Bulb and stem nematatode in garlic

The bulb and stem nematode is a microscopic worm-like organism that is causing extensive damage to garlic across the province. It's a parasitic nematode, which becomes active in the spring. Once active, it enters plants through the roots or wounds on the bulb/clove; however, research has shown that they can also enter stems or leaves close to the soil surface during periods high moisture. Once in/on a suitable host (e.g. garlic), the nematode injects a toxin into the plant as it feeds, causing lesions and resulting in distorted growth.

Under warm soil conditions (15-18°C), it takes between 19-23 days from egg to adult. Adults live from 45-74 days with the female capable of laying between 200 - 500 eggs during her lifetime. The fourth juvenile stage (J4) is able to enter a survival stage/stage to survive adverse conditions. Research has shown that under field conditions, J4s can survive in this state anywhere from 3-5 years. In museums, previously dried J4 specimens as old as 23 years could be 'awakened' and became viable nematodes.

Above ground symptoms include yellow leaves, which dry prematurely and stunted plants (Fig 1). In many cases, diseased bulbs/cloves are then invaded by other fungi and bacteria resulting in secondary diseases and rots. Infested garlic bulbs tend to be soft, shriveled, discoloured and lighter in weight. The basal plate and roots of severely infested bulbs may also appear to have a dry rot and can be easily separated from the bulbs, mimicking symptoms of Fusarium basal plate rot.

Fig. 1. Above and below ground symptoms of bulb and stem nematode feeding activity.
Top photo - yellowing of oldest leaves is a symptom of bulb and stem nematode. Bottom photo - rotting of basal plate and root loss.

Bulb and stem nematode can spread within the field and to other fields via a variety of vehicles (Fig. 2):

• infected seed stock
  
• contaminated soil (in field, on equipment, on people)
  
• irrigation/rain water
  
• volunteer plants

Fig. 2. Nematode dispersal. A) on/in infected bulbs; B) in free water; and C) within leaf tissue.
Source: http://plpnemweb.ucdavis.edu/nemaplex/images/G042S14.jpg

So, what can you do? There are two key points, prevent the introduction into your fields, and, where present, keep populations below economic thresholds. In Ontario, we use a threshold of 100 nematodes per kg of soil. In Russia, the threshold is 10 nematodes per 500 cubic cm.

Ways to reduce damage:

• test your soil before planting a host crop; if high, don't plant
  
• use clean seed - obtain and plant seed from a reputable seed supplier
  
• use a hot water treatment on seed stock before planting (Research has shown that when cloves are submerged in hot water (between 44 - 50?C) for an hour, there is a reduction in the number of nematodes present; but remember, bulbs/cloves are living things and there is always a risk that the cloves may experience some damage during this process) and;
  
• rotate and implement a 3-year crop rotation with non-host crops.

If you have an issue with nematodes, you may also want to consider growing a Brassica green manure (e.g. Indian mustard) which releases allyl isothiocyanates, a natural fumigant, when broken down and worked into the soil.