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Stem and Bulb Nematode

Stem and bulb nematodes (Ditylenchus dipsaci) are microscopic (0.9 - 1.8 mm long) roundworms that live in soil and plant tissues. They feed on stems, leaves and bulbs by puncturing and sucking cell contents with a needle-like mouthpart called a stylet. There are thought to be about twenty morphologically similar forms of D. dipsaci, each with a specific host group. Throughout Europe this organism attacks more than 1,200 species of wild and cultivated plants.

Except in cold weather, reproduction occurs throughout the year with a typical life cycle taking between 19 and 23 days at 15°C. When sexually mature, the nematode lives for between 45 and 75 days, each female being capable of producing up to 500 eggs. It is the fourth stage juvenile that is the nematode survival stage and it can go into a state of diapause ('slowed life') on or below the surface of plant stems, petioles and bulbs for up to five years.

The stem and bulb nematode is a migratory parasite and at the beginning of a new season the fourth stage juvenile will enter young tissues especially seedlings just below the soil surface. Migration on plant parts above ground level requires water and juveniles and adults will take advantage of rain and irrigation water to travel above the soil.

Symptoms: Garlic infested with stem and bulb nematode has distorted and swollen tissues with a spongy appearance. The plants become stunted with shortened and thickened leaves often with brown or yellowish spots. The bulb tissue begins softening at the neck and gradually proceeds downwards; scales appear grey and the bulbs desiccate and split at the base under dry conditions. Under wet conditions secondary invaders such as bacteria, fungi and onion maggots may induce soft rot and decay of the bulbs.

Management & Control: It is important to know the cropping history of the land to be planted; if the field is known to be free from stem and bulb nematode then planting should be done with clean, uninfected cloves. Where infestation of planting stock is evident or suspected then treating bulbs with hot water (46°C for 2 hours) has been shown to be effective in eradicating nematodes.

Bulb & Stem Nematode damage ( photo: VIARURAL)

Rotation with non-host crops is the most effective way of reducing nematode populations. Two years of rotation with crops such as carrots, lettuce and potatoes should reduce nematodes to below levels that would cause economic losses. Also, avoid infesting new land by cleaning machinery and equipment thoroughly and by preventing the movement of infested soil.

Stem and bulb nematode can also be controlled by the use of Fenamiphos (Nemacur 15G) at a rate of 7 - 14 Kg/acre applied with the garlic cloves at planting.

An organic alternative for areas heavily infested with nematodes is to grow dwarf French marigolds on the infested land. It has been shown that solid planting of marigolds suppresses nematode populations and greatly reduces the numbers found in the roots of susceptible hosts. Compounds toxic to nematodes have been identified in the growing roots of French marigolds and these have been shown to be released by the plants throughout the growing season and for the benefits to be evident after a period of three to four months. For maximum benefit marigolds should be planted 30 cm apart in May and grown through until autumn when the tops should be removed and the roots ploughed into the soil.

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