Question
My potatoes had a 'heart attack'. They were going big and strong and about two weeks ago they keeled over. When I took them out of the ground I had potatoes of course, but I had many, many little white bugs on the roots, nematodes. How do I treat my soil to prevent this from happening. I am using all organic seeds now so I pretty sure they will affect my other plants: cukes, beans, tomatoes, etc. By the way I am in central Florida.

Answer
Daniel:

As I have not had nematode problems before I did a little research. The below article is a little long but gets to the point. I agree with his findings. Crop rotation and nematode resistant vegetables are the only answers.

Plan Ahead For
Effective Garden Nematode Control
Ed Perry
Farm Advisor



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Root knot nematodes are one of the most troublesome garden pests, especially in annual vegetable crops. The nematodes, which are tiny eel-like roundworms that feed on plant roots, injure the plant抯 ability to take up water and nutrients and cause a general decline of plants, usually without killing them directly. Most gardeners aren抰 aware of what may be causing the problem until they pull the affected plant out, then notice the distinctive swellings, called galls, on the roots. There are currently no chemical controls for nematodes in garden soils, so gardeners need to learn how to control these pests using a variety of cultural techniques.

One way to reduce nematode populations is to leave the soil unplanted for a season or more, a practice known as fallowing. You must keep the garden perfectly weed free to keep the nematodes from surviving on alternate hosts. You should also keep the soil moist, which induces nematode egg hatch. Without roots to feed on, the pests starve. Fallowing the soil for one year will usually lower root knot nematode populations enough to successfully grow a susceptible annual crop. Better yet is to fallow for two years, which lowers the population even further.

You will have to repeat the fallowing from time to time, especially when you again find roots with lots of galls. A good way to carry out a fallowing program is to split the garden into thirds and fallow one-third every 1 or 2 years on a rotating schedule.

Solarization is a good method of temporarily reducing nematode populations in the top 12 inches of soil, allowing you to successfully grow shallow-rooted crops. It will not provide long term protection for susceptible fruit trees or vines, however. For the solarization process to be effective, thoroughly moisten the soil, then cover with a clear plastic tarp; leave the tarp in place for 4 to 6 weeks during the hottest part of the summer (July-August). Root knot nematodes, including egg stages, are readily killed when soil temperatures exceed 125 degrees F for 30 minutes or 130 degrees F for 5 minutes.

Crop rotation, or growing crops which do not allow nematodes to reproduce, is another cultural control method. While crop rotation works well in controlling species like the cyst nematode, it unfortunately doesn抰 work as well for the root knot nematode. Root knot nematodes are the most common garden nematode pests, and attack too wide a range of weed and vegetable crop species. However, with careful long term planning, you can use rotation in combination with fallowing or solarization to manage most problems. Annual crops that help to reduce root knot nematode populations include small grains such as wheat and barley, sudangrass, and resistant tomato and bean varieties. Other crops, like corn and onions, are not seriously damaged by root knot nematodes, but still allow the pests to build up in the soil; such crops would not be good choices in a rotation program.

A good way of managing nematodes, or at least to lessen the impact of nematodes on your crops, is to add organic matter to your soil. Materials such as compost, manure and peat moss may contain substances that are harmful to nematodes. The difficulty is in getting these materials deep enough into the soil to reach the target nematodes. Organic amendments are probably more useful in their ability to increase the water holding capacity of soil, especially in sandy soils, where root knot nematodes tend to be a greater problem. Plants that are water-stressed are more seriously injured by root knot nematodes. Improving the soil抯 water holding capacity helps to prevent the drought stress; there may be just as many nematodes in the soil, but they will cause less damage.

One of the simplest ways of managing nematodes is to use vegetable varieties or tree rootstocks that are not susceptible to damage by nematodes. Various levels of tolerance and resistance are available. for example, stone fruit and almond trees can be protected from root knot nematode damage with Nemaguard rootstock, and tomato varieties with VFN after their names are resistant to root knot nematodes. Root knot resistant grape varieties include Harmony and Freedom. Unfortunately, no resistance is available for the majority of garden crops.



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Index for Home Horticulture


The author is Ed Perry, Farm Advisor,
University of California Cooperative Extension.


March 15, 1999