Photos by Vern Gruginger unless otherwise noted.
Many garlic growers have heard about the garlic bloat nematode (also called bulb and stem nematode) that attacks their valuable crop. However, it's worth making sure that everyone who buys or sells garlic for seed is well-informed about this pest and understands the practices that can prevent it from becoming a much bigger problem than it already is.
Steven Johnson and David Fuller of the University of Maine Cooperative Extension recently prepared an excellent fact sheet (www.umaine.edu/publications/1205e) about the history and life cycle of this pest, what it does to crops, and what growers can do about it. Much of the following information comes from that document, and is used with permission.
The garlic bloat nematode (Ditylenchus dipsaci) is a microscopic non-segmented roundworm. In the U.S., it was first reported on onions in 1929 and on garlic in 1935. It has only became a significant threat in our region recently, probably after it was imported from Canada on garlic intended for food use that was planted as seed garlic. The problem then spread through the sale of garlic seed, because there is no certification program for seed garlic.
Crops attacked by the nematode include other Allium species, such as leeks, onions and chives, as well as flower bulb crops like iris and gladiolus. The nematode can cause severe damage to garlic, whether grown in backyard gardens or on commercial farms.
Symptoms of infection are stunted and yellow leaves, and plants may die prematurely. Initial bulb infection may show light discoloration. Later, entire cloves or bulbs will become shrunken and soft, darken and eventually decay. Mild or new infections may show no symptoms initially. The absence of roots, often on one side of the basal plate, is a symptom of infection. Advanced symptoms include distinct swellings or "bloat," along with underdeveloped or deformed bulbs. Infected bulbs are susceptible to secondary invaders such as fusarium and other fungi, soft rot or other bacteria and bulb mites. Heavily infected garlic cloves may not store well, and nematode reproduction and damage to bulbs will continue and may increase during storage.
Symptoms of infection by garlic bloat nematode include brown, spongy tissue and lack of roots on bulbs. Testing is the only way to confirm an infection. Anyone selling garlic seed should have it tested before offering it for sale. Regardless of the cause, it is a bad idea to plant unhealthy-looking garlic seed.
The bloat nematode swims limited distances on its own, but is easily spread with infected plant material; water runoff; contaminated equipment, clothes or shoes; or other means of transporting infested soil. Most often, the bloat nematode is introduced to a field by planting infected seed bulbs. Understanding the garlic bloat nematode and how it is spread will help to minimize the serious and long-lasting effects of this harmful crop pest.
Nematodes reproduce via mating, which takes place within plant tissue. The entire life cycle can be completed in about 21 days. After fertilization, females can deposit as many as 500 eggs over a 50-day period. The first molt is in the egg; the nematodes that actually hatch out of the egg are second-stage juveniles. The juveniles undergo two additional molts to produce third and fourth-stage juveniles. Although the last three stages of juveniles are capable of attacking plants, the fourth-stage juvenile is the most destructive. They are able to withstand drying conditions, which allows them to survive for several years in plant tissue or soil, becoming active when a host and moisture are present. Fourth-stage juveniles can readily travel up the garlic plant in a film of water, then penetrate and feed on leaves and shoots.
Cool, moist conditions are ideal for nematodes to invade plants; they migrate to the soil surface after rains. The nematodes can penetrate through stomata, cracks or leaf axils. After entering the plant, the fourth-stage juveniles go through the final molt, where they differentiate into mature males and females capable of breeding. Several life cycles may be completed within a season in the outer layers of leaf and bulb tissue. In late stages of plant decay, fourth-stage juveniles can collect on or near heavily infected tissue and form "nematode wool."
Management of this nematode must take into consideration that it is a long-term pest and can spread quickly from seemingly clean-looking seed stock. A combination of approaches is the best way to avoid the risk of infection. These include: testing garlic seed, planting only clean garlic seed, cleaning soil off of equipment between uses, using a long rotation between garlic crops, and sowing biofumigant cover crops as part of that rotation.
Garlic bulbs, plants and field soil can be tested for the nematode. However, a test is no better than the sample taken. Clean test results are not a guarantee that the entire farm is free from bloat nematode, only that it was not detected in the sample. Periodic retesting of seed garlic makes sense. Seed and soil testing for bloat nematode is not the same as a certification program.
