2009 - The Year of Bulb and Stem Nematode in Garlic

This season garlic growers are dealing with an aggressive, extremely damaging pest - the bulb and stem nematode. The microscopic worm-like nematode enters plant material through the roots or through wounds on the bulb. Once inside, it happily feeds on plant tissue and injects a toxin into the plant resulting in distorted growth.

Often, in garlic, the severity of damage isn't obvious until harvest; however, this season the above ground symptoms of bulb and stem nematode have been quite pronounced. Early above-ground symptoms include a yellowing of the oldest leaves, which spreads to newer leaves as the season progresses. Plants exhibiting these symptoms start to wilt and can be easily pulled out of the soil. Inspection of the basal plate reveals missing roots, and dry rot of the base of the bulb (Figure 1).

Figure 1.

Although bulb and stem nematode has a wide host range, known to feed on other alliums, peas, pumpkin, beets, rhubarb and a host of weed species (e.g. common chickweed, prostrate knotweed, wild buckwheat etc.), the biggest economic impact appears to be on garlic. The reason for this is how garlic is produced. Garlic is grown from cloves saved as seed stock. And, although seed stock may appear clean and healthy to the naked eye, bulbs grown in fields with bulb and stem nematode are at risk of carrying the nematode into future plantings.

If you grow garlic, you need to prevent the introduction of this pest into your fields or, if present already, you need to manage populations to minimize economic losses. Sounds easy, but it requires a lot of hard work. Firstly, you need to test your soil and planting stock before planting. If you are purchasing seed stock, have a discussion with your supplier. A frank discussion now will save you the headaches and agony of losing your crop later. If either your soil or seed stock tests positive, then you will need to think about crop rotation, biofumigation (e.g. green manures), hot water treatments at time of planting, and purchasing new stock or finding new land. Although there are no quick fixes for this pest, there is some hope. Currently, researchers at the University of Guelph are working on a clean seed project, the goal of which is to produce virus free, nematode free seed stock.

Until that time, due diligence is required to minimize the spread and losses associated with this pest.

For more information on the bulb and stem nematode, or the hot water treatment I mention contact me, Jennifer Allen, Vegetable Crop Specialist, OMAFRA, Guelph.