December Q & A
by Michelle Le Strange, UC Master Gardener Advisor

Q. Last year my neighbor gave me 10 small tomato plants, which I set out in my garden. Only half of them grew to full size and produced fruit. The rest looked sick all summer long. When I finally dug them up, I noticed their roots were infested with nematode galls. I've always heard nematodes were hard to get rid of. Can you give me any ideas on what to do?

A.Nematodes are microscopic worms that infest the soil and attack susceptible plant roots. You can find nematodes in all soils, however they seem to cause most damage in sandier soils. Nematode populations vary widely in the soil even in the same backyard.

Some good news is that nematodes are not active when soil is cold. You can raise winter crops on land with a high nematode population. The danger time is when soil warms up in the spring. Detecting nematode damage is difficult in the beginning. When you finally do see signs of trouble it is usually summer or early fall. By then, roots are badly damaged and the population of nematodes will have increased greatly.

The first piece of advice is to not plant tomatoes, peppers, potatoes, or okra where you had tomatoes before. These vegetables are all in the nightshade plant family and are extremely susceptible. Nematodes increase most rapidly if you grow susceptible plants repeatedly. So move those summer veggies to a new spot in the yard.

Another way to reduce a nematode population is to leave the soil bare for one season or more, a practice known as fallowing. The area must be kept absolutely weed free to keep nematodes from surviving on alternative plant hosts, and works best if the ground is moist. Fallowing the soil for one year will lower root knot nematode populations enough to successfully grow a susceptible annual crop; 2 years of fallow will lower populations still further.

An important tip is to keep nematodes out of your garden. Purchase plants from reputable nurseries and look for tomato and pepper varieties resistant to nematodes. Be careful not to bring nematodes into your yard with topsoil or manure containing crop residues.

Solarization can be used to temporarily reduce nematode populations in the top 12 inches of soil, allowing successful production of shallow-rooted crops. For the solarization process to be effective the soil must be moistened and the clear plastic tarps must be left on for 4 to 6 weeks during the hottest part of the summer.

There are no pesticides for nematodes that the homeowner can purchase. Professional applicators can apply metham and sometimes methyl bromide in the small garden. Both are biocides that kill more than just nematodes.

For many years it has been suggested that planting marigolds suppressed nematodes. Turns out that sometimes they do and sometimes they don't. More important is that marigolds contain an abundance of water-soluble compounds which are biocides and nematicides, so that in addition to killing the nematodes, the biocides sometimes affect the plants that follow the marigolds.

If you can't fallow, solarize, or fumigate your ground, then try crop rotation. Certain plants are more resistant to nematodes that others. You can grow these without injury. After 2-3 years of non-susceptible crops nematode populations have diminished so that you can try a susceptible crop again. Garlic, onions, and sweet corn are examples of vegetables that are non-susceptible to nematodes; most others are susceptible. Ornamentals listed as tolerant of nematodes include azalea, camellia, ferns, daffodils and tulips. Susceptible ones are begonia, pansy, petunia, rose, and zinnia. Adding compost prior to planting helps dilute the damage that nematodes can cause and provides more of an advantage to the plants.

Q. Last spring I read that oak trees were dying from a mysterious disease. Has there been any progress in determining the cause?

A. Since 1995, tan oaks, coast live oaks, and black oaks have been dying in large numbers in Marin, Santa Clara, Santa Cruz and Monterey Counties, while none of the common oak trees in the valley were inflicted with this unusual malady. Dubbed "Sudden Oak Death" because large mature trees virtually died in a few months time, researchers were stumped as to the cause. The first noticeable symptom is the appearance of a dark thick sap that oozes to the surface of the bark. New growth droops, yellows, and before long the entire top of the tree is wilted.

Researchers presumed the cause was a common soil inhabiting fungus called Phytophthora, however the disease did not match any of the currently known 60 species. Last summer a UC plant pathologist finally isolated the Phytophthora organism doing the damage. This species is different from most of its relatives because the spores are airborne, not soilborne, so spread is more frequent and rapid.

December 21, 2000