Stem and Bulb Nematodes in Garlic

In mid-summer of 2008 garlic growers across Saskatchewan noted that their garlic did not look right.  The plants lacked vigor and the leaves were turning yellow beginning at the bottom of the plant.   Within a few weeks the tops were dead and the bulbs were distorted and decayed.    Assays of the diseased plants showed the presence of several known pathogens – one of which (Fusarium oxysporum f.sp. cepae) causes an array of symptoms very similar to those observed in the 2007 crop.  However the assays also showed that the affected bulbs and stems were loaded with microscopic nematodes which could also been causing or contributing to the problem.   Of particular concern is the Garlic Stem and Bulb nematode (Ditylenchus dipsaci).   This nematode is already widespread in temperate regions but infections of specific fields can usually be traced to the planting of garlic cloves or onion sets that came from nematode-infested fields.   Once introduced into a field the nematode can persist in a dormant state for many years until a suitable host crop is planted.  While these nemas prefer garlic and onions they can survive on a range of vegetable and grain crops.  When conditions are right (saturated soil) the nemas swim from the soil into the leaf sheaths of the young garlic plants.  They then move towards the base of the leaves where they begin to feed.   As they feed they inject a toxin into the leaf base which causes the affected tissues to die – leaving a characteristic yellow spotting of the leaf base (Fig. 1).    The nemas reproduce quickly during warm summer weather, with populations increasing by a factor of 1000 fold over the course of a single growing season.  As the bulbs begin to form the nema feeding results in loose, distorted growth with the outside of the bulbs showing a roughened corky texture.  Affected bulbs are prone to splitting and tend to separate from the roots at the basal plate (Fig. 2) – this opens the bulb to infection by the previously mentioned Fusarium pathogen.

Problems with Stem and Bulb Nematodes are common in many of the major garlic production areas of the world (Ontario, California and China).   As the problem is introduced on planting materials growers should be cautious about the health of any garlic they plan on using as seed.   Where possible the garlic should be tested for the presence of nemas prior to purchase.   Heat treatment will reduce nema populations on the seed garlic (cloves immersed for 2 hours in 44-50oC  water), but may also damage the garlic resulting in reduced vigor.   Once the nemas have been introduced into the field they are very difficult to eradicate.   A four year rotation out of host crops is recommended.  Rotating with crops such as mustard, millet or marigold may further reduce nema numbers as residues of these crops appear to be toxic to the nemas.   While nemas can be effectively eliminated using chemical fumigants this practice is expensive, environmentally unsound and useless if the field is then replanted with nematode infected seed garlic.   Having the soil tested for nemas prior to planting is strongly recommended if there the area has a previous history of garlic/onion production.   While cool moist production conditions and fine textured (clay) soils are most favourable for the development and spread of the nemas, damage to the crop seems to be worse in warm, dry years.   While there are no lines of garlic that are truly “nematode resistant”, some vigorous growing lines appear to be better able to tolerate the nemas than others.
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