Favorable Conditions
o   Most severe in cool, wet, humid weather.
o   Often widespread under favorable conditions but usually causes minimal damage.
Control Measures
o   Plant resistant cultivars (e.g., Algonquin).
o   Rotate with cereals or grasses to eliminate overwintering inoculum.
o   Note: Windborne spores from adjacent infected fields can still cause reinfection.
 
4.     Root & Crown Rot: (Winter Injury and Soil Borne Fungi)
Symptoms
o   Brown discoloration of internal crown tissue.
o   Plants become stunted, chlorotic (yellow or pale green), and lack vigor.
o   Some plants die immediately; others decline over several years.
o   Most severe in older stands.
Favorable Conditions
o   Includes fungi such as:
o   Coprinus spp. (cottony snow mold)
o   Plenodomus meliloti (brown root rot)
o   Fusarium spp.
o   Active in late fall and early spring when soil temperature near crown is close to 0°C.
Control Measures
o   Plant hardy, well-adapted cultivars.
o   Maintain vigor with balanced fertility.
o   Rotate with summer fallow or cereals for 3 years after breaking alfalfa.
                                   
5.     Crown Bud Rot: Soil-borne fungi: Rhizoctonia solani, Fusarium roseum, Phoma medicaginis.
Symptoms
o   Destruction of bud tissue on crown and upper root area.
o   Disease progressively worsens over time.
o   Leads to reduced regrowth and stand decline.
Favorable Conditions
o   Occurs mainly in irrigated alfalfa or moist soils.
o   Often worsens in fields with winter injury, insect damage, or mechanical injury.
Control Measures
o  Maintain plant vigor with balanced fertilization based on soil tests.
o  Allow plants to recover in spring before cutting.
o  Avoid late-season cutting to prevent depletion of root reserves and winter injury.
o  Rotate with cereals or grasses for 2–3 years to reduce soil-borne fungi.
o  Use resistant varieties: Anchor, Beaver, Thor.
 
6.     Verticillium Wilt: Alfalfa and Sweet Clover Fungus: Verticillium albo-atrum.
Symptoms
o  Initial wilting of upper leaves, progressing to entire plant.
o  Wilted leaves and stems turn white or yellow.
o  Base of stem may turn grey or black due to fungal presence.
Favorable Conditions
o   Via infected seed or machinery carrying infected debris/spores.
o   More severe in irrigated fields.
o   Infected stands can die out within 3 years in high-risk areas (e.g., BC); slower progression in Manitoba.
Control Measures
o   Treat alfalfa seed with Thiram fungicide to control seed-borne infection.
o   Plough down infected fields and rotate with cereals or grasses for at least 2 years.
o   Avoid planting potatoes, sunflowers, and sweet clover in rotation (also susceptible).
 
Red Clover
7.     Northern Anthracnose: Fungus: Kabatiella caulivora.
Symptoms
o   Early: Translucent, water-soaked spots on stems and leaves.
o   Lesions enlarge, becoming dark brown with lighter sunken centers.
o   Affected stems and petioles curl over like a shepherd’s crook.
o   Plants survive but lose leaves, stems, and flowers → reduced hay and seed yields.
Favorable Conditions
o   Fungus overwinters on diseased stubble.
o   In spring, white or pink spores form in old lesions and spread mainly by splashing rain.
o   Most active in cool, wet weather.        
Control Measures
o   Plant resistant cultivars (e.g., Norlac).
o   Rotate with cereals or grasses for 2 years.
Alsike & White Clover
8.     Sooty Blotch: Fungus: Cymadothea trifolii
Symptoms
o  Dark brown to black lesions on the underside of leaves.
o  Upper leaf surface appears pale green or chlorotic.
o  Leaves eventually shrivel and die.
o  Can prevent flowering.
o  Infected foliage may be toxic to livestock, causing mouth ulcers.
Favorable Conditions
o   Most prevalent in cool, moist fall weather.
Control Measures
o   Rotate with non-clover crops for 3 years to break the disease cycle.
 
 
Forage Grass Diseases
  • Ergot (Claviceps purpurea): Black sclerotia replace seeds; toxic to livestock. Found most frequently in: brome grass, wheat grasses and fescue grasses. Occasionally it is found on timothy and reed canary grass.
    Control: Cut before heading, rotate crops and rotate with broadleaf crops.
  • Helminthosporium Leaf Spots: Brown lesions and blotches; reduces hay yield. Most prevalent brome and wheat grasses
    Control: Avoid dense stands and avoid overfertilization with nitrogen, rotate with non-grassy crops.
  • Purple Spot (Timothy): Purple-brown leaf spots; severe in sandy soils.
    Control: Resistant varieties, proper fertility. Varieties such as Bounty and Climax have better resistance
  • Snow Mold: Dead patches in spring; common under snow cover with cobweb-like fungal mycellium. Pink snow mold usually occurs late in the fall with cool moist weather.
    Control: Fall fungicide for turf, crop rotation.
  • Rust (Puccinia spp.): Reddish pustules on leaves; weak growth. Attacks bluegrass.
    Control: Fertilize, mow regularly. Post-harvest burning in bluegrass seed crops.
 
Nutrient Deficiencies
Macro element deficiencies in forage crops are usually easy to observe. Micro nutrient deficiency symptoms are less common and more challenging to identify correctly. 
Legumes -  Legumes are considered to be much more sensitive to micro nutrient deficiencies than grasses.
  • Nitrogen: Yellowing older leaves which drop off; correct with inoculation.
  • Phosphorus: Stunted growth, dark green/purple leaves, roots are light brown.
  • Potassium: White spots on leaf margins that turn yellow and die; mostly affects the lower, younger leaves, common in high-yield stands.
  • Calcium: Collapse of petioles, leaflets will become reddish purple; prevent with liming.
  • Magnesium: Interveinal chlorosis, Margins of leaves may turn a reddish brown color; Prevalent on acidic soils and sandy soils in wet conditions, correct with dolomitic lime.
  • Sulfur: Pale green/yellow younger leaves; most likely to occur in coarse textured, well drained soils, apply gypsum or ammonium thiosulfate.
  • Micronutrients:
  • Boron (yellow/red leaf tips) occurs on sandy, high pH level or low organic matter soils.
Copper (wilting young leaves) common on peat and muck soils
Molybdenum (similar to N deficiency) deficient on acid soils.
Iron deficiencies are very rare due to low availability of iron rather than lack of it. Iron may not be available in alkaline or calcareous soils.
 
Grasses
  • Nitrogen: Pale green/yellow older leaves; rapid response to N application.
  • Phosphorus: Slow regrowth, dark green/purple stems.
  • Potassium: Yellow streaks, leaf tip scorch.
  • Magnesium: Low on acidic soils, low magnesium levels in forage grasses may result in a nutritional disease  - grass tetany, an imbalance of the ratio of potassium to the total amount of calcium plus magnesium [ K / (Ca + Mg)].
  • Calcium: Rare but important for animal health.
  • Sulfur: Chlorosis of younger leaves; correct with sulfur fertilizers.
 
Key Management Practices
  • Use soil and tissue tests for accurate diagnosis.
  • Apply nutrients proactively to avoid yield loss.
  • Rotate crops and select resistant varieties for disease prevention.
  • Maintain balanced fertility and proper cutting schedules.