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Re: Question for Dr. Grewal

  • Subject: Re: Question for Dr. Grewal
  • From: "Bill Meyer" <njhosta@hotmail.com>
  • Date: Thu, 10 Jan 2002 11:08:49 -0500

Hi Parwinder,
          It's a pleasure to have someone to ask the questions to. To have
someone with your knowledge and experience with nematodes step into hosta
circles is a real help to those of us who know hostas but not nematodes.
Over the last five years or so foliar nematodes have spread like a biblical
plague through the community of hosta collectors and breeders. The rule
before was always free exchange of plants with little to worry about with
pest transmission. Now many nurseries both wholesale and retail routinely
sell nematode infested plants to their customers, largely because of timing.
They get stock in and watch it most carefully during the time of year that
nematode damage doesn't show and pay much less attention later in the summer
when sales slack off. By then, many more infested plants have been sold. The
atmosphere now is one of secret words being passed about which nursery has
them and which ones don't. From what I've seen personally, as many as half
do.
          OK, more questions. Dan Nelson and I were at Alex Summers' garden
one day in September when I noticed that there was a large planting of
anemones (at least five years old) that appeared to be heavily infested with
foliar nematodes. Dan then noticed that the hostas rubbing leaves with them
were not showing any signs of infestation, although both groups of plants
were growing together for years. Is this the same nematode? Are their more
than one species of nematode causing problems in hosta, and if so, would
there be different treatment for each?
           Another question comes from the ones you just answered. If the
nematode found in hosta is named for strawberry plants, I would assume it is
a serious pest for strawberry crops. In hostas these nematodes don't do well
if there is a lot of sunlight and heat on the plant, but strawberries are
grown in full sun. Any idea what this would mean?
          Is there any relief in sight as far as the government goes in
allowing effective chemicals to be used at least in nurseries to stop this
growing problem? These days I think more hosta gardens have nematode
problems than not, and the situation is worsening each year, as more and
more people buy or trade infested plants. Hosta societies from the local
level to the national are very dependent upon the revenue raised by plant
auctions and sales, and the likelihood now is that several (at least)
infested plants are passed at every one. It has been suggested that this
problem was caused by the government-mandated move away from broad-spectrum
pesticides to more specific ones, before effective specific ones were
developed. Can we expect better pesticides in the near future, both for
nurseries and home use?

