Daylily Rust in the United States
presentation by Dr. Jean Williams-Woodward
notes by Rebecca Board
On September 15, 2001, following the Region 15 fall meeting,
Dr Jean Williams-Woodward gave a presentation on the research
being done on Daylily Rust at the University of Georgia. I thought
it was a great presentation and discussion, and I'm very glad I made the
trip for it. In some cases the information was new to me, in others it
increased my understanding of what I already knew in part. Below are
my notes describing that meeting. I don't have training in plant science,
though, so I don't claim that that my notes perfectly
match the speaker's intent, though they should be close.
I should also mention that the notes were made from memory, so they
do not reflect the speakers exact words, though eceryone who has
read them tells me the match what they remember also, at least for
content.
I have tried not to use words that weren't in my vocabulary prior to
the meeting. I am making these notes available to people who
couldn't attend in the hope that they may prove useful.
- The rust spores will only live a couple days if they are unable to
germinate. They can only germinate on green daylily tissue.
- Rust spores require a period of 100% humidity for as little as 5
hours to germinate.
- Rust spores require moderate temperatures to germinate -- approx.
40F (<10C) to approx 90F (30C).
- After a spore has germinated and penetrated the plant tissue, it
can lie dormant and nearly invisible if the environment becomes
unfriendly towards it either due to fungicide use or temperatures.
This is the stage the plants are in during the hot summer or
as clean-looking new arrivals that develop spots later.
- It appears that only the leaves, not the plant crown or roots, become infected.
- The appearance of rust can vary quite a bit from one cultivar to another.
Some plants may have very large obvious yellow fuzzy spots, and
in others the spots may be smaller and/or darker orange in color.
- Fungi are hard to kill and we shouldn't be surprised that multiple
treatments are required to kill it. As with antibiotics, the
worst thing to do is treat it only a little while and stop before
it's really all gone.
- Rusts are usually very plant specific - one kind of plant rust may look
like another but they are actually different and do not cross
over into new plants.
- Cutting back infected leaves to the ground and removing from the garden
is recommended to reduce the number of spores present, remove most of
the infected but not yet fully mature infections, and remove most
of the green surface needed for germination. Plants will need to be
sprayed as they grow out to finish the last remaining infections and
prevent the remaining spores from germinating.
- Rust should be unable to survive a cold winter if the plants are _completely_
dormant, showing no green anywhere, and if there is no alternate host
present. This is based on the experience of only one winter (a cold
one) in the speaker's own garden with only dormant varieties.
- It was recommended to cut and remove all foliage from the garden in the fall,
and possibly to continue to cut back evergreens during the winter
months to reduce the odds of rust surviving the winter.
- The more foliage is removed on new arrivals, the less chance of rust, though
obviously one cannot reduce the odds to 0 without killing the plant.
- Quarantine new arrivals from the rest of the garden. Initial suggestion was
3 months, but this was increased to 6 months during the discussion.
- Things like ZeroTol and bleach solutions will kill spores on contact, but
are not contact fungicides. I've been calling them disinfectants
since the meeting, which some of you have done all along. There is
no residual effect on the plant after the initial application/dip,
and there will be no effect on fungal growth inside the leaves.
These were identified as safe products that do no harm and may help,
but cannot solve the problem.
- Contact fungicides prevent spore germination of spores which contact the
leaf surface. These fungicides do remain active on the surface of
the leaf for a time (a week in most cases), but do not penetrate into
the plant tissues. They have no effect on the fungus already growing
inside the plant.
- Older contact fungicides like Daconil and Fore are highly effective, readily
available, and unlikely to cause a resistance to be built up against them.
(Seems to me like this is what one wants to be using for preventative
spraying.)
- Systemic fungicides enter into the plant and remain active for longer (about
two weeks). The fungicide can move from the application site along
the leaf in the growth direction only. If you don't spray the base
of the leaves, they aren't going to be protected there.
- Systemic fungicides may prevent spore germination, but their primary job
is to attack the already germinated fungus growing inside the leaf.
One application isn't going to be enough to destroy the fungus at
this point.
- Some fungicides (like Heritage) also move through the leaf from the top
surface (where the spray is likely to hit) down to the underside
(where the rust generally is living).
- Heritage requires a non-ionic spreader-sticker/wetting agent to be highly
effective.
- All the products which have been discussed for daylily rust are labeled to
permit home use, though many are packaged only in large amounts for
the commercial market.
- The contact fungicides can be tank mixed with the systemics and applied at
the same time. This was recommended - spray affected plants once
a week rotating through the systemics (Heritage, Banner Maxx, Systhane)
and using a contact fungicide (Daconil, Fore)
mixed with it in the tank each time.
- What we know of the daylily rust life cycle comes from literature. Many
rusts have a complicated lifecycle involving two host plants and
a large number of different types of spores performing different
tasks during the cycle.
- Two kinds of spores are produced on daylilies. One kind reinfects the
daylily itself, which is what we are observing in our gardens,
and the other kind of spore is what should infect patrinia. Once
the patrinia is infected, the spores it produces will reinfect
daylilies. (Simplified version.)
- Researchers have not yet succeeded in infecting patrinia much less
observing the complete life-cycle.
- The literature suggests that hosta might be susceptible, but
researchers have thus far been unsuccessful in attempts to infect
hosta with daylily rust in a greenhouse. This may mean that hosta
is not susceptible, or that it is only susceptible to the spores
produced during a different stage in the rust life-cycle.
- All the rust spores examined so far are genetically the same, implying
that rust in this country mostly likely came in as a single source
(possibly in 98) and has been spreading asexually (without using
the alternate host) ever since.
- As long as the rust does not complete the sexual life cycle, there is an
excellent chance of breeding resistance into our daylilies. If
rust develops genetic diversity through sexual reproduction, then
breeding resistance will become extremely difficult.
- It is now thought that rust probably entered the country on a single
plant back in about 1998.
- When the state Ag folks become aware of the problem, they worked with
the nurseries in their states to control it, and these measures
have been largely effective. The problem is, the nurseries we
are talking about here are not the daylily gardens of collectors
and hybridizes! We're talking about normal plant nurseries
that sell a variety of plants in containers. Remember those?
- It was only later that the state agencies really became aware of and
began to understand the network of daylily collectors. And only
then that daylily growers not closely connected with the world
of the general nursery became aware of the problem.
- Rust has escaped into the non-nursery and roadside gardens in Florida,
and most states are coming to realize that the problem is too
widespread to control by quarantine.
- Advice that you see from state agencies is largely written for the
world of the large general nursery. It's not surprising that
it doesn't map well onto our daylily gardens with expensive
collections but no nursery infrastructure.
- Our hybridizes are not responsible for introducing this stuff, nor
for creating susceptible plants. The species came with the
susceptibility, and no one knew to breed against!
--Rebecca, 9/25/01, last modified 9/30/01
