Managing the Bulb and Stem Nematode Menace in Garlic

The bulb and stem nematode (Ditylenchus dipsacci) is a microscopic worm-like organism that can be a very destructive pest of garlic, onion, and leek as well as many other host crops. Unfortunately, this pest has been spreading recently on garlic cloves used for seed. Regardless of the crop being grown, the bulb and stem nematode has the potential of causing complete crop failure. The nematodes can spread through irrigation water, in infected bulbs or cloves, as well as on contaminated equipment, footwear and clothing.

The bulb and stem nematode survive freezing or extremely dry conditions in a dormant state in infested plant debris, infested soil and between the scales of infested bulbs of Allium crops (onion, leaks, shallots, garlic). Under wet conditions the nematodes become active and swim in the film of water in soil or on wet plant surfaces. They feed on leaves and bulbs (but rarely roots) by piercing plant cells with their stylet or hypodermic needle-like mouth part. During feeding, they inject saliva containing an enzyme into the cells which can cause distorted growth of the tissue. Leaves of severely infected plants turn yellow and dry prematurely resulting in stunted plants. Once the nematodes have caused initial damage to the growing garlic plant, hot dry weather will often exacerbate symptoms even though this type of weather does not favour the spread or activity of the pests. Other fungi and bacteria often enter wounded bulbs and cause further degradation. Infested garlic bulbs tend to be soft, shriveled, discoloured and lighter in weight. The basal plate and roots of severely infested bulbs may also appear to have a dry rot and can be easily separated from the bulbs, mimicking symptoms of Fusarium basal plate rot.

Bulb and stem nematodes prefer wet soils and are not likely to cause damage during hot dry seasons unless the fields are over-irrigated or heavily infested seed was planted. Prolonged periods of rain and cool temperatures tend to favour the activity, reproduction and spread of this pest (such as the weather conditions experienced last year). These nematodes complete their life cycle within 19 to 23 days under optimum conditions and during this time a single female can lay up to 500 eggs. Several generations can occur over one growing season resulting in a rapid and dramatic population explosion. Although it was very hot and dry in many areas of Ontario this past season, the increase in infested garlic gloves planted from last years crop in the fall of 2004 impacted many garlic growers this year. The impact of this pest on the 2005 garlic crop would have been worse if the weather remained wet and cool again this year.

Slightly infested cloves used for seed often do not show any symptoms although the presence of nematodes can be detected by examining tissue under a microscope. Using garlic from contaminated fields as seed will most likely result in the introduction of this nematode into clean fields or further spread of this pest. Obtaining and planting nematode-free seed from a reputable seed supplier is the best way to prevent further spread of this pest. However, hot water treatment of garlic bulbs used for seed has been shown to be an effective method for reducing, and in some cases completely eliminating the nematode from seed. This technique is only effective when the thermal tolerance of the nematode is less than that of the plant material. Temperatures above 50°C may injure the garlic cloves resulting in poor germination and establishment, while temperatures less than 44°C for 1 hour may not kill all the nematodes.

For best results:

• Select only healthy, disease-free bulbs for hot water treatment and planting. Unhealthy bulbs may not survive the hot water treatment which could lead to poor germination and establishment.
• It is recommended to presoak the bulbs in 0.1% detergent and water solution for about 1 hour at room temperatures before dipping them into the hot water bath.
• Remove the bulbs from the detergent solution and submerge them immediately into the hot water bath maintained at 44°C for 1 hour ensuring that all bulbs are completely submerged.
• After one hour, remove the bulbs and submerge them in a cool water bath for another 15 minutes or until the bulbs have cooled down completely.
• Remove the bulbs and spread them on a clean surface to dry.

As well as planting clean seed, implementing a 3-year crop rotation with a non-host crop and burying or burning infested or contaminated seed and plant debris are also necessary to reduce reintroduction and spread of this pest. Cleaning soil and debris off of equipment footwear and or clothing before moving between fields as well as cleaning debris from storage areas will also help reduce the potential spread of this pathogenic nematode. Selecting fields that have been tested for bulb and stem nematode and determined to be nematode free is also important to ensure that the crop being planted will not be infested by soil borne nematodes. Soil should be sampled and sent to a Pest Diagnostic Clinic qualified to extract, identify and enumerate nematode population levels, well before the crop is planted so that appropriate actions can be made. Fumigating soil in conventional agriculture production systems with a registered soil fumigant late in the summer or early in the fall before planting will also reduce the potential of severe losses and further spread of the bulb and stem nematode.

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