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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 ¼ 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
70°F 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 (¼
to ¾") 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.
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