Bulb and Stem Nematodes In Garlic

The stem and bulb nematode, (Ditylenchus dipsaci) is one of the most destructive pests of garlic in North America and much of the world. They are a microscopic worm-like parasite that damages plants and are very difficult to control once  established in a growing area. The arrival of this pest can be devastating to a grower and make it very difficult to continue growing garlic or other alliums in the future.

Sources 

Nematodes can be found in seed cloves, bulbils,  plant debris, soil or in water. They are most often introduced into a field from infected garlic cloves, but can also arrive with soil on equipment, from water sources used for irrigation or from overland flooding.   

Once present on a farm, nematodes can remain dormant in the soil, survive in plant residues and on weed hosts for many years and sometimes indefinitely. If garlic is planted again, even a small number of nematodes can reproduce very rapidly, resulting in a population explosion that can cause significant damage.

Symptoms

Infected garlic plants become swollen, leaves appear twisted and malformed, young roots and bulbs rot, severely infected plants turn yellow and die. Plants that do survive have bulbs that are deformed, and have shorter leaves that turn brown prematurely at harvest. Severely infected garlic bulbs are soft, discoloured and deformed, with portions of their root system missing. 

Often times a low-level infestation will go unnoticed, but the nematodes can multiply in storage if the bulbs are not kept at low temperatures. 

Nematode infected bulbs are easily infected by secondary pathogens. This is why nematode infections are often mistaken for fusarium basal rot or other diseases and why it is a good idea to have diseased bulbs tested for their presence.

Prevention & Control 

Planting clean nematode free garlic seed into nematode free soil is the primary control method. Using long rotations out of Alliums for 4 to 5 years will help keep populations low if they are already present. As well, removal of weeds and volunteer garlic plants every year is important even when growing other crops.

Planting cover crops of mustard, rapeseed, oilseed, radish or sorghum-Sudan grass for one to two years before growing garlic has also been shown to help.

Proper sanitation should always be followed to prevent nematodes from being transferred from infested to non-infested fields. Tools and equipment should be carefully cleaned before using them to another location.

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About the Author: John Côté owns and operates John Boy Farms with his family who has been farming the same land for over 140 years. As an agronomist and experienced farmer, he helps other growers learn how to grow garlic successfully. He has written many articles and is the author of The Master Guide to Growing Big Garlic.