Nothing scares a garlic grower like the bloat nematode. With the potential for almost total destruction of a crop, the bloat nematode (Ditylenchus dipsaci) is most commonly introduced via infested seed. Once present, nematodes can survive in seed, crop debris, and soil for several years. Populations can remain at virtually undetectable levels before growing up to a thousand-fold in just one season, causing significant, and at times, complete crop loss.
Given its ability to survive in live seed, D. dipsaci poses the greatest threat to garlic crops, but also feeds on onions, leeks, and chives, as well as a variety of other plant hosts. The bloat nematode has been studied in Europe since 1877 and was first reported in the United States in 1929. In 2010, bloat nematode was found to be the cause of up to 80% crop loss in garlic-producing areas in western New York state and Ontario, Canada. A new pest to the Northeast, D. dipsaci is forcing growers in our area to take necessary precautions and carefully evaluate seed sources.
Resembling the shape of a worm, the D. dipsaci is less than 1.5 mm long. Their life cycle can be completed in 19-23 days. After fertilization by their male counterparts, mature females deposit 10 eggs per day for 20-50 days. Juveniles feed on young tissues of leaves, stems, and bulbs of Allium crops. The bloat nematode moves very little on its own but can be spread through infected seeds or planting materials, irrigation water or surface run-off, and contaminated equipment. Known to withstand near desiccation conditions in soil for several years, infestation is a long-term problem with very few organic options for control.
Signs of Infestation
The most severe infections of D. dipsaci cause garlic plants to exhibit stunted growth, yellowing and collapse of leaves, and premature defoliation. The bulbs first show slight discoloration but can later become entirely dark brown in color, shrunken, soft and light in weight. The activity of a number of opportunistic saprophytic soil organisms leads to further decay. With continued nematode reproduction, damage to bulbs can be exacerbated in storage.
Just Say No
Effective control of the bloat nematode starts with preventing its introduction into production areas. The importance of planting nematode-free seeds can not be overstated. Here at High Mowing Organic Seeds, we take this very seriously and have rigorous testing mechanisms in place to monitor all of our seed sources. If you’re saving your own garlic seed, do not replant any garlic from an infested site. If you’re interested in having your own seed tested, Cornell University and Michigan State University accept samples.
Sanitation is key to the prevention of D. dipsaci introduction. Aside from infested seed, bloat nematode can spread through contaminated tools, equipment, and plant debris. Thorough cultivation and removal of all crop debris can eliminate host material for dormant juvenile nematodes. Sanitation of equipment is recommended, especially that which is used in known infestation areas.
There is considerable literature available on various hot water treatments, which have been shown to reduce populations of nematodes in seed garlic. The water temperatures recommended range from 100-120 degrees Fahrenheit. We do not treat any of our garlic seed and view this as a last resort with its own risks, given that temperatures over 122 degrees Fahrenheit appear to injure garlic tissues.
In order to eliminate the build-up of nematode populations in the field, practicing at least a four year rotation is recommended. Avoid planting any Allium crops (garlic, onions, leeks, chives) as well as celery, parsley, salsify, and Shasta pea. Weed hosts include hairy nightshade and Canadian thistle. Even with the best crop rotation, we recommend testing any at-risk or previously infected soil before planting garlic.
A handful of cover crops are purported to reduce levels of bloat nematodes in the soil when incorporated as green manures. After harvesting garlic, plant mustard, sorghum-sudan grass hybrids, rapeseed, or oilseed radish, which all produce toxic metabolites during decomposition. Bloat nematodes thrive in dry conditions, so keep your fields where garlic was recently grown moist.