Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
A bioassay is the best way to determine potential injury from Root Knot Nematode on vegetable crops, such as lima beans, from spring sampling. It can also be used to survey the distribution of Root Knot Nematodes in the field.
To conduct a bioassay, divide your field into grids of one or two acres and take soil samples to a 12 inch depth for each grid so that you have at least enough soil to plant several 3-4 inch pots. Keep soil from grids separate.
Thoroughly mix composite soil sample for one grid and place in two 3-4” pots with drainage holes. You can use lettuce seedlings, or direct seeded cucumbers as indicator plants. After filling pots, plant with 2 lettuce seedlings or cucumber seeds. Maintain in a greenhouse or on a workbench under lights, watering daily or as needed for 4 to 6 weeks. Fertilize once a week with a solution of complete fertilizer.
After 4 to 6 weeks, remove plants from the pots/containers and wash roots free of soil and rate for root gall severity using the scale below:
Root gall severity ratings should not exceed 2 for carrots or 3 for other vegetable crops without treating with a registered nematicide. If levels are high across the field, consider rotating to a crop that is not a good host, such as wheat, barley, or corn.
Below is an example of a bioassay result from a Delaware field. Grids are 2 acres in size. Expect significant yield losses where root gall severity ratings are greater than 3.
Visit this Cornell site for more details on bioassays for Root Knot Nematode: http://veg-guidelines.cce.cornell.edu/Rootknotnemahowto.pdf