COLE CROPS, ROOTS, BULBS AND LEAFY VEGETABLE EDITION

Prepared by:  Jennifer Allen, Vegetable Crop Specialist

Fig 1.  Globular springtail.
© 2009  Lynette Schimming.   
 www.bugguide.net/node/view/254638

Globular springtails are tiny (1 mm/ 1/16^th inch), primitive wingless insects (Fig. 1).  Females lay their eggs in moist soil, with a preference for areas of high organic matter (Fig. 2 – life cycle).  The young often only differ from the adults in colour.  Both lifestages spring away when disturbed using a structure called a furcula, located on the underside of their abdomen (hence the name springtails).  As an aside - these little critters can propel themselves a distance of up to 20 times their body length!  

Fig. 2. Springtail life cycle
Source: http://www.pfc.cfs.nrcan.gc.ca/diseases/nursery/images/full_images/fig82.jpg

Most springtails are considered beneficial soil scavengers, feeding on decaying matter, fungi, moulds and other soil microbes; however, there are a couple of species, which feed on plants.  An example is the garden springtail (Fig. 3).  Young garden springtails are orange in colour, while adults are reddish-black.

Fig. 3.  Adult garden springtail – actual size (left) and 15x magnification (right).  Special thanks to Bryan Mailey for all garlic photos.

In general, springtails thrive in moist soil environments where there is an ample supple of decaying plant material (e.g. leaf litter, decaying wood, high levels of organic matter, soils amended with compost).  Species that spend most of their time in the soil are very susceptible to desiccation.  Based on what I’ve observed, this may not be true for the garden springtail.  In garlic, both immature and adult springtails were happily feeding in the heat of the day under a clear blue sky with temperatures reaching 28˚C.  With that said, they could leave the plant at any time and take refuge under ground cover.   

Damage caused by the garden springtail can be seen below (Figs. 4 and 5).  In most cases, damage is only observed on the oldest leaves.  Feeding sites are pale yellow in colour, and appear as though the springtails have ‘sucked’ out the chlorophyll.  Lesions may be mistaken for botrytis   leaf blight.  Botrytis is often found throughout the field and affects young and old tissues alike.  Also, botrytis lesions are often larger than those caused by the springtails.  And finally, springtail feeding removes the chlorophyll but doesn’t appear to change the leaf texture.  Botrytis lesions cause death of tissue and have a different texture than healthy leaf tissue.  In addition to the feeding damage, I also observed a lot of leaf yellowing of the oldest leaves.  When these plants were pulled, we found that springtails weren’t the only critter munching away - bulb and stem nematode was also present.  The question now is – which came first?  Are the springtails more attracted to the garlic due to activities of the nematode, or vice versa?  Are the decaying garlic bulbs, attacked by the nematodes releasing chemicals that are attractive to the springtails?  Are plants fed on by springtails more susceptible/attractive to the bulb and stem nematodes?

Many questions!  


Fig. 4.  Springtail feeding damage on garlic leaves (left and right). 

Fig. 5.  Springtail damage (leaf lesions) and leaf yellowing caused by bulb and stem nematode (top and bottom photos).

Bulb and stem nematode

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. 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.  

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:

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

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:  

1. test your soil before planting a host crop; if high, don’t plant
2. use clean seed - obtain and plant seed from a reputable seed supplier 
3. 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;
4. 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.