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Re: getting ready for winter

  • Subject: Re: getting ready for winter
  • From: "Bill Meyer" njhosta@hotmail.com
  • Date: Tue, 25 Oct 2005 10:04:29 -0400

Hi Everyone,
     Foliar nematodes are a very widespread problem in the US. Basically if
you do not have them yet, as most of us do, you wil be lucky if you can avoid
them. This is a result of changes in laws from the EPA that removed effective
nematicides from ornamentals growers, and caused a huge outbreak that it seems
no one was expecting. Until they ease their restrictions or new nematicides
are developed, there is little we can do to stop the spread of foliar
nematodes or eliminate them from gardens.
     As we have all learned in recent years, foliar nematodes are highly
efficient spreaders that are very good at moving through hosta gardens. In my
experience with them, I have repeatedly been surprised by their ability to
spread from one area to another and I have come to think that the primary way
they move beyond a small area is by dead leaves blowing around. Dead leaves
that hosted foliar nematodes are essentially like seed packets that will sow
nematodes wherever they land. This is not just a problem in the Fall. Late in
the summer, infested plants start losing the worst infected leaves and these
drop to the ground with a good number of living foliar nematodes still inside.
These leaves can move to another spot in the garden carrying the infestation
to a new location. The load of dead leaves in the fall in an infested garden
will be like hundreds of "seed packets" ready to sow nematodes wherever they
lie when it warms up in the Spring.
     This being the case, it is never a good idea to leave old dead hosta
leaves lying around where they can release their load of dormant nematodes
come spring. All dead hosta leaves should be carefully cleaned up each fall so
as to keep the nematode population as low as possible to aid in their control.
No benefit from those leaves' small contribution to fertility can outweigh the
harm their load of dormant nematodes will do to the plants. Good fall leaf
cleanup is essential to keeping them in check. Simply leaving dead leaves of
nematode-infested hostas in place will allow them to blow around and spread
the infestation to new areas.
     It is also virtually impossible to tell which hostas are infested and
which are not. Only when the plant has high numbers of nematodes does damage
start to show. By then there will always be nearby plants that are lightly
infested but not yet showing damage. These will usually show damage the
following year. When you see a nematode-infested hosta, you are looking at the
epicenter of a twenty-foot wide (or larger) infestation. Removing it will
leave behind maybe 20-30% of the nematodes, so you will hardly eliminate them
that way. The idea of disposing of nematode-infested hostas is largely
pointless - like throwing the baby out with the bath water. While it is good
for those who sell hostas, it will have no effect in ridding the garden of
foliar nematodes. By the time you see the problem, they will be in other
plants, the soil, dried leaves, etc., and you will not get rid of them. All we
can do is try to control them in the garden so they do not damage the hostas
too badly.
    Natural ecosystems eventually find balance, but a hosta garden in the US
is anything but a natural ecosystem. With a highly artificial ecosystem
comprised of plants that evolved in different parts of the world, there will
be problems that will not be solved naturally, at least not any time soon.
Using Southern Blight in hostas as an example, hostas might naturally evolve
defenses against it in 10,000 years. This would happen through a process of
natural selection as those with weaker defenses died off without offspring and
those with better defenses had seedlings with slightly better resistance. Each
generation, those with the least resistance would be culled until a seed
strain with very good resistance emerged. Few of us really want to wait for
that to happen, though.
    Pretty much every reference I have found makes it very clear that a prime
method of overwintering for foliar nematodes is in dead plant material. A
sampling is below.

                      .........Bill Meyer


Here, the bulletin from the Department of Crop Sciences at the University of
Illinois Urbana/Champaign says:

"Unlike other plant-parasitic nematodes, foliar nematodes can live in the soil
for only a short time.Therefore,very few are found in the inorganic fraction
of the soil beneath infected plants. However, foliar nematodes possess a
remarkable ability to survive for relatively long periods in an inactive state
in dry plant tissues, especially under cool conditions. Hence, dry plant
debris from a prior crop, indoors or out, serves as a source of infection in
later crops."

"As infection progresses, there is a gradual transition from healthy to dead
leaves, a shortening of the stems, and the production of abnormal flower buds.
Dead leaves usually fall to the ground. The nematodes emerge through the
stomata and cracks in the tissue and migrate back up the stem, completing the
cycle is a gradual transition from healthy to dead leaves, a shortening of the
stems, and the production of abnormal flower buds."

"All infected plants and fallen leaves should be carefully removed from the
garden or greenhouse and burned."


Here, from the Department of  Plant Pathology at Iowa State University:

"Disease cycle.
Nematodes overwinter in dead, infested leaves on the ground or between the
scales of infected buds. In the spring, the nematodes become active and swim
up the plant stems. The stems must be covered with a film of water, from rain,
irrigation, or humidity, in order for the nematode to move upward. The
infected leaves then develop spots that coalesce to form the large V-shaped
lesions. The leaves from infected buds may be small, distorted, and fail to
develop. Eventually infected plants parts collapse, turn brown, and die. The
dead and infected leaves that fall to the ground can then serve as an
overwintering source of inoculum to repeat the disease cycle."


Here, Janna Beckerman, extension plant pathologist at the University of
Minnesota recommends:

"Third, remove and destroy infected leaves, and remove all dried leaves and
stems during fall clean up. "

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