.........Bill Meyer


> Bill,
>
> You have made some excellent observations on the exposure of nematode
> infected hostas to the sun.  I have made similar observations also.  In
> fact there are two factors, temperature and ultraviolet (UV) radiation
that
> affect nematodes.  Nematodes are very sensitive to ultra violet
> radiation.  In our tests with UV light emitted from a lamp in the
> laboratory, most nematodes were killed in the leaves with just one hour
> exposure.  Thus putting nematode infected hosta plants in the sun (exposed
> areas) is likely to reduce nematode infection.  If most leaves are
exposed,
> it could actually cure the plants of nematode infection.  We are doing
such
> experiments.
>
> Yes, temperature could be important as well, but hot temperatures may also
> kill the plant, so I would not recommend to place plants in closed
> cars.  Exposure of plants to the sun on driveways for a few days is a good
> idea.
>
> Yes, the nematodes have the ability to move up and down the plant, but you
> are also right that successful reproduction occurs only under favorable
> temperature and minimal UV light.
>
> Yes, different stages of nematode life cycle show differences in tolerance
> to temperature and nematicides.  Eggs are most tolerant.  Nematodes in the
> dry leaves can enter into what we call anyhydrobiosis (life without water)
> and then become very tolerant of both the high and low temperatures.  This
> is one of the ways this nemtaode can survive in the dry plant material.
>
> It is possible that animals moving through wet plants can carry nematodes
> and aid in their dispersal in the garden, but I shall not consider them as
> the major source of dissemination.  Perhaps the biggest source would be
> water (rains, floods, and watering).  Treating plants as soon as the
> nematode symptoms become evident is always a good idea.
>
> Keep firing away the good questions.
>
> Parwinder Grewal
>
> At 04:58 PM 01/09/2002 -0500, you wrote:
> >        I've noticed that in hostas that receive a lot of direct
sunlight,
> >nematode damage in the upper, more exposed leaves is rarely seen. If the
> >plant is infested the damage shows only on leaves that are shaded by the
> >outer leaves. This would indicate that they are sensitive to the
> >temperatures that can develop in a leaf that is heated by the sun, and
will
> >avoid those leaves. It would also seem to indicate that they can move
> >through the plant down at the crown level, where the leaves join. Would
you
> >say this is so? Or are they randomly entering leaves and only reproducing
> >successfully where the temps stay cooler? Do you think impromptu
treatments
> >such as leaving infested plants on a black driveway for a few days in hot
> >weather or putting them inside closed cars in same would have much
effect?
> >Are different stages destroyed at different temperatures?
> >         If you're not tired of answering, one more---- What possibility
is
> >there of nematodes spreading around a garden carried by animals running
> >through wet leaves after a rain? This seems a possibility to me if
> >populations are high and thriving in one area, and a strong argument for
> >treating immediately when you first discover them.
> >         Sorry about the endless string of questions, but many hosta
> >gardeners still think they are harmless and not worth bothering to treat
> >for. I have been breeding hostas and collecting sports for some time now,
so
> >disposing of plants, especially seedlings, is not a choice.
> >
> >...........Bill
> >
> >
> >
> > > Bill,
> > >
> > > I replied to a very similar question yesterday.  I hope you got the
answer
> > > to your question.  The nematodes become active soon after the plants
start
> > > sprouting. Due possible injury (phytoxicity) to young plants, I would
wait
> > > spraying until early May.
> > >
> > > Parwinder
> > >
> > > At 02:16 PM 01/08/2002 -0500, you wrote:
> > > >Another thing most people seem confused about is when in the spring
they
> > > >become active. Because symptoms do not show in hosta until midsummer,
it
> >is
> > > >impossible to tell without a microscope if a plant or bed is
infested.
> >This
> > > >causes most gardeners to think in terms of beginning control measures
at
> > > >that time. When do nematodes come out of dormancy in the spring, and
when
> > > >should treatment begin?
> > > >
> >.........Bill
> > > >Meyer
> > > >
> > > >
> > > > > Yes, the rate of spread is about right and it depends mainly on
the
> >soil
> > > > > type, slope and water.
> > > > >
> > > > > No nematicide provides 100% control of nematodes, but there are no
> >reports
> > > > > on the development of resistance in nematodes under field
> > > > > conditions.  Furthermore, the mode of action of ZeroTol is such
that
> > > > > resistance development is not really possible.
> > > > >
> > > > > Regular applications of ZeroTol to infected plants will reduce
spread
> >of
> > > > > the nematodes in a garden.  Plucking of infected leaves as soon as
the
> > > > > symptoms of nematode infection become clear followed by a thorough
> > > > > clean-out in the fall can substantially reduce nematode spread.
> > > > >                 - Parwinder Grewal
> > > > >
> > > > > At 01:13 PM 01/08/2002 -0500, you wrote:
> > > > > >Hi Dr.Grewal,
> > > > > >              Thank you for answering. From what I've seen, they
seem
> >to
> > > > > >spread at a rate of about ten feet a year in all directions, more
if
> > > > > >downhill. Would this seem about right? Your findings indicate
that
> > > >ZeroTol
> > > > > >kills about 80% when plants are in the ground, if I remember
right.
> >Would
> > > > > >this make it likely that a resistant strain would develop? Also
would
> > > >using
> > > > > >ZeroTol regularly control the spread, or would they continue to
> >spread
> > > > > >through the rest of the garden at more or less the same rate?
> > > > > >
> > > > > >.........Bill Meyer
> > > > > >
> > > > > >
> > > > > > > Hi Bill,
> > > > > > >
> > > > > > > I know that most effective nematicides are no more available
to
> > > >control
> > > > > > > foliar nematodes and there are many restrictions the use of
other
> > > > > > > chemicals.  In our research, we have discovered that ZeroTol,
> >which is
> > > > > > > currently used as a general sterilant/fungicide, is an
effective
> > > > > >nematicide
> > > > > > > against foliar nematodes.  This chemical can be applied by
home
> > > > > > > owners.  Our findings on ZeroTol and other chemicals to
> > > >control/suppress
> > > > > > > foliar nematodes were published in the Spring issue of Hosta
> >Journal
> > > >in
> > > > > > > 2001.  There are also other useful tips and preventive
measures
> > > >described
> > > > > > > in that article.
> > > > > > >
> > > > > > > Yes, foliar nematodes can eventually kill hosta plants if they
are
> > > >ignored
> > > > > > > for long.  Overtime nematode populations build up on plants
and in
> >the
> > > > > >soil
> > > > > > > around plants.
> > > > > > >
> > > > > > > Parwinder Grewal
> > > > > > > Assistant Professor
> > > > > > > Department of Entomology
> > > > > > > OARDC
> > > > > > > The Ohio State University
> > > > > > > 1680 Madison Ave
> > > > > > > Wooster, Ohio 44691, USA
> > > > > > > Phone (330) 263-3963
> > > > > > > Fax (330) 263-3686
> > > > > > >
> > > > > > >
> > > > > > > At 11:31 AM 01/08/2002 -0500, you wrote:
> > > > > > > >Hi Dr. Grewal,
> > > > > > > >           A question that is commonly asked on the lists is
what
> >can
> > > >the
> > > > > > > >average gardener who does not have access to restricted
chemicals
> > > >like
> > > > > > > >Nemacur do about foliar nematode infestation. Because of
changes
> >in
> > > >the
> > > > > > > >pesticide laws, many of us find ourselves with these pests
> >running
> > > > > >rampant
> > > > > > > >and we are not permitted to use the chemicals that best
control
> >them.
> > > > > >What
> > > > > > > >would you say is the most effective treatment we could use?
> > > > > > > >           Another somewhat related question is ------What
would
> >you
> > > >say
> > > > > >the
> > > > > > > >long-term effects on hosta are of untreated or poorly treated
> >foliar
> > > > > > > >nematode infestations? Can they kill plants eventually?
> > > > > > > >
> > > > > > > >..........Bill Meyer
> >
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