The stem and bulb nematode is a pest that can cause significant damage to garlic crops. The cool wet weather experienced last fall after planting and again in the early part of this spring 2016 has resulted in the spread of this pest within fields if infested garlic seed was 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. 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 conditions. The short period of time between egg hatch 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 plate. During wet weather some nematodes leave the infested garlic and swim to neighbouring healthy garlic plants. They enter the neighbouring garlic 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 bulbs and cloves to plant in the fall.
Managing bulb and stem 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 getting this pest. Unfortunately the nematode has a very extensive host range with more than 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, recent 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 in the rotation before planting garlic can help keep this pest suppressed in soil.
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 47°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.
Michael Celetti is plant pathologist, horticulture crops for the Ontario Ministry of Agriculture, Food and Rural Affairs, based in Guelph, Ontario.