Biological Control: Combating nematodes — Can it be done in less time? Print
Root lesion nematodes are a serious problem in many nursery crops. They can, however, be controlled quite effectively with Tagetes (Marigolds).  Two problems with this approach are that it is very easy to introduce weeds and that the control crop requires three summer months.  PPO (Plant Pathology Research) started research to determine whether it was possible to reduce the control crop time and still maintain proper control.

The cropping of marigolds is an eco-friendly nematode control method.  A good crop of Tagetes patula can virtually eliminate root lesion nematodes (Pratylenchus penetrans) in a field with benefits continuing measurably for several years.

However, take note that not all Tagetes varieties are good at controlling root lesion nematode. Tagetes minuta and Tagetes erecta either have either no or considerably reduced effect.  Marigolds are not a ‘silver bullet’ as, unfortunately, there are several downsides to their use as a nematode control.

Drawbacks

BecauseTagetes  develops slowly initially, it is possible for weeds to get an early foothold in the crop.  And because root lesion nematodes are capable of multiplying on the roots of many weeds, the control from the crop may be drastically reduced.  Several cultivars now have allelopathic properties to combat the establishment of weeds.

Another significant problem is that the grower has to take a field out of production for the entire growing season.  Tagetes needs to be seeded between mid-May and mid-July and needs three months to maximize effectiveness.  This method has been used widely, to generally favorable  results.  The big question of course is: Will a shorter control crop time reduce the resultant nematode suppression?

Effective against nematodes

PPO has conducted research to determine the effectiveness of a shorter crop time for nematode control.  The widely used cultivar Tagetes patula ’Ground Control’ was used for the tests.  In 2006, three different crop times were tested:
  • Standard (from early June to winter),
  • Three months (early June to early September) and
  • Two months (from early June until early August).
All three crop times produced good results.  At the start of the trials approximately 1000 root lesion nematodes were present per 100 ml of soil.  This is a large and damaging population.  After the two month crop only 12 root lesion nematodes per 100ml remained.  The three month crop brought this down to five and the standard crop to two.

In 2008 the Tagetes crops were tested from mid-June until mid-November.  Soils were tested for nematodes after nine weeks and after 11 weeks.  Another sample was taken a month after the plough-down.  These tests were compared to summer fallow (no weeds were allowed to grow) and a crop of oilseed radish (a crop where root lesion nematode multiplies readily).  The infection with root lesion nematodes dropped quite rapidly — from a high of 400 per 100ml of soil at the start to roughly 50 by August (at nine weeks) and September (11 weeks), and ultimately to eight in December.

In the hot summer of 2006, the short crop time of the Tagetes was very effective.  In the cool summer of 2008 it was less effective.  It is not apparent whether the 50 root lesion nematodes found in August would be sufficiently low to allow for the production of  sensitive nursery crops.

Two months are sufficient

The crop in 2008 was not entirely optimized as the plants established themselves rather slowly, and in the end the stand was only reasonable to good.  Based on the results obtained, it is reasonable to state that a Tagetes crop of two months may be sufficient for good control of root lesion nematodes.  Whether a sensitive crop can be grown without problems still needs to be tested.

source:  De Boomkwrkerij
 

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