Maybe
Nematodes are Nibbling at Your Roots?
by Michelle Le Strange, UC Master Gardener Advisor
If plants in your garden just don't seem to grow as big or as
robust as they should, then perhaps your soil is infested with
nematodes. Nematodes are tiny (microscopic) eel-like roundworms,
which live in soil and feed on plant roots. These nearly invisible
pests create an increasingly serious problem each season in many
home gardens. Unfortunately many plants are susceptible including
vegetables, flowers, trees, vines, berries, shrubs, and even weeds.
PLANT DAMAGE
Plants affected by nematodes may be off-color or stunted in growth.
In hot weather, infected plants often wilt or die because damaged
root systems cannot supply enough water. Annual and herbaceous
perennial plants may be killed by nematodes, but woody plants
rarely are. Injury to woody plants is less obvious and more difficult
to diagnose. Infested fruit and nut trees have reduced growth
and yields. Ornamental shrubs and trees have reduced growth and
branch tip dieback, and may defoliate earlier than normal.
NEMATODE FACTS
Root knot nematode is the most troublesome species in the garden.
It forms small knots/galls on young feeder roots, causing plants
to lose vigor and appear stunted. These swellings do not rub off
and should not be confused with the beneficial nitrogen-fixing
nodules on the roots of legume plants.
You can find nematodes in all soils, but population numbers vary
greatly from place to place even in the same garden. Root knot
nematodes cause most damage in sandier soils. They are not active
in cold winter months and overwinter as eggs. Nematode numbers
increase rapidly, if susceptible plants are grown repeatedly,
especially during summer months. Summer irrigation encourages
nematode activity. When nematodes become too numerous in one spot,
then many annual plants fail.
KEEP NEMATODES OUT
Although nematodes occur naturally in many soils, people increase
the problem. We carry nematodes from one area to another on garden
implements, plants, shoes caked with mud from infested soil, or
by bringing in infested topsoil.
PLANT RESISTANT VARIETIES
One great way to manage nematodes is to plant vegetable and flower
varieties and fruit tree rootstocks that are resistant or tolerant
of nematode injury. Varieties with VFN after their name are resistant
to most root knot nematodes, as are 'Nemaguard' rootstock used
for stone fruit and almond trees, and 'Harmony' and 'Freedom'
rootstock used for grapes. Citrus trees growing on 'Troyer' and
'Trifoliate' rootstocks are resistant to the citrus nematode.
FALLOW
Fallowing is the practice of leaving the soil bare for a period
of time. Fallowing for 1 year will lower nematode populations
enough to successfully grow a susceptible annual crop. Two years
of fallow will lower nematode numbers even further. When fallowing,
keep the soil moist to encourage egg hatch and control weeds on
which nematodes may survive. Eggs will hatch but the nematodes
will die if there is nothing to feed on. Try dividing the garden
into thirds and fallow one-third every year (or two) on a rotating
basis.
SOIL SOLARIZATION
Like fallowing, solarization temporarily reduces nematode populations,
but in less time. For effective solarization, moisten the soil
and cover it with a clear plastic tarp. The tarp must be left
in place for 4 to 6 weeks during the hottest part of summer. Sustained
temperatures above 125 °F will kill eggs and adult nematodes.
ORGANIC SOIL AMENDMENTS
Adding peat moss, manure, compost, and humus to soils greatly
improves the water holding capacity of the soil and provides some
food for nematodes, which help dilute the effects of nematode
damage to plant roots.
ABOUT MARIGOLDS
Marigolds have been proven to reduce nematode populations, but
all marigolds are NOT alike. Some marigold varieties suppress
nematode activity and other marigold varieties foster nematode
activity. French marigolds 'Nemagold', 'Petite Blanc', 'Queen
Sophia', and 'Tangerine' are effective. Signet marigolds (Tagetes
signata or tennifolia) should be avoided. No marigold works well
against the northern root knot nematode.
The effect of marigolds is greatest when grown as a solid planting
(like a lawn) for an entire season. When planted with annual vegetables
with a row here and a row there, nematode control is not good.
Marigold plantings should be mowed before flowers open, so their
seed does not become a weed in next year's garden.
PESTICIDES
There are currently no chemical nematicides or soil fumigants
available to home gardeners for nematode control in backyard gardens
and home landscapes. Visit www.ipm.ucdavis for more information
about nematodes.
February 6, 2003