Bloat nematode is introduced and perpetuated primarily by planting seed that is infected with the nematode. If bloat nematode has been confirmed in a field, do not replant any garlic from that field. Bulbs from an infested field may appear symptomless, but could harbor low levels of bloat nematodes and will serve to perpetuate the issue. It is unknown whether the nematodes could be harbored in bulbils formed at the top of scapes, so avoid planting bulbils from an infected crop. Hot water treatment of garlic planting stock is not recommended as a solution to infected seed garlic unless professional equipment is used. Introduction of garlic planting stock from a source that is not known to be bloat nematode-free is fraught with risk.
Testing for Garlic Bloat Nematode
The University of Maine offers a low-cost test for garlic bloat nematode. This service is key to reducing the spread of this pest in the state and the region. Plants, bulbs and/or soil samples can be tested for the presence of the garlic bloat nematode.
Plant samples: Collect five entire living plants between June 15 and July 30 in varying stages of decline. Do not submit totally dead plants; try to send weak or stunted plants. Cut the tops off the garlic, leaving the bulb and about 1 foot of aboveground plant.
Harvested bulb samples: Collect five bulbs to be tested.
Soil samples: Collect about 1 cup of soil from under and around plants in varying stages of decline after July 1.
If garlic is planted in multiple fields separated by 100 feet or more, each field would require separate samples. All samples should be placed in a sealed sandwich bag and kept cool until they are shipped - early in the week so they arrive at the lab promptly. Include a check for $20 per sample ($15 per sample for five or more samples at one time), payable to The Maine Garlic Project. Ship to: Steven B. Johnson, Ph.D., Maine Garlic Project, Cooperative Extension, Aroostook Farm, 59 Houlton Road, Presque Isle, Maine 04769.
You must provide the following information for each sample: Your name, mailing address, phone, email, and a description of the garlic variety.
Cornell University also offers bloat nematode testing at a reduced rate for New York growers. Visit http://cvp.cce.cornell.edu/submission.php?id=59 for information.
No seed garlic should be planted or sold for planting from a field that has tested positive for bloat nematode. A garlic field that is separated from any infested field by at least 100 feet could be marketed for seed, assuming that it does not test positive. Bulbs from infested fields may be sold for table use, but they should be labeled as such.
Clean and disinfect all equipment that is used in garlic fields. The bloat nematode can be transported on clothing, shoes and anything else that moves soil. Changing shoes, power washing equipment and sanitizing it using quaternary ammonium materials offer some defense against the introduction of the bloat nematode into previously clean areas. Crop sanitation is also important; infected bulbs and plants should be bagged and discarded in a landfill, not onto fields or in compost piles.
Use long rotations between susceptible crops. The long-term survival potential and wide host range enables the nematode to persist in infested fields for long periods of time. Primary management tools include:
- Long rotations out of Allium crops (garlic, onions, leeks, chives), celery, parsley or salsify
- Keeping weeds such as Canada thistle and hairy nightshade to a minimum
- Starting with bloat nematode-free planting material
A five-year rotation (four years between host crops) is a minimum. Bloat nematodes can also live in the soil and on alternative hosts.
A plow-down biofumigant crop will not eliminate the need for the long rotation, but there is some evidence that a mustard cover crop can suppress bloat nematode populations and subsequent garlic infection. Research in Ontario concluded that oriental mustard (Cutlass or Pacific Gold, for example) appears to suppress bulb and stem nematode soil populations, though it will not eradicate them. Other mustard varieties that are high in glucosinolates (the active ingredient) should also be effective.
The cover crop can be used in the crop rotation, with two sowings in the year prior to planting garlic in the fall. Getting a good mustard stand requires a firm seedbed; use a seed drill and pack after seeding. Sow at 6 to 7 pounds per acre, perhaps 10 if broadcast instead of drilled. Seed in the spring and again in August. Early weed control is essential for good cover crop establishment. Bare fallow ahead of and in between the mustard crops if need be. If possible, chop the cover crop, and then incorporate as green manure just prior to flowering, as that is when it has the maximum glucosinolate content.
Garlic is a high-value crop that has been grown successfully for many years on farms across the Northeast. With proper management, growers can help keep garlic bloat nematode from becoming a threat to that success.
The author is a vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.