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Nematodes: Getting to Know
the Enemy Better
Bill Meyer, Woodbury, Connecticut
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Many
consider foliar nematodes to be the #1 hosta pest and there is good
reason for thinking so. These microscopic worms have proven to be the
most difficult pest to eradicate in any environment from nurseries to
home gardens. The heavy restrictions placed on pesticides in the latter
part of the 20th century unleashed a plague of pests that were under
control previously to the point that most had forgotten them. |
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From the tiny nematodes
to the watermelon-seed-sized bedbugs, old insect enemies are back
and flourishing and the weapons that once vanquished them are nowhere to
be found. Most of us are used to thinking that we can eliminate
a particularly bad pest from our garden, but to date the foliar
nematode has won every battle. While the pesticides of old offered a
chance of eliminating them, nothing available to the home gardener
these days has offered more than a reasonable chance of keeping
their numbers and the resulting damage to hostas down to an
acceptable minimum.
It must be understood
that controlling them does not mean getting rid of them. It is
theoretically possible to get rid of them with the pesticides we are
no longer allowed to use, but so far it seems totally impossible to
eliminate them with what remains available even to the nursery owner
in most states. New pesticides have not appeared to replace the old
effective ones and for now it seems foliar nematodes have scored a
clear victory.
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The key to effective
control lies in understanding them better, because knowing how
they survive from year to year and how and when they move back
into the hostas gives us our best chance of hitting them when they
are most vulnerable. That's where the great 2005 study by Drs.
Ganpati B. Jagdale and Parwinder S. Grewal comes in. Prior to this
study, we really had little more than guesswork for how they
overwintered, when and how they moved into the plants in spring,
and how they moved from one hosta to another. This study is a bit
easier to read than most scientific papers and most OLJ readers
should get the gist of it without too much effort. What they
learned in the study falls into two areas - how they overwinter
and how they spread from plant to plant in the summer. In the
sections below we'll look at what they learned and what it means
to both gardeners and nurseries. |
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Hosta Nematode
Facts |
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Name |
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Aphelenchoides
fragariae |
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Size |
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Under .8
millimeter long |
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First
Discovered |
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1890
by Ritzema Bos |
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Reproduction |
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Sexually
reproducing; males required |
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Life
cycle |
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10-13
days from egg to reproduction |
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Egg
Production |
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Female
produces about 30 eggs |
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Time
to Hatch |
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4
days |
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Survives
in soil |
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About
3 months |
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Overwinters |
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Soil,
Dead Leaves, Dormant Buds |
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Number
of Host Species |
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Over
700 from 85 plant families |
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Overwintering |
In their study, titled "Infection
Behavior and Overwintering Survival of Foliar Nematodes, Aphelenchoides
fragariae, on Hosta" Drs.
Jagdale and Grewal found that nematodes overwinter in soil, dead leaves,
and the dormant buds of hostas, but not in the roots. This indicates
that fall cleanup of dead leaves will reduce the number of overwintering
nematodes, as will disposing of old potting soil and replacing with new
soil in the spring while it is still cold. Because some are also
overwintering between the nascent leaves inside the dormant buds,
however, it is impossible to prevent them from remaining in the plants
and setting up shop again in the spring. Even a systemic pesticide
probably cannot reach into the spaces between the tiny leaf initials on
the dormant buds, and hiding there even offers good protection from
fumigant-type pesticides. The nematodes really couldn't have found a
better place to hide.
While fall cleanup does not offer any hope of elimination,
it should be the first step in efforts to control them. Removing all
dead leaves before a spring warm-up will reduce the numbers that
overwinter significantly, leaving fewer to try to kill by other means
later. Pesticides are unlikely to work during the cold months because
the nematodes have a dormant stage in which they are toughened and more
resistant.
The presence of foliar nematodes in the soil also makes it
clear that control schemes like heating the plants, which can
successfully kill all nematodes in the plants, are pointless in the
garden because they will be re-infected when they are returned to the
ground. Optimal times and temperatures for heat treatment probably vary
significantly with different size plants and probably with different
cultivars as well. To date no study has been done to determine for
example the difference in heat treatment between a huge plant and a mini
- one would expect that temperatures and times that would kill the mini
might not even warm up a massive plant.
Other interesting results from this and other studies are
that nematode eggs do not overwinter - only adults and juveniles - and
that cold, even at levels found only at the Poles, has no real effect on
them. |
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Infection |
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The nematodes overwintering in the dormant buds were found to be
hiding between the nascent leaves of the buds and not internally
in the rhizome, roots, or other parts of a dormant hosta. As the
soil warms in spring and the hostas emerge from dormancy the
nematodes climb the outside of the petioles to enter the leaves.
To climb and enter the leaves they require high humidity or wet
conditions, which indicates that plants in controlled conditions
such as greenhouses could be bottom-watered with ventilation
keeping the humidity down as a step in controlling them.
Applications of somewhat-effective modern pesticides made
regularly by bottom watering could successfully kill nematodes in
the pots before they can climb up into the plants. |
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Typical
nematode damage in a hosta leaf |
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Future studies hopefully can
pin down the times and temperatures when they become active again.
Outdoors, this information is still somewhat helpful in understanding
when the nematodes may be ascending the outside of the plant and
re-infecting it.
Foliar nematodes do not seem to be terribly difficult to
kill when they are on the exterior of the leaves and petioles, and even
weak (and inexpensive) control measures like insecticidal soap and
hydrogen peroxide can have good effect in reducing the numbers
significantly. We still don't know how long the period of migration is,
which is a piece of the puzzle that would be greatly helpful in moving
towards actual elimination of the nematodes. The timing and length of
the migration from soil level up into the leaves is likely very
dependent on temperatures and how wet the exterior of the plant is, but
a better understanding of these conditions should help define when
pesticide applications will have the most effect. Killing them on the
outside of the plant before they can get up to the leaves is the best
shot we have today with our reduced chemical arsenal.
Hopefully further research in that area will reveal more,
as control measures during the migration stage hold promise to be the
most effective. After all, it is at that time that their population is
at its lowest, so killing them then will have the most impact on their
numbers. Once the nematodes have entered the leaves by mid-spring or so,
control becomes much more difficult as systemic or translaminar agents
like Pylon are then necessary and the best of these are now restricted primarily to
farming in most states, and are priced beyond what most gardeners are
accustomed to paying. |
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Nematode photo above courtesy of Jason
D. Stanley
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