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Disease Management in Clovers & Alfalfa

Tom Kucharek

Seedling Blight

Cause: Fungi ( Pythium spp., Fusarium spp., and Rhizoctonia spp.)

Symptoms: The seed, preemerged seedling or postemerged seedling becomes discolored and maybe dies. Discoloration will vary depending on the organism(s) involved. Rhizoctonia infection will tend to result in surface or cratered lesions that are red to orange to brown in color. Pythium infection will result in a watery, gray rot. With infection from Pythium the outer part of the root (cortex) will slip off of the inner part of the root (stele). Fusarium infection can appear similar to Rhizoctonia symptoms but usually the tissue will have an overall reddish cast. Also, Fusarium infection is more apt to occur in the vascular tissue of the roots and stem resulting in two parallel discolored streaks if a root or stem is cut lengthwise.

Host Range: Many crop and weed species.

Control: Use certified seed. Avoid planting deep and in cool soils. Wait to plant until plowed down green matter has rotted; this usually means 30 days or more after plow down. Avoid inadequate management practices such as excess fertilizer, excess moisture etc. Use a seed treatment fungicide.

Root Rot, Crown Rot and wilt:

Cause: Fungi ( Pythium spp., Rhizoctonia spp.,

Fusarium spp., Cylindrocladium spp., Macrophomina phaseolina , Phytophthora spp., Sclerotinia sclerotiorum , Sclerotium rolfsii )

Symptoms: No one symptom can describe this complex problem. Often more than one fungus will be involved. Plants will wilt with leaves turning yellow, brown or other colors. Lower stem tissue will be off color.

If Cylindrocladium crotalariae is involved, the lower stem will be black and small red structures may be seen at the base of the stem. Foliage may turn brilliant, red, orange or brown.

Root tissue on healthy plants should be white. Discolored roots indicate some sort of infection is present.

Wilting and rotting caused by Sclerotinia will be accompanied by white mycelium on and in the stem. Also black sclerotia to 1 inch in length may be seen.

Desmodium sp. is very susceptible to root knot nematode.

If Sclerotium rolfsii is involved, a white mycelial growth with mustard seed-size sclerotia may be seen near or on the soil, particularly near the main stems.

Host Range: Many crop and weed species.

Control: Use certified seed. Avoid planting deep and in cool soils. Wait to plant until plowed down green matter has rotted; this usually means 30 days or more after plow down. Avoid inadequate management practices such as excess fertilizer, excess moisture, etc. Use a seed treatment fungicide. Also, maintain adequate fertilization. Rains leach nitrogen, potassium and other essential elements below the root zone thereby weakening plant and predisposing plant to infection. For perennial peanut, controlled burns may reduce inoculum for disease if ample residual fodder remains after the last seasonal cutting and after winter kill of fodder. Break up hardpans prior to planting. Cylindrocladium has been found in commercial alfalfa in Baker County.

Stemphylium Leaf Spot, Target Spot:
or Zonate Leafspot

Cause: Fungus ( Stemphylium botryosum )

Symptoms: Brown, somewhat round, lesions up to 3 to 4mm, with or without concentric rings, occur in leaflets, petioles, peduncles and stems.

Host Range: Clover, alfalfa.

Control: None available except cut hay before severe defoliation occurs.

Anthracnose:

Cause: Fungi ( Colletotrichum spp.)

Symptoms: Any part of the plant can be infected but in Florida, stems, petioles, leaves and crowns are infected. Large stem lesions, tan in color, diamond in shape, with a pink spore mass in the lesion center or cushion-like structures in the lesion. Among the pink spore masses are black cushion-like structures. Dark spines can be seen in these structures with the aid of a hand lens.

Host Range: Many legumes including Stylosanthes.

Control: No control available except early harvesting.

Aerial Blight:

Cause: Fungus ( Rhizoctonia spp.)

Symptoms: Symptoms are various and will occur during or after periods of heavy or frequent rains during warm to hot weather. All plant parts are susceptible. Usually nondiscript, irregular, lobed tan lesions surrounded by a dark border occurs in leaflets. Brown linear lesions in petioles and stems may also be present. Ruminants consuming infected forage salivate and urinate more, develop diarrhea and stop eating.

Host Range: Most crop and weed species.

Control: Avoid planting in poorly drained fields. Avoid excessive fertilization that promotes excessively thick stands.

Cercospora Leafspot:

(Summer black stem)

Cause: Fungus ( Cercospora medicaginis )

Symptoms: Leafspots are brown, angular and deliniated by the veins. In alfalfa, lesions are round.

Host Range: Clover, Siratro, Centrosema, alfalfa.

Control: Cut hay before severe defoliation occurs. Some copper fungicides are labelled.

Rust:

Cause: Fungi ( Uromyces spp.)

Symptoms: Orange brown pustules will be seen in leaves, petioles and stems. Severe cases will result in leaf yellowing and defoliation.

Host Range: Forage legumes. The alfalfa rust ( Uromyces striatus var. medicaginis ) has certain Euphorbia sp. as an alternate host for the pycnial and aecial spore stages.

Control: Harvest hay as soon as possible. Sulfur applications can be used legally if labeled but the economics of such is questionable. Do not apply sulfur when crop is over 3 inches high.

Powdery Mildew:

Cause: Fungus ( Erysiphe polygoni )

Symptoms: A white powdery growth usually begins on the upper leaf surface and later lower leaf surface. Infected leaves will turn yellow.

Host Range: In excess of 300 plant species are susceptible including forage legumes.

