Phytophthora Root Rot - Phytophthora medicaginis
Description: Plants turn yellow, wilt and die. Areas of light brown discolouration up to 5 cm long occur on the taproot up to 30cm below the crown. The Taproot below the discoloured area will rot away completely.
Incidence: Occurs throughout Australia, particularly in heavy and/or poorly drained soils, and in wet conditions. The disease can be severe, killing large numbers of seedlings or scattered plants or large patches in mature seeds. In irrigated stands plants can survive. The taproots immediately below the crown is keptalive by the availability of water but forage yields are reduced.
Spread: The fungus spreads rapidly in water over considerable distances.
Management: Use resistant varieties and spell the paddock from lucerne. Do not rotate with chickpeas.
Avoid waterlogging irrigated stands on heavy soils.
Colletotrichum Crown Rot or Stem Anthracnose -
Description: Brown-Black spots on the stems develop into well defined boat shaped lesions that are up to 25 mm long, dark around the edges with pale centres, and covered in raised dark spots. The fungus can also enter the crown causing a blue-black discolouration, 5 - 8 cm into the taproot. In mature stands, the dead stems are white or straw coloured with a sheppard’s crook appearance. Plant death occurs gradually. Incidence: Occurs throughout Australia in warm environments with high humidity. It is more severe during late summer to autumn. It is less likely in drier and cooler climates.
Spread: Spores spread in warm, wet weather from plant debris and from the crown of infected plants by raindrop splash and harvesting equipment.
Management: Use disease resistant varieties and, if crown rot and anthracnose have been severe, rotate the crop every three years with non-host plants.
Spotted Alfalfa Aphid (SAA) - Therioaphis trifolii
Description: Adults are pale yellowish-green, 2 mm long, with six or more rows of black spots along their backs. Adults may have wings. Nymphs are smaller and wingless.
Damage: Adults and nymphs suck sap from the stems or the undersides of lower leaves. SAA inject a toxin that can kill seedlings and mature plants. Prior to that, leaf veins become yellow or white and the leaves curl and drop off. Honeydew excreted by SAA causes foliage to become sticky and develop a black, sooty mould.
Incidence: Occur throughout Australia in dry conditions, mainly in the spring and autumn.
Management: Plant resistant varieties. Monitor beneficial insects. Irrigate or graze the stand to reduce SAA numbers. In irrigated hay stands, use insecticides if the infestation is heavy.
Bluegreen Aphid (BGA) - Acyrthosiphon kondoi
Description: Adults vary from pale green-grey to dark green-blue and are 3 mm long and have tube-like projections on either side at the rear of their bodies. Adults may have wings. Nymphs are smaller and wingless.
Damage: Adults and nymphs suck sap from the leaves and stems at the growing points, causing shortened internodes between the leaves at the top of each stem, stunted plants, leaf curling and leaf yellowing. Honeydew excreted by BGA make the foliage sticky and affects hay and pasture quality. BGA do not kill mature plants.
Incidence: Occur throughout Australia & most active during the cooler months, particularly dry conditions.
Management: Plant resistant varieties. Monitor beneficial insects. Irrigate or graze the stand to reduce BGA numbers. In irrigated hay stands, use insecticides if the infestation is heavy.
Pea Aphid (PA) - Acyrthosiphon pisom
Description: Green in colour, though some may be yellow or pink. They are 4 - 5 mm long with dark bands around the antennae and spine-like projections on both sides at the rear of their bodies. Adults may have wings. Nymphs are smaller and wingless.
Damage: PA suck sap from the leaves causing wilting, stunting and curling, and odd-shaped plants. The top leaves often turn light green while the lower turn yellow and die. Honeydew excreted by PA makes foliage sticlky, affecting hay and pasture quality. PA is a significant carrier of alfalfa mosaic virus.
Incidence: Common in southern Australia, Western Australia and New South Wales during dry conditions in spring and autumn,although economic levels of damage are rare.
Management: Monitor beneficial insects. Irrigate or graze the stand to reduce PA numbers. In irrigated
hay stands, use insecticides if the infestation is heavy.
Stem Nematode - Ditylenchus dipsaci
Description: Microscopic eel-worms that are individually difficult to see with the naked eye. Sometimes they mass on or just below the surface to form visible ”eel-worm wool”. These can survive desiccation and be transported in hay to start new infestations.
Damage: Plants are dwarfed and distorted, with swollen shoots. Leaves are distorted and clustered towards the ends of stems. Plants die in patches.
Incidence: Occur in southern Australia, common in irrigated stands on river flats, with greatest severity in the spring.
Management: Sow resistant varieties, plough out badly infested stands and practice crop rotation.
Bacterial Wilt - Clavibacter michiganense ssp.insidiosus
Description: Yellow and stunted plants with small leaves are scattered through the stand. The inner bark of the taproot is white while the exposed root centre is yellowish.
Incidence: Common in southern Australia, but has not been reported in the southeast of South Australia. It often occurs in autumn in irrigated stands. It is not found in the dry, inland subtropics of Queensland and northern New South Wales.
Spread: The bacteria persist in soil for more than 10 years. The disease is spread by stem nematodes and through hay and machinery.
Management: Sow certified seed of resistant varieties.
Fusarium Wilt - Fusarium oxysporum f.sp.medicaginis
Description: Initially, plants are stunted with wilted shoots and yellow leaves.The infection then bleaches the leaves and stems, eventually causing plant death. Dark red-brown streaks develop in a layer under the bark at the base of the stem forming a reddish-brown ring in the centre of the root.
Incidence: Fusarium wilt is not common. The Fusarium fungus is widespread, but rarely causes wilt. Fusarium wilt has not been identified in New South Wales.
Spread: The fungus survives for long periods in decaying plants. It invades small roots or wounds in the taproot during warm, wet weather.
Management: Controlled by crop rotation and resistant varieties.