Coastal Gardener – Nov 14, 2002
COASTAL GARDENER
Franklin Laemmlen, Ph.D.
Q:
I was thinning out the dianthus in my flower bed when I came upon the enclosed
specimen. The plant appeared to be growing OK, but as you can see, the roots
have numerous galls and are very deformed. What is the problem?
A:
Your dianthus has a severe infestation of root knot nematodes. There are many
species of nematodes of which a few hundred species attack plants and animals.
Root knot nematode is a microscopic worm that is non-segmented, i.e., if you
magnified it many times, it would look like a snake. The female lays eggs which
hatch into a juvenile nematode, which has the ability to penetrate plant roots.
Once inside the root, the nematode pierces a cell with a very fine needle-like
mouth and begins drawing nutrients from the cell. The injury to the cell is so
slight that the cell continues to function normally. In fact, in the case of root knot,
it functions better than normal, and secretions from the nematode stimulate the
cell to grow and cause other cells in the immediate vicinity to also become super
active. This results in the gall formation you see. Each gall has one to several
nematodes feeding inside it.
Root knot nematodes are a common and serious landscape and garden problem in
the sandy soils of the Central Coast. Anytime you have plants that are growing
poorly, losing vigor, declining or dying prematurely, or are smaller than nearby
plants of the same kind, you should suspect the presence of nematodes. Galled
and deformed roots are the underground symptoms of the presence of root knot
nematode.
Control of this pest is difficult. One of the reasons strawberry growers fumigate
their fields is to control nematodes. Soil fumigants and some other soil-applied
chemicals are the only effective way to control nematodes, and this control is only
temporary. Another means of control is to grow plants that will not be a host to
the nematode. Unfortunately, there are very few plants on which root knot
nematode does not feed. Grasses and marigolds are good plants to use where
nematode populations are found in your garden. There are a few vegetables which
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Coastal Gardener – Nov 14, 2002
have resistance to nematodes. The N in VFN tomato varieties indicates resistance
to nematode attack. Finally, use of high amounts of organic matter as a soil
amendment can help suppress nematodes and help stimulate healthy plant growth.
Unfortunately, I know of no comprehensive list of nematode-resistant plants, so
the gardener is pretty much left to trial and error in developing a landscape that
will grow and do well in the presence of nematodes.
Send your landscape and garden questions to: The Coastal Gardener, 624-A West
Foster Road, Santa Maria, CA 93455.
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