Diseases of Lucerne - 5: Fungal Stem Diseases
Note Number: AG0730
Published: November 1999
Spring black stem, caused by the fungus Phoma medicaginis (Ascochyta imperfecta) is a common disease of lucerne which can cause reductions in yield and forage quality.
All parts of the plant may become infected. Early symptoms are small, dark-brown or black spots on the leaves, leaf petioles and stems. As the disease develops the lesions on the stems rapidly enlarge and join together until most of the stem is blackened. The infected areas on the stems may extend for several centimetres or may entirely girdle it. Severely infected stems and petioles wither and die. The leaves turn yellow and drop off if harvest is delayed.
Survival: The fungus carries-over on infected plant debris, on the seed coat and in the soil. New shoots become infected as they grow up through the infected crop debris.
Environmental conditions: Cool moist conditions are ideal for disease development.
Dispersal: Spores released from the infected crop debris are primarily spread by rain splash, but also by wind and insects.
Host Range: Phoma medicaginis occurs on lucerne, medics, subterranean clover, red clover, trefoil and common vetch.
Cut hay as early as possible to reduce losses through leaf drop.
Seed from drier areas are less likely to be infected.
There are no resistant cultivars nor chemicals.
Sclerotinia crown and stem rot
Sclerotinia crown and stem rot, caused by the fungus Sclerotinia trifoliorum, occurs in most lucerne growing areas. The incidence and severity of this disease varies from year to year; there may be just scattered plants affected or when severe, whole paddocks. The greatest damage occurs when seedlings are affected.
The fungus infects the base of the stems and crown branches, near the soil line, causing death of these tissues and rapid wilting of the plant tops. A white cottony fungal mould develops on infected tissue during moist weather. Small, hard, black sclerotia form in the mould and inside the stems and crown.
Survival: The black resting bodies, sclerotia, enable the fungus to survive the hot summers on plant debris or in the soil, where they can survive for at least five years.
Environmental conditions: Cool, moist weather and luxuriant plant growth are ideal conditions for the development of this disease.
Dispersal: Under moist conditions, in autumn, the sclerotia germinate to produce a saucer shaped spore bearing body (apothecia). The spores are ejected from this body and are carried onto nearby leaves by wind to set up the initial infection. Secondary spread is via fungal growth from plant to plant.
Host Range: Sclerotinia trifoliorum occurs on lucerne, red clover, white clover, subterranean clover and vetch.
Close grazing of clover in the autumn and early winter will slow the spread of the disease by reducing the humidity within the stand.
Deep ploughing will bury sclerotia and reduce their chances of germinating.
Use seed that is free of sclerotia.
Use rotations of at least four to five years between susceptible crops. There are no resistant cultivars.
Charcoal rot, caused by the fungus Macrophomina phaseolina is unpredictable in occurrence and sporadic in appearance within the crop. When severe it can cause considerable thinning of stands due to plant deaths.
Dry, light brown lesions occur on the crown and roots which later become soft and spongy. After a period of hot dry weather the conducting tissues become covered with very small black resting bodies, sclerotia. By this time the tops of the plant will have wilted and died.
Survival: Macrophomina phaseolina carries-over as resting bodies, sclerotia, in the soil, on plant debris and on the seed coat.
Environmental conditions: Charcoal rot is most destructive when excessively hot conditions (35°C to 39°C) follow periods of good growth. The fungus invades immature, unthrifty, wounded or senescent tissue. A nutritionally balanced plant growing in good physical conditions is not likely to be severely affected.
Dispersal: After the resting bodies (sclerotia) germinate and the fungal growth (mycelium), invades the plant, fruiting bodies (pycnidia) form. Spores from the pycnidia are dispersed by rain splash. Spread from crop to crop may occur via wind-borne infected plant debris.
Host Range: Macrophomina phaseolina occurs on lucerne, subterranean clover, lupin, sunflower, soybean, maize, sorghum, garlic, potato, cotton, groundnut and conifers.
Minimise moisture and nutritional stress where possible.
Crop rotations and deep burial of infected plant debris will help reduce inoculum levels.
Rhizoctonia stem blight
Rhizoctonia stem blight, caused by the fungus Rhizoctonia solani can cause a slow thinning to the point where the stand is uneconomical.
The fungus infects the crown and stem bases just below the soil line. Dark sunken elongated cankers form near the base of the stem, often girdling it and causing the leaves to yellow and wilt.
Minute black resting bodies, sclerotia, may be found, just visible to the naked eye, on the dying roots.
Survival: The fungus carries-over as resting bodies, sclerotia, either in the soil or in plant debris.
Environmental conditions: High soil temperatures (25° to 30°C) and moisture favour disease development. Under flooding or irrigation, plant parts above the soil may be affected. Mechanical damage to roots and crowns and unfavourable soil conditions are also associated with this disease.
Dispersal: Under moist soil conditions the resting bodies, sclerotia, germinate and fungus moves through the soil until it finds a nearby susceptible host.
Host Range: Rhizoctonia solani has a vast host range including: weeds, lucerne, red clover, white clover, subterranean clover, pasture grasses, grain legumes, cereals and oilseed crops.
No control measures are known.
Limit machinery movement through the stand to minimise damage to crowns.
Improving drainage may be useful.
This Agnote was developed by Rod Clarke, November 1999.