In Queensland there are several fungal root and crown diseases that, together with insect and physiological disorders, are responsible for poor lucerne persistence. Unlike most of the leaf and stem diseases, root and crown diseases have a permanent effect, killing infected plants and thereby reducing the plant stand. Of the diseases, phytophthora root rot and colletotrichum crown rot account for most losses.
Phytophthora Root Rot
Phytophthora root rot
Cause
• The fungus Phytophthora medicaginis.
Occurrence
• This is a very common disease, severe in heavy soils in wet conditions, infecting scattered plants or isolated patches.
Symptoms
• Infected plants yellow and wilt. The taproots have lightbrown areas up to 5 cm long, up to 30 cm below the crown. The roots below the discoloured area rots away and the plant may die.
Infection and spread
• The fungus survives as oospores or vegetatively in infected plant tissue, and, under wet conditions, moving spores are produced which are attracted towards lucerne roots. The fungus spreads rapidly in water, over considerable distances.
Control
• Avoid planting lucerne in very heavy soils, especially where drainage is impaired.
• Ensure adequate surface drainage to avoid waterlogging.
• Do not replant a paddock immediately after the stand has been ploughed in at the end of its economic life.
•Use disease-resistant cultivars. Note that planting infected paddocks with chickpea will result in serious chickpea losses.
Colletotrichum Crown Rot or Anthracnose
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Colletotrichum crown rot
Cause
• The fungus Colletotrichum trifolii.
Occurrence
• This is a very common disease, severe in most lucernegrowing districts.
Symptoms
• The fungus causes well-defined, elliptical spots, up to 25 mm long on the stems. They are brown-black at first, but later become bleached in the centre and dotted with small, dark fruiting bodies. Badly affected stems are killed.
• The fungus can enter the crown, where it causes an internal, bluish-black discolouration extending 5-8 cm into the taproot. There may also be dark lesions on the outside. Badly affected plants die.
Infection and spread
• The fungus survives in debris in the stand, on harvesting equipment and in crowns. Spores are spread from these sources in warm, wet weather by rain and irrigation. The disease reaches its peak from late summer to autumn.
Control
• Crop rotation (at least every three years) is recommended.
•Use disease-resistant cultivars.
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Violet root rot
Cause
• The fungus Helicobasidium purpureum = Rhizoctonia crocorum.
Occurrence
• This common disease is only occasionally severe.
Symptoms
• Above the ground, the plant turns yellow and dies. Small, circular patches of dead plants appear in the paddock in increasing numbers. The root system is covered with light brown to purple fungal growth extending from the crown to about 15 cm below the surface. The roots finally become rotten and shredded.
Infection and spread
• The fungus can survive in the soil for up to 20 years in the form of small, black resting structures (sclerotia). Infection is more severe under warm, wet conditions.
Control
• Do not replant areas where the disease has recently occurred.
• Do not renovate affected areas.
•Improve drainage.
Rhizoctonia Canker or Root Rot
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Rhizoctonia canker
Cause
• The fungus Rhizoctonia solani.
Occurrence
• This is uncommon but occasionally severe in drier areas.
Symptoms
• Dark-brown, elongated spots may appear on the stems, especially near the base. Sunken, elliptical cankers, coloured tan to buff with dark outlines, form in the taproot near the laterals. They kill the plants if they encircle the root. On the crown, brown lesions first appear on buds and young shoots, at or below ground level, eventually killing them.
Infection and spread
• The fungus survives in the soil or the infected plant material. Those strains causing stem lesions cannot infect crowns and roots. High temperatures are favourable to the disease, irrespective of soil moisture levels.
Control
• There are no known control measures.
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