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Parasitic nematodes

Agronomist, James Jess (Western Ag), has observed canola seedlings that have been damaged near Derrinallum, in the western district of Victoria. The stems of canola plants in a few paddocks at the 1-2 leaf stage are displaying signs of damage that resembles ring barking at, or near, the ground level. This is a common symptom of several possible pests including false wireworms, some weevils and plant parasitic nematodes. James reported that a single false wireworm larva was detected within the paddock. The only other possible culprit identified after extensive searching on two separate occasions (including a direct search at night) were tiny ‘worm-like’ larvae. These were found upon close inspection of affected plants that had swollen stems, and are likely to be parasitic nematodes. They were only discovered after peeling back the outer plant tissue in the swollen parts of the canola stems. James says the plants are now growing away from the damage.

Stem nematodes (Ditylenchus dipsaci) are relatively common plant parasites and are widespread in temperate areas of Australia. They have previously been detected in crops in South Australia, Victoria and New South Wales. There are many races of stem nematodes. The ‘oat race’ is one of the most problematic in broadacre crops and known to affect oats, faba beans, field peas, canola, lentils and chickpeas, along with some horticultural and ornamental plants. 

Stem nematodes can tolerate desiccation and they become inactive in dry soil, hay and seed, sometimes for many years at a time. When significant moisture is available (usually in autumn) they become active again and can invade young growing stem tissues of plants, especially seedlings that are still below the soil surface. Stem nematodes prefer heavier soil types and cooler, moist conditions.

Severe cases of stem nematodes can eventually lead to plant death. Some symptoms may be confused with herbicide damage, waterlogging or nutrient deficiency. Controlling stem nematodes is difficult once they have invaded plants, so control efforts should focus on rotations involving non-host crops (e.g. wheat and barley) or controlling volunteer hosts in paddocks and avoiding the spread of infested plant material, hay, seed and soil.

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