Nematodes are microscopic
roundworms found everywhere but quite common in managed soils. A fertile soil
may contain billions per acre. Most don’t cause plant damage. The ones that do
are called plant parasitic nematodes
because they feed and rely on the energy and nutrients derived from plants.
Some parasitic nematodes are beneficial
such as the so-called entomopathogenic
nematodes that parasitize insects. They parasitize many different types of
soil insects including so-called “grubs” like white grubs and other larvae of
butterflies, moths, beetles, and flies. Some parasitize adult crickets and
grasshoppers as well. These can be found marketed under several different trade
names.
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| One of several products that use beneficial nematodes |
Nematodes are not a huge
problem in most turf and landscapes. Some may never encounter them. But when
they are present, they draw a lot of attention because they are difficult to
control. Nematode damage falls into that category of “out of sight out of
mind”. But once plant damage from nematodes is identified, they are no longer
“out of mind”.
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| Root knot nematode on tomato |
There are two primary groups
of nematodes that concern us in horticulture; those that feed upon plant roots and
those that feed on plant foliage. Those that feed on plant roots live their
entire lives in the soil. Those that feed on plant foliage spend most of their
time above ground, feeding on leaves and succulent stems.
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| Root knot nematode on mulberry |
Most nematodes do not just
attack one type of plant but might feed on a number of different plants. Generally
speaking, nematodes that feed on plant roots can damage turfgrass, ornamentals,
nursery plants, houseplants or tropicals and greenhouse plants. Nematodes that
feed upon plant foliage are, for the most part, restricted to ornamentals, nursery
and greenhouse plants.
Sometimes we discover soil
dwelling, plant parasitic nematodes when infested roots are exposed during soil
preparation. The most common soil dwelling nematode is the root knot nematode.
They leave behind root nodules or “swellings” on the roots.
But most of the time we see above
ground plant symptoms which cause us to inspect the roots. Aboveground clues to
a nematode attack to the roots include leaf yellowing and scorching, leaf drop
and poor or stunted growth.
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| Mulberry infested with root knot nematode but kept going with plenty of water, compost and fertilizer |
Sound familiar? Nematode
damage can be confused with nutrient deficiencies, drought, salt problems, root
damage, under or over fertilizing and plant disease.
However, depending on the type
of nematode, root damage may vary from the presence of galls to the stunting
and decaying of roots. In some cases, nematode damage might be confused with
root disease.
Types of root damaging nematodes
include the stunt nematode, lesion nematode, ring nematode, cyst nematode,
spiral nematode, and lance nematode which produce other symptoms. These include
shortened or stubby roots, malformed roots that are multi-branched, darkened or
browning lesions which resemble plant disease which frequently accompanies
nematode damage.
Damage from nematodes that
feed on foliage are easier to identify since plant symptoms are easier to directly
trace back to nematodes. This type of damage frequently occurs in greenhouses. Why?
Nematodes need a moist environment to survive and spread. The higher humidity
of greenhouses and the presence of surface water on plant leaves contribute to these
types of nematode problems.
Most references refer to the
presence of “angled lesions” that result from the feeding of foliar nematodes.
Perhaps a better description than “angled lesions” is “brown spots on newly
attacked leaves that are not round but longer than they are wide”. In advanced
stages, severely attacked leaves may turn brown and die which masks the
presence of these lesions. In cases like this, search for leaves that are more
recently attacked to verify these “angled lesions”.
Nematode damage to turfgrass
is common in warm climates and may resemble some turfgrass diseases, soil
compaction, nutrient deficiencies, herbicide injury among others. Symptoms from
nematode damage may gradually enlarge as much as three feet per year. Machinery
that comes in contact with soils, such as aerators and hand tools, may spread
nematode infestations with equipment. A common symptom occurring due to
nematodes is a lack of a response from applied fertilizers.
How to control nematodes?
Nematodes are nearly young impossible
to eliminate using traditional pesticides without killing infested plants.
Prevent nematodes from entering the property through exclusion. Most problems
develop when soils, composts, soil mixes and plant materials are brought in
from unreliable sources. Reduce the spread of nematodes through sanitation.
Clean equipment and tools between worksites that have been in contact with infested
soils.
Recognize that the presence of
nematodes is not always bad. In the past, the general recommendation was to
improve plant and soil health so that plants “grow ahead” of their damage. There
is quite a bit of evidence that increasing soil organic matter through the use
of compost and organic surface mulches, particularly in arid and desert soils,
helps keep nematodes in check.
It is thought that compost
from organic matter stimulates micro and macroorganisms antagonistic to
parasitic nematodes. A population of nematodes antagonistic toward plant
parasitic nematodes is an important tool used to keep undesirable nematode
populations in check.
Amendments with a low carbon/nitrogen
ratio (C:N of 20:1 or lower) seem to be the most effective types of organic
matter for keeping nematodes in check. For low organic matter content soils,
such as arid or desert soils, there is a direct relationship between
controlling nematodes and the nitrogen content of the soil due to additions of
compost or chemical fertilizers.
North America is estimated to
be the largest market for nematicides; pesticides aimed specifically to control
or kill nematodes. Nematicides sales are predicted to dominate the agrochemical
industry from 2015 to 2020. That’s the size of our problem.
Chemical control of nematodes
is becoming more difficult because of environmental problems associated with their
application. There is one traditional pesticide that remains available for
nematode control. This product, Nemacur, an organophosphate pesticide, is being
phased out. Only the remaining inventory on shelves will be sold. When this
inventory is gone, there will be no other traditional pesticides available for
use against plant parasitic nematodes.
Bob Morris is an independent consultant for ViraGrow, Inc.
This article appeared in Southwest Trees and Turf, a regional trade magazine.Viragrow Delivers!
