Diseases of Lucerne - 4: Nematodes
Note Number: AG0729
Published: November 1999
Alfalfa stem nematode
Alfalfa stem nematode (Ditylenchus dipsaci) is the most important problem nematode on lucerne, capable of causing a steady decline in plant numbers. The presence of this nematode in a stand, can increase its susceptibility to Bacterial Wilt.
The nematode enters the plant through developing buds. Infected plants are stunted, the stems enlarge and usually become discoloured, the nodes swell and the internodes are short. Lower stem necrosis occurs as the nematode numbers within the stem increase. Stems may blacken from the crown up for 300 mm or more. The number of stems per plant are reduced as the crown is destroyed and then the plant eventually dies.
When infestation is heavy the nematodes may move out onto the leaves causing a curling and distorting of the leaf tissue.
Some affected shoots, particularly regrowth after the first hay cut may be devoid of green colour (white flagging), but only slightly smaller. Roots are occasionally infected and the nematodes cause internal cavities or gall-like outgrowths.
Survival: The nematode may survive as larvae for years in infested plant debris, hay and in lucerne crowns and soil.
Environmental conditions: The female nematode lays between 75 and 100 eggs with optimum reproduction and infestation during cool, 15° to 20°C, wet weather.
A life cycle can take 19 to 23 days under these conditions. This nematode has been reported to be a problem in heavy soils, in areas with high rainfall and irrigated areas.
Dispersal: The larvae move over the plant surface in a film of water. Spread from paddock to paddock may occur via farm machinery and free flowing ground water. Re-used irrigation water probably spreads the nematode most effectively.
Host Range: Ditylencyhus dipsaci may survive on lucerne, medics, red clover, white clover, subterranean clover, oats, wheat, beans and onions.
Use resistant cultivars. Use crop rotations with 2 to 3 years between non host crops. Re-contamination by machinery and irrigation water can negate the rotation effect.
Root knot nematode
Root knot nematode (Meloidogyne spp.) is probably the most widely distributed plant parasitic nematode in the world, and capable of causing a gradual thinning of the stand.
This nematode can build up large populations in lucerne, but usually causes less damage to the lucerne than to other more highly susceptible crops that follow. The presence of this nematode may increase the susceptibility of plants to infection by fungal and bacterial pathogens.
Infected plants are yellow and stunted, roots are branched excessively due to the nematodes feeding on the root tips. There are small galls on these roots that are difficult to see.
Survival: Meloidogyne spp. may survive as egg laying females in old root galls in the soil.
Environmental conditions: Root knot nematodes are favoured by sandy loam soils. The life cycle on lucerne takes about 30 days at 25°C.
Dispersal: Spread from paddock to paddock may occur via farm machinery and free flowing ground water.
Host Range: Meloidogyne spp. occur on lucerne, medics, white clover, red clover, subterranean clover, wheat, onions, bean and lupins.
Use resistant cultivars. Crop rotations are not practical because of the nematode’s wide host range.
This Information Note was developed by Rod Clarke, November 1999.