Control: Harvest hay as soon as possible. Sulfur applications may be used legally, if labeled, on alfalfa and clover but the economics of such is doubtful.

Sooty Blotch:

Cause: Fungus ( Cymadothea trifolii )

Symptoms: At first, in lower leaf surfaces tiny olive green dots appear. These dot-like structures (spore bearing structures) become thicker and darker eventually appearing as a velvety black cushion. Later shiny structures are produced. The leaf may yellow, turn brown and then die.

Host Range: Clovers.

Control: None available.

Common Leafspot:

Cause: Fungus ( Pseudopiziza medicaginis )

Symptoms: Circular, dark spots, about 2mm in size occur in leaflets. There tends to be a sharp line between the spot and non-infected tissue. On older spots with the aid of a hand lens you can see a tiny, brown, cup-shaped structure (apothecium) on upper leaf surface.

Host Range: Alfalfa.

Control: None available except cut hay before severe defoliation occurs.

Bacterial Leafspo:

Cause: Bacterium ( Xanthomonas campestris pv alfalfae )

Symptoms: A hot weather disease. Leaf lesions are irregularly shaped with narrow water-soaked margins (more pronounced in lower surface of leaves) usually surrounded by diffuse chlorosis. Severe defoliation is common. Stem lesions may coalesce to form elongate water-soaked lesions that may extend for several internodes. Older lesions become light to dark brown.

Host Range: Primarily alfalfa.

Control: None available except cut hay before severe defoliation occurs. Resistant varieties are being developed.

Bean Common Mosaic Virus:

Cause: Bean common mosaic virus.

Symptoms: Leaf mottling and distortion.

Host Range: Many legumes including Siratro.

Control: No control available.

Alfalfa Mosaic Virus:

Cause: Aphid-borne virus

Symptoms: Symptoms vary from practically no symptoms (latent) to plant death. This is due to numerous strains of the virus and numerous reaction types with different varieties and environmental influences. Symptoms include interveinal yellow-green mottle with stunting. Leaf or petiole contortions may be combined with leaf yellowing or mottling. Root necrosis can also occur. Over time stunted plants are overgrown by healthier neighboring plants. Yield is reduced, possibly without any symptoms (latent). Symptoms are most visable when temperatures are below 70F during daytime. Can be seedborne.

Host Range: Extensive.

Control: Use virus free seed if such is available. Use resistant varieties if available.

Bean Yellow Mosaic Virus:

Cause: Bean yellow mosaic virus

Symptoms: Diffuse shades of yellows and greens along with stunting and plant death. Branches and leaves may be small. Some plants are yellow and others reddish in color. Brown streaks may occur on one side of stems.

Host Range: Extensive (not on alfalfa and can be serious on yellow Lupines).

Control: None.

Lepto Spot:

Cause: Fungus ( Leptosphaerulina briosiana )

Symptoms: Usually occurs in young leaves, but all above ground parts are susceptible. Lesions begin as black spots which may remain (pepper spots) or enlarge to oval to round "eyespots" 1 to 3mm. Such lesions have light brown to tan centers with dark brown borders, often surrounded by a yellow ring. Lesions may coalesce and cover extensive areas on a leaflet. Dead leaflets may remain attached to plant. Dark ascocarps may be seen within lesions, particularly from dead leaves that are moist.

Host Range: Alfalfa, clovers.

Control: Cut hay early. Some copper fungicides are labelled.

Leaf Scorch:

Cause: Leptosphaerulina crassiasca

Symptoms: Often in tip or margins of leaflets. First sign is yellowing and then death (necrosis) of infected area. Necrotic are ( to ") surrounded by yellow halo often with dark circular area or concentric rings in dead tissue. Stem lesions are black and may be over 6 inches long.

Host Range: Peanuts, perennial peanuts

Control: Avoid excessive irrigation and retention of moisture on leaves.

Sclerotinia Root and Crown Rot:

Cause: Fungus ( Sclerotinia sclerotiorum )

Symptoms: This disease will occur during cool, wet weather. A lower stem watery rot causes stunting, wilting and death of plants. White mycelia may be evident on and in plant. Black sclerotia may be present on or in plants or soil. This disease may be more serious in damaged tissues if weather is favorable. Small brown spots in leaves or stems may preceed wilting or advanced symptoms.

Host Range: Extensive.

Control: No control available.

Downy Mildew:

Cause: Fungus ( Peronospora trifoliorum )

Symptoms: Occurs during cool and wet periods. Yellow spots on leaflets or large areas in leaflets turn yellow. A grey-purple downy growth may be seen on undersides of leaflets. Leaflets may be twisted and leaflet margins may curl down.

Host Range: Alfalfa and possibly other legumes.

Control: Not common in Florida. Found in Holmes County. Use resistant varieties if available.

Dollar Spot:

Cause: Fungus ( Sclerotinia homoeocarpa )

Symptoms: Tan to white lesions, surrounded by distinct purple to crown border occur on leaflets and stem. Lesions in leaflets commonly seen on margins of leaflets. Usually appears in clusters of adjacent plants. Plants may exhibit a dieback symptom if severity is high.

Host Range: Perennial peanut and many turf and forage grasses.

Control: Frequent cutting for hat or heavy grazing. Controlled burns may be beneficial.


This document is PDMG-V1-08, part of the 1997 Florida Plant Disease Management Guide, Volume 1, Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised March 1997.

Tom Kucharek, Professor, Plant Pathology Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

"Serious problems cannot be dealt with at the level of thinking that created them."
 Albert Einstein
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