Common Crown Rot
Acrocalymma crown and root rot, and Stagonospora crown and root rot
(sometimes called common crown rot)
Cause
• A number of fungi including Acrocalymma medicaginis (common) and Stagonospora meliloti (rare).
Occurrence
• Occurs commonly. Plants under stress are likely to be affected.
Symptoms
• Both fungi cause similar symptoms. A reddish speckling and streaking of the internal tissues of the crowns and roots is the first sign of disease. Affected tissues later darken and dry out, while the bark splits. Secondary diseases commonly attack the crown, killing the plant.
Infection and spread
• The fungi probably survive on lucerne residues, and the spores are spread by rain. Unlike other crown rots, these cause a slow decline of plants over a period of 3-4 years. Plants under stress are more likely to be affected.
Control
•Avoid undue stress and excessive wounding of the crowns, e.g. through renovation.
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Fusarium wilt
Fusarium Wilt
Cause
• The fungus Fusarium oxysporum f.sp. medicaginis.
Occurrence
• Fusarium wilt is not common, but is severe when it occurs. However, this fungal organism occurs commonly in the soil and can contribute to the expression of common crown rot.
Symptoms
• Initial signs are stunted plants with wilting shoots and yellow leaves. The infection then bleaches the leaves and stems, and finally kills the plant. If bark at the base of the stem is removed, dark, red-brown streaks can be seen in the water-conducting tissue.
Infection and spread
• The fungus survives for long periods in the decaying plant. It invades small roots or wounds in the taproots, growing through the water-conducting tissues. The diseas develops slowly and, although only scattered plants show symptoms at any one time, overall loss may be severe over several years. Infection occurs during warm, wet weather.
Control
•Crop rotation at least every five years is recommended.
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Sclerotium blight and Sclerotinia rot
Sclerotium Blight and Sclerotinia Rot
Cause
• The fungi Sclerotium rolfsii and Sclerotinia trifoliorum.
Occurrence
• Common diseases that can infect many other hosts.
Symptoms
• The symptoms of both diseases are very similar.
• Infected plants usually have a white, cottony growth on the outside of the stem base and crown. Internally, a brown rot extends from the stem base into the roots. Small, brown resting bodies or sclerotia develop on the cottony growth.
Infection and spread
• The fungus survives in the decomposing plant, causing disease mainly in wet summer conditions.
Control
• Ensure that decayed plants are completely decomposed before an area is replanted.
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