The stem and bulb nematode is a pest that can cause significant damage to garlic crops. The cool, wet weather experienced this spring has resulted in the spread of this pest within fields where infested garlic cloves were planted last fall.

The stem and bulb nematodes can survive in garlic cloves used for seed as well as in the soil. In fact they are often introduced into a field of garlic by planting infested cloves that otherwise look healthy. One stage (4th juvenile) of the nematode is particularly adapted to resist desiccation and freezing and can persist for many years under dry or cold conditions. A single female can lay up to 500 eggs within her life span and several generations can be produced within one growing season. It only takes 19 days for these nematodes to develop into mature adults when temperatures average around 15°C. They can live for 45 to 75 days depending upon the environmental conditions. The short period of time between emergence from the egg and maturity together with the frequency of reproduction often results in an explosion of this pest population under cool, wet conditions.

Stem and bulb nematodes feed on cells near the basal root plate of the garlic plant. As they feed they inject enzymes into the cells which break down cell walls resulting in a rotting around the root plated. During wet weather some nematodes may leave the infested garlic and swim to neighbouring healthy garlic plants. They enter neighbouring plants by getting in between the scales of the garlic bulb near the soil line. Under wet conditions, the nematodes can swim a short distance up leaves of small emerging plants in the spring and then move down between the leaves in films of water left from rain or dew. Later in the season, the nematodes can infect garlic plants through scales of the bulbs. If infection is closer to harvest, the nematodes may not cause noticeable damage to the mature bulbs. Growers may unknowingly select these infested bulb and cloves to be used as seed in the fall.

Managing stem and bulb nematode is not easy once it is introduced and becomes established in a field. Planting clean, nematode-free seed into non-infested soil is the best option to avoid this pest. Unfortunately the nematode has a very extensive host range with over 450 species of plants that can be infected. However, there are several races of this nematode, each with a specific limited host range. Although the entire host range for the Ontario race of stem and bulb nematode is not known, studies at the University of Manitoba indicate that the Ontario race can also infect and multiply in yellow pea, as well as pinto, kidney and navy bean. Once introduced, a four year crop rotation with non-susceptible crops such as a cereal crop, fumigating soil or planting a nematode suppressing cover crop such as oriental mustard before planting garlic can help suppress this nematode in soil.

Before planting garlic this fall, growers should have their garlic seed cloves and the soil in their field tested for stem and bulb nematodes. In a recent OMAFRA study, the stem and bulb nematode was found to inhabit the top 5 to 7.5 cm (2 to 3 inches) of the soil profile. When sampling soil to determine if stem and bulb nematodes are present, it is best to sample the top 5 to 7.5 cm of the soil profile using a 2.5 cm (1 inch) diameter soil tube probe. If the field or area where garlic is to be planted is less than 500 square meters, take at least 8 to 10 soil samples; between 500 square meters and half a hectare, take 25 to 35 soil samples; and between half a hectare and 2.5 hectares, take 50 to 60 samples and place in the bucket. The soil sample should represent no more than 2.5 hectares. Keep the soil sample in a cool place and submit to a pest diagnostic lab qualified for nematode extraction, identification and enumeration as soon as possible and no later than 1 to 2 days after sampling. Do not let the sample heat up or dry out before submitting to the lab.

Planting clean, nematode-free seed is the most important practice in managing this pest. If clean, nematode-free seed is not available, growers can try dipping infested cloves in a hot water bath at 49°C for 20 minutes; however, this is a very tricky technique and must be performed carefully to prevent damage to cloves. If the temperature drops below 49°C, the effectiveness of the hot water to kill nematodes in the cloves is significantly reduced. If the temperature of the hot water bath increases above 50°C, the garlic may be damaged resulting in poor emergence. Other management options are currently being investigated by the Ontario Ministry of Agriculture Food and Rural Affairs, the University of Guelph and Agriculture and Agri-Food Canada.

Figure 1: The basal plate of garlic bulbs (the region of the bulb where the roots attach) severely infested with the stem and bulb nematode appear rotted and can be easily separated from the bulbs.