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Scientific Name: Ditylenchus dipsaci

Although there are more than 20 biological races of bulb and stem nematode, only a few races are known to occur in Canada and the federal government regulates the importation of potential hosts of exotic races to prevent their entry. Some biological races are host or crop specific while others are capable of invading more than one host. Crops infected by bulb and stem nematode include onions, garlic, leeks, chives, shallots, peas, beets, carrots, celery, tomato, cucumber, narcissus, hyacinth, tulip, oats, rye, alfalfa, red clover, phlox and strawberry. Onions, and other alliums, are susceptible to several races of bulb and stem nematode.   

Identification

• The base of infected seedlings appears swollen and leaves appear twisted, malformed and may bear slightly raised pimple like spots
• Severely infected plants eventually turn yellow and die.
• Scales on older infected bulbs often split and appear swollen or bloated and inner layers tend to bulge through

Often Confused with
Onion maggot damage

Biology
Bulb and stem nematode activity begins early in the spring.  In onions, the nematodes will move down inside the leaf sheath until they reach the bulb and feed between onion scales. During feeding, the nematode injects saliva containing a toxin into plant cells, creating lesions and distorted growth.

Bulb and stem nematode prefer moist muck soil and are less likely to cause damage during hot, dry seasons. However, irrigation can encourage the invasion of susceptible crops during drought periods. The nematodes can spread through irrigation water, infected bulbs or seed, and on contaminated equipment. They can survive freezing or extreme dry conditions in a dormant state in dried infested plant residues.  In fact, in museum collections, dried specimens have been ‘resucitated’ with water more than 23 years after drying.

Period of Activity
In Ontario, nematode populations are usually at their highest in May-June and again in September-October.  Soil populations of most plant parasitic nematodes tend to be highest in September and October after crops have senesced and died.

Scouting Notes
A crop may be attacked by the combined action of two or more kinds of nematodes.  In some cases, the presence of nematodes exacerbates some fungal diseases.  In areas where plants are stunted and/or the stands tend to have an uneven or wavy appearance, dig up plants and examine the roots and developing bulbs.   If you suspect nematodes are present, send samples of affected and healthy bulbs to the diagnostic lab for analysis.  Also, take eight to 10 soil cores from areas where plants are unhealthy and another eight to 10 soil cores separately from areas of healthy growing plants for comparison. 

The number of soil core samples required to estimate population levels, depends on the size of the area under investigation (Table 1). The chart below is a guide of how many cores are necessary to make up a representative sample. If soil type changes within the field, take separate samples from each soil type. Send the soil samples to a pest diagnostic clinic or laboratory that is qualified to isolate, identify and enumerate nematodes.

Table 1. Number of soil core samples/area required to estimate nematode populations.

AREA	NUMBER OF SOIL CORES/SAMPLE
< 500 m2	8-10
500 m2 - 0.5 ha	25-35
0.5 ha - 2.5 ha	50-60

Thresholds
The threshold for bulb and stem nematode in onions, garlic and other alliums is 100 nematodes/kg of soil.

Management Notes
How to soil sample for nematodes:

• Soil and root samples can be taken at any time of the year that they soil is not frozen. 
• Sample soil to a depth of 20-25cm (8-10 in.).  
• If living crops are present in the sample area, take samples within row and from the area of the feeder root zone (with trees, this is the drip line).
• Take soil and root samples from the margins of the problem area where the plants are still living.
• If possible take both soils and root samples from the problem and healthy areas in the same field.