When scientists in the United States first began working to identify
the fungus attacking U.S. and Costa Rican daylilies, they did not have
a modern description or illustrations of P. hemerocallidis to
consult. Researchers at SBML, including José Hernández
and Lisa Castlebury, as well as Palm, have since rectified that situation.
They compared American daylily rust specimens with P. hemerocallidis
samples collected in China, Japan, Russia, and Taiwan. Their study included
both microscopic examination and DNA sequencing.
. . . and Convicted
Daylily rust fungus is known to produce five different types of spores.
Two types are found on daylilies in the United States and Costa Rica:
urediniospores and teliospores. Because rust fungi can usually be identified
by the shape of their spores, SBML researchers tried to identify the
American fungus by comparing samples taken from U.S. and Costa Rican
daylilies with samples taken from daylilies grown across the Pacific.
They observed differences between the samples and made note of some
general trends.
For example, the wall thickness of the Asian urediniospores was consistent,
while it varied in those from the United States and Costa Rica. Also,
the American specimens contained more one-celled teliospores than the
Asian specimens, which were far more likely to contain two-celled teliospores.
Although variations existed between fungal samples, the researchers
found that all the specimens were of the same species.
Their investigation was aided by the U.S. National Fungus Collections,
part of SBML, which houses more than a million fungal specimens from
around the world. Scientists use these reference specimens to help identify
and characterize agriculturally important fungi. But they don't just
compare them by their physical appearance.
Castlebury says, "We also use DNA sequences to identify and characterize
new and previously described species and to sort out relationships among
various groups of plant-pathogenic fungi. Knowing how these fungi are
related to one another makes it possible to predict how they might behave
as pathogens."
Hernández explains the differences they noted between the Asian
and American rust fungus specimens. He says, "This fungus can replicate
itself on daylilies, but another type of plant host is necessary for
it to complete its life cycle. In Asia, Patriniathe alternate
plant hostis common. In the United States and Costa Rica, it's
not. The differences seen between Asian and American rust specimens
may be due to the fungus' apparent lack of sexual recombination in the
United States and Costa Rica. It has more limited genetic variability
here."
Palm believes the fungus was introduced into the United States accidentally.
She says, "Last year APHIS made more than 1,400,000 plant interceptions
at ports of entry, in the mail, and through our preclearance and predeparture
programs. But we need the cooperation of industry and the traveling
public to further enhance our safeguarding efforts."
The journal Plant Disease has accepted a paper that Hernández,
Palm, and Castlebury submitted for publication. The article provides
a modern description, photomicrographs, and DNA sequences of P. hemerocallidis.
Plant quarantine workers, diagnosticians, and daylily growers will be
able to use these descriptions and sequences to identify future cases
of daylily rust and help prevent its spread.By Amy
Spillman, Agricultural Research Service Information Staff.
This research is part of Plant Diseases, an ARS National Program
(#303) described on the World Wide Web at http://www.nps.ars.usda.gov.
José R. Hernández,
Mary E. Palm, and Lisa
A. Castlebury are with the USDA-ARS Systematic
Botany and Mycology Laboratory, 10300 Baltimore Ave., Bldg. 011A,
Beltsville, MD 20705-2350; phone (301) 504-5364, fax (301) 504-5810.
"Daylilies Under Attack" was published in the November
2002 issue of Agricultural Research magazine.
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