Managing stem and bulb nematode in garlic starts in the fall
Soon it will be time to plant garlic for next year's crop. Growers are advised to be careful not to plant cloves infested with the stem and bulb nematode. Several fields of garlic in Ontario have been completely destroyed by stem and bulb nematodes in recent years. This microscopic worm-like pest has been spreading from field to field unknowingly in contaminated garlic cloves sold and used for seed. Although growers attempt to discard bulbs severely infested with stem and bulb nematodes (Figure 1), those that appear healthy and without symptoms growing adjacent to or near severely infested plants become infested in the field.
Figure 1 . Garlic bulbs severely infested with stem and bulb nematodes.
If the cloves of theses bulbs are used for seed this fall, the plants that grow from cloves of the healthy looking but contaminated bulbs will develop symptoms the following season. Furthermore the nematodes from this infested seed will eventually infect healthy plants growing nearby and the pest will spread.
Obvious symptoms of stem and bulb nematode damage often do not appear until late June or early July. Severely infected plants appear stunted, turn yellow, dry prematurely and are easily pulled from soil leaving the rotted region of the bulb where the roots attach, called the basal plate, in the soil (Figure 2). Infested garlic bulbs tend to be soft, shriveled, discoloured and lighter in weight. Often bacteria, fungi, maggots and mites will invade severely infested bulbs causing them to become mushy with soft rot and decay.
Figure 2. Garlic plants infected with stem and bulb nematode appear stunted, turn yellow, dry prematurely and are easily pulled from soil.
For a successful garlic crop next year, growers are strongly advised to plant garlic cloves that are free from disease and stem and bulb nematodes this fall. If symptoms appeared in the garlic crop this past season that is also being kept for seed to plant this fall, it may be wise to discard the seed no matter how good it looks and start over. Obtain seed from a reputable source that does not have stem and bulb nematode. If that is not possible, another option is to purchase clean seed from the clean seed program developed at the University of Guelph New Liskeard Research Station and now sold through August's Harvest.
Growers could attempt to soak their seed in hot water to kill the nematodes that are deep in the cloves as follows:
Use extreme caution when performing the hot water treatment. It is advised that good quality and accurate thermometers be used to monitor the temperature of the hot water bath. If the water gets up to 52°C or higher, the cloves will cook and not germinate. If the water does not get hot enough, the nematodes will not be killed and infestation will occur the following spring.
Growers should plant their clean garlic or hot water treated seed in a field that had not grown garlic for at least 3-4 years, since the nematodes will survive in soil from many years. In a recent study conducted at the University of Manitoba, the stem and bulb nematode strain that has infested Ontario garlic was also found to infect and reproduce on yellow peas, beans and chickpeas. Based on this information, garlic should not be planted in fields that had peas or beans in the rotation.
What is a nematode?
Nematodes are microscopic eel-like organisms that live in soil and water. Most soil dwelling nematodes are beneficial organisms that play a role in the break down and release of nutrients from organic matter. Some beneficial nematodes prey on other nematodes as well as soil-borne insect, fungal and bacteria pests. Unfortunately there are several species of nematodes that feed on or in roots, stems or bulbs resulting in significant yield reduction in both field and horticulture crops grown in Ontario. Examples of these include stem and bulb nematode, root knot nematode and root lesion nematode to name a few.
Figure 3. The stem and bulb nematode Ditylenchus dipsaci.
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