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DNA fingerprinting has revealed that the primary cause of recent late
blight epidemics of U.S. and Canadian potato and tomato plants are new
genotypes of the fungus Phytophthora introduced from Mexico. Late
blight, a disease caused by one of the more than 50 species of
Phytophthora, is one of the most destructive plant
diseases known. It caused the Irish potato famine in the 1840's. In 1994
alone, U.S. potato and tomato crop losses from late blight were estimated
at over $100 million, and another $100 million or more was spent to
control the disease. ARS and Cornell University scientists tested 251
isolates of P. infestans for their sensitivity to the
fungicide metalaxyl that usually controls the late blight pathogen. The
isolates came from 15 states and one Canadian province from 1987 through
1993. Analyses with molecular markers revealed the primary cause of these
epidemics was new, recently introduced genotypes of the fungus--most
likely from northwestern Mexico. Resistance to metalaxyl was detected at
about the same time as the migrations occurred. Field experiments
confirmed that metalaxyl alone has no effect on the new, resistant
genotypes. Since resistance to metalaxyl was almost certainly introduced,
there was nothing U.S. potato growers could have done to prevent its
occurrence. Seed potatoes should be monitored for the new genotypes to
limit their migration and fields containing them should not be sprayed
with metalaxyl.
Crop Production and Pest Control Research, West Lafayette, IN
Stephen Goodwin, (317) 494-4635
A new test has been developed to screen geraniums for newly discovered
viruses. Geraniums are one of the most rapidly expanding ornamental
garden crops in the United States. From 1970 to 1990, total U.S. potted
geranium sales increased from $25 to over $130 million, with an additional
$30 million for bedding geraniums in 1990. Since geraniums are propagated
mainly from cuttings, more than fifteen different viral diseases threaten
their production and quality, severely reducing vegetative growth and
plant and flower quality. Often mixed viral infections cause severe
disease, causing confusion about the exact identity of the culprit. ARS
scientists isolated two of the top four viruses from geraniums. After
studying their biological, biochemical and serological properties, the
scientists called them pelargonium line pattern virus and ringspot virus.
Next, they developed a polyclonal antisera to these viruses and showