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Garlic Growers Alert: Stem/Bloat Nematode

Posted: November 4, 2011

Garlic growers in Canada and several northeastern states have seen crop losses of 80-90% in sections of fields infested with the stem/bloat nematode (Ditylenchus dipsaci). Pennsylvania growers obtaining garlic seed/bulbs from Canadian and other northeastern sources may unknowingly introduce this serious pest into their fields.
bulbs (left) and infested garlic bulbs (right) with bloat nematode

bulbs (left) and infested garlic bulbs (right) with bloat nematode

The stem/bloat nematode attacks a variety of hosts including garlic, onions, chives, leeks, celery, peas, lettuce, hairy nightshade, Canada thistle, flower bulbs, and many other plant species. Garlic plants infected with the stem/bloat nematode exhibit foliar symptoms like stunting, yellowing and collapse of leaves, as well as premature defoliation. Bulbs of infected plants may display a light discoloration at first in the bulbs. As symptoms worsen, the cloves of the garlic will become dark brown in color, shrunken in appearance, and may exhibit cracking and/or decay symptoms that are often associated with the colonization of plant tissues by other naturally occurring soil dwelling saprophytic fungi. On onions and other hosts, growers may also see leaf deformation (twisting), distinct swellings of plant tissues, and even plant death. Nematode damage to onion and garlic from this diminutive pest often increases after harvest resulting in significant bulb damage in storage.

Growers wishing to prevent the establishment of stem/bloat nematodes on their farms should first and foremost plant only nematode-free seeds. While this would seem to be a rather easy task, infected garlic seed (bulbs) remains the chief source of infection on farms and growers need to make sure that their suppliers are truly providing nematode-free seed (bulbs).

If clean nematode-free garlic seed is not available, growers may consider a hot water treatment on their garlic seed. Hot water treatments are not 100% effective and if water temperatures above 1220 F are used the garlic bulb tissues will be injured. As a rule, most researchers are recommending that garlic bulbs be dipped in 1200F water for 20 minutes.

In some cases, growers may have inadvertently introduced the stem/bloat nematode into their fields from infected garlic seed. Research has shown that populations as low as 10 stem/bloat nematodes per 500 cc of soil will cause crop injury/losses. If nematode assays determine that populations of stem/bloat nematode exist in a field a grower should consider either soil fumigation, a four year crop rotation using non-host plants like corn and wheat, or the use of biofumigant crops like rape, mustard, and sorghum-sudangrass hybrids. Growers should conduct nematode assays prior to planting susceptible crops to make sure that your management strategies have been effective.

If you suspect a stem/bloat nematode problem in your garlic and/or onion field please contact your local Penn State Extension office regarding the submission of plant samples to a testing facility.

Source: Abawi, George S., Moktan, K. 2010. 2010 Bloat Nematode Problem on Garlic: Symptoms, Distribution, and Management Guidelines. http://www.hort.cornell.edu/expo//proceedings/Onions%20&%20Garlic/Abawi%20bloatSum.pdf