by Michael Celetti, Plant Pathologist – Hort Crops, OMAFRA
The bulb and stem nematode (Ditylenchus dipsacci) is a very destructive pest of garlic, onion, leek as well as many other host crops.
The nematodes begin their life cycle as eggs that hatch and develop through several stages to become adult nematodes. The 4th juvenile stage can survive extreme conditions for many years in either the soil or infested bulbs by going into a resting state. Under wet conditions the eggs hatch or the dormant 4th juvenile stage nematodes become re-activated and swim in water filled pours in soil to other neighbouring plants or swim up the wet stems and leaves of garlic plants in the film of water left after a rain, dew or irrigation event. As they swim on the wet plant surfaces, they feed on leaf tissue by piercing plant cells with their needle-like mouth part sometimes leaving a small yellow spot or pimple. The nematodes move down between the leaves of garlic continuing to feed, develop and multiply. They can actively live from 45 to 75 days and begin to reproduce within 19 days after hatching when temperatures average around 15oC. A single female can lay up to 500 eggs within her life span and several generations can be produced within one growing season.
Severely infected plants turn yellow, dry prematurely and appear stunted. Often infested plants on which the nematodes have been feeding and reproducing in will appear to have thicker leaves and necks. Eventually the nematodes reach the bulb of the garlic plants. Severely infested garlic bulbs tend to be soft particularly around the neck, eventually become shriveled, spongy, discoloured and lighter in weight. The basal plate (the region of the bulb where the roots attach) of severely infested bulbs may also appear to have a dry rot which can be easily separated from the bulbs. Bacteria, fungi and onion maggots will invade nematode infested bulbs causing soft rot and decay (Figure 1). Often symptoms of bulb and stem nematode in garlic look very similar to symptoms of Fusarium basal plate rot.
Although bulb and stem nematode has a huge host range with more than 1,200 species of plants, there are over 20 races of this nematode pest. Each race looks morphologically identical but each has a different but narrower host rangers. For example, research has shown that the race of bulb and stem nematode that attacks garlic can also attack onions and maybe leeks, but not spinach or chicory, were as the race that attacks spinach and chicory does not attack garlic or onions. This is important when considering management practices such as crop rotation.
A 3 year crop rotation with non-susceptible crops such as a cereal crop, soil fumigation or planting a nematode suppressing cover crop such as oriental mustard or French marigolds in the rotation and before planting garlic have all been shown to keep this pest under control in the soil. However, if infested seed is planted back into fields that have been rotated with a non-host crop for several years, or fields that have been fumigated, or where nematode suppressive cover crops have been grown, the nematodes will be reintroduced into the field negating the effects of crop rotation or fumigation and the damage caused by this pest will continue. Some garlic bulbs from infested fields may not appear to have symptoms but are probably infested. Planting garlic seed harvested from contaminated fields, no matter how good it looks, will result in the introduction of this nematode into clean fields. In fact, bulb and stem nematode has been spreading within Ontario through the distribution of what appears to be non-infested garlic seed that was harvested from infested fields. It is important to have garlic tested at a pest diagnostic lab for bulb and stem nematode before using it as seed the following season, particularly if the seed came from a field that had plants with symptoms. If the tests results indicate the presence of bulb and stem nematodes, the bulbs should not be used for seed. Garlic growers should only purchase garlic from a reputable garlic seed supplier. Always ask the supplier if the garlic has been tested for bulb and stem nematode by a qualified pest diagnostic lab.
The Ontario Garlic Association (OGA) has provided funding over the past few years to the Northern Horticultural Research/SPUD Unit at the University of Guelph New Liskeard Agricultural Research Station to propagate nematode-free garlic seed for the OGA members. They have recently released a small quantity of nematode-free garlic seed to the OGA for distribution to their members to begin growing and bulking nematode free garlic seed for a wider distribution in the future. This is the first step in managing the spread of this menacing pest, however, planting nematode free seed in fields that have bulb and stem nematodes, will only result in the garlic from these fields becoming contaminated. Therefore it is important for garlic growers to ensure that this small quantity of nematode free seed be planted in fields that have been tested and determined to be free of bulb and stem nematode. Controlling the spread and damage caused by bulb and stem nematodes in garlic can only be achieved through the combination of planting nematode free seed in non-infested fields or fields that do not have a history of the nematodes.