Growing Magazine - February, 2012

NORTH FEATURES

Battling Garlic Bloat Nematode

Time to test in seed and soil
By Kara Lynn Dunn

In June of 2010, Cornell Cooperative Extension (CCE) Vegetable Specialist Christine "Christy" Hoepting saw something she didn't like in a western New York garlic crop.

"The field I saw had as much as 90 percent crop loss in some sections. A colleague advised me to 'Get it tested for bloat nematode,' which he knew to be just over the border in southwestern Ontario," Hoepting says.


The bulb on the right is healthy; the bulb on the left shows GBN damage
PHOTOS BY CHRISTINE HOEPTING.

Hoepting sent a sample to Cornell University nematologist Dr. George S. Abawi. He confirmed that the damage Hoepting saw was caused by the microscopic and parasitic stem and bulb (bloat) nematode (Ditylenchus dipsaci). It is commonly called bloat nematode or garlic bloat nematode (GBN).

Since then, Abawi's lab at the New York State Agricultural Experiment Station (NYSAES) in Geneva, N.Y., has identified GBN in garlic seed or soil samples from at least 17 New York counties.

GBN damage was also reported in garlic crops in Vermont and Massachusetts in 2010.

GBN has been studied in Europe since 1877. It was first reported on garlic in the U.S. in 1935 in California. In 1929-30, it was reported that the GBN was causing damage in an onion crop on a farm in Canastota, N.Y.

By the end of 2011, Abawi said, "Through testing samples and conversation with colleagues, I am now aware of this nematode also occurring in Maine, Connecticut, Pennsylvania and western U.S. states."

Some growers have suffered up to 80 percent crop and economic loss. Damage can occur with an infestation as low as 10 nematodes per 500 grams of soil.

Stunted growth with limp, yellow and sparse foliage is a sign of GBN activity. Bulbs first show light discoloration, then go soft, turn dark brown, shrink and split.

Infection by GBN makes the garlic vulnerable to secondary infections by soil organisms, resulting in various forms of decay symptoms. The Ontario Ministry of Agriculture adds secondary soft rot, underdevelopment of bulbs, and roots absent on one side of the basal plate to the list of symptoms of GBN infection.

Growers may confuse GBN damage with symptoms of fusarium basal rot. Abawi says, "Cutting open an infected clove vertically will show a brownish discoloration of the stem plate from the outer layer and upward. In later stages, fusarium causes stem plates to become pithy, but the cloves show only dry rot in contrast to GBN infection, which affects the whole bulb and the leaves."

The garlic leaves, stems and bulbs attract juvenile nematodes. Once the infected plant tissues become rotted and desiccated, the adults move into the soil and can overwinter in dry soil and in harvested crops in storage (primarily in the fourth juvenile stage).

Tips for Managing Garlic Bloat Nematode (GBN)

  • Test your seed and test your soil
  • Plant GBN-free seed in a site not cropped to garlic in the past four years
  • Plant biofumigant cover crops after harvest; the green manure of mustard, rapeseed, oilseed radish, sorghum-sudangrass hybrids and other cover crops is thought to possibly help reduce GBN populations
  • Keep fields moist
  • Practice aggressive field, equipment and storage sanitation
  • Treat infested fields with a conventional fumigant-type nematicide
  • Use a five-year planting rotation: rotate away from allium species, other known GBN host crops and control nightshades for at least four years. Carrots, potatoes, spinach, corn and wheat are among the poor hosts of GBN

Non-fumigant nematicides have not been tested or registered for use on garlic in New York.

From "Managing Bloat Nematode in Garlic" by Crystal Stewart, Christy Hoepting and George Abawi, Cornell Cooperative Extension, September 2010.

GBN can exist in low levels for several years, and is often not discovered until high levels of crop damage are seen. Infestations can increase a thousandfold in just one growing season.

State grant supports GBN testing

The New York State Department of Agriculture and Markets awarded a $69,122 specialty crop grant to Abawi and Cornell Cooperative Extension vegetable specialists Crystal Stewart (Capital District) and Robert Hadad (western New York) to provide diagnostic expertise to growers statewide in 2012.

The grant funds will be used for garlic seed analysis, will help determine the current range of GBN in the state, and will facilitate outreach regarding the management of this pest by the vegetable industry.

Testing of garlic samples usually involves selecting 5 to 20 grams of fresh (not frozen) tissues from a sample of 10 bulbs or more. The selected subsample of garlic tissues is incubated for three to four days to extract and identify GBN in the sample, if any exists. Of the samples tested in the Abawi lab in 2010, some showed infestation levels as high as 987 nematodes per gram. A number of the soil samples processed also tested positive for GBN.

Hoepting says that Abawi's diagnostic analysis is critical to managing bloat nematode and the cost of its damage.

Stewart says 10 percent of all New York vegetable farms grow garlic as a high-value crop. New York's acreage of garlic has grown from 11 acres reported in 1992 to 330 in the 2007 Ag Census.

New York is now the fourth largest garlic producing state in the U.S.

"Garlic is a financially important aspect of our vegetable industry. Annual gross sales are worth $19.6 to $24.5 million," Stewart says.

Garlic Seed Foundation Director David Stern says, "The bloat nematode issue has brought the New York state garlic industry together to fight a real problem that can decimate your crop. New York is extremely fortunate to have Dr. Abawi, his nematode lab and the Cornell Extension vegetable team working to help growers."

While GBN can spread on field equipment, in planting material and crop debris, on boots, and in field runoff, Abawi says the parasite is spreading primarily through infected seed and soil.

"Clean seed must go together with clean soil. Growers do not help themselves by planting clean seed in infested ground," he says. "This problem will worsen if we do not identify both the infected seed and the infested soil throughout the production regions."

He encourages growers to review and enhance their agricultural practices to reduce the risk of infection - from planting site selection to harvesting and storage methods.

"It is critical that growers do not transport infected seed, bulbs or plant material," Abawi says, "and do not replant any of the garlic from a lot that is known to be GBN-infested, even if the bulbs appear normal."

Stern has issued a Garlic Seed Foundation Alert, which notes that Canadian growers have been watching GBN spread in Ontario for the past five years. He advises growers: "If you've been growing your same garlic for many years and have not introduced any new stock, it is doubtful you would have this pest. However, if you purchased/brought in new planting material on a regular basis, you may have this pest."

Although test results from small samples may not suggest 100 percent nematode-free production, Abawi says, the lab testing gives growers an opportunity to implement best practices to limit the distribution and damage of bloat nematodes.

Best management practices include crop rotation, keeping soils moist (GBN likes dry conditions), properly removing crop debris, and hot water washing of field equipment.

"It is known that the bloat nematode can survive longer in dried crop debris than it generally can in wet soil. The dormant juveniles lodge in crevices on the seed surface and in the outer sheath layers of bulbs and cloves," says Hoepting. "Do not dump infested bulbs or debris in production fields or in cull piles. Do bury the infected bulbs deep in the soil outside production fields or burn where possible."

Confirm the source; certification program in the future?

Abawi would like to see an official certification program started to help growers ensure that the seed they buy, plant and/or sell is clean.

"For both the sellers of seed and the sellers of fresh garlic, being able to say your product is GBN-free would be a tremendous marketing advantage," Abawi says.

A November 2011 press release from the New York State Department of Agriculture and Markets quotes Commissioner Darrel J. Aubertine: "Garlic is an important crop in New York. It is a valuable crop that has seen production increase. But, this pest [GBN] presents a real threat to our garlic crop. Growers need to understand the threat and take precautions."

The release states: "To help prevent stem and bulb nematode from entering the country and impacting our local crop, growers should always require a valid phytosanitary certificate when they purchase foreign seed."

All seed garlic shipments into the U.S. from Canada are now being tested for GBN.

U.S. Customs and Border Protection (CBP) inspections of imported shipments of garlic seed found that 40 to 50 percent were infected with GBN. Ann Marie Paul, CBP assistant director of field operations at Buffalo, N.Y., said, "We were surprised at the volume of shipments found to be infested with this nematode, given how clean and disease-free the garlic cloves appeared."

While bloat nematodes prefer garlic, they can also affect onions, leeks, chives, celery, some varieties of peas and lettuce, as well as other plants. More than 120 plants can serve as a host.

Abawi's research team has developed guidelines for submitting seed and soil samples to the Cornell lab. Contact Crystal Stewart at 518-775-0018, cls263@cornell.edu; Robert Hadad: 585-739-4065, rgh26@cornell.edu; or Dr. George Abawi: 315-787-2374, gsa1@cornell.edu.

Kara Lynn Dunn is a freelance writer who writes from her farm in Mannsville, N.Y.