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Seed rot, damping-off, and seedling blights, caused by species of Pythium,
Rhizoctonia, Fusarium, Phytophthora, Phoma (Ascochyta), Mycoleptodiscus,
Sclerotinia, and other fungi occur wherever alfalfa and clovers are grown.
Sometimes the diseases reduce the stand, especially when alfalfa or clovers
are grown in acid, poorly-drained soils with a high content of organic
matter, or when prolonged, wet weather follows planting.
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Symptoms
Three types of injury occur.
1. With preemergence damping-off, seed may decay or seedlings may become
blighted and be killed before they emerge. The stand is thin and "patchy,"
especially in low, wet areas of fields.
2. With postemergence damping-off, infection commonly occurs as the seedling
emerges, and the disease develops slowly afterward. The seedlings emerge,
only to wilt, collapse, dry up, and die from a rot at the oil line and
below.
3. Young roots and stems decay, causing varying degrees of stunting,
but the infected plants survive the early seedling stage. Later, some
plants may turn yellow, wither, and die while the remainder recover.
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Disease Cycle
Some organisms (species of Pythium, Fusarium, Rhizoctonia, and Phytophthora)
are soil inhabitants and persist in and on plant residues as oospores
and sporangia (Pythium, Phytophthora), dark sclerotia (Rhizoctonia, Sclerotinia),
chlamydospores and mycelium (Fusarium) for two years or longer without
a suitable host plant. When conditions are favorable, the fungi attack
the seed and young seedlings–especially when germination and emergence
are delayed. Other fungi (Phoma and Mycoleptodiscus) invade the soil and
survive in crop debris only until the plant crowns, stems, and leaves
become decayed. A rotation that excludes clovers and alfalfa and closely
related legumes for at least two years will eliminate the Phoma and Mycoleptodiscus
fungi from the soil. Several seed-decay, damping-off, and seedling-blight
fungi are seed-borne diseases. These include the fungi that cause yellow
leaf blotch (Leptotrochila medicaginis) and spring black stem (Phoma medicaginis
var. medicagnis). The sowing of certified, fully-mature seed produced
in an arid climate should eliminate these fungi as a source of infection.
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1. Sow high-quality, certified, fully-mature seed produced in arid areas.
Plant the varieties recommended for your area. Avoid seed grown in the
southern states or in foreign countries. See Illinois Agricultural Pest
Management Handbook for alfalfa varieties recommended for Illinois.
2. If possible, plant in a fertile, well-prepared seedbed where soil
drainage is good. Acid soils should be limed to get a pH of 6.5 or 7.0.
Practice balanced soil fertility based on a soil test.
3. Treating alfalfa and clover seed with a seed protectant fungicide
or mixture will often reduce the amount of decay and preemergence damping-off
in soils infested with Pythium, Phytophthora, Rhizoctonia, and certain
other fungi. The final stand may be somewhat improved in some cases. The
results of numerous field tests, however, indicate that treating alfalfa
and clover seed rarely increases the yield of forage or hay. A fungicide
treatment of small-seeded legumes is not justified in Illinois under normal
conditions. A seed treatment may help control seed rot and damping-off
where the soils are wet and poorly-drained, or where stands fail consistently.
The seed-treatment fungicides cleared by the federal EPA for treating
alfalfa and clover seed are given in Illinois Agricultural Pest Management
Handbook. All seed-treatment fungicides should be used strictly according
to label directions.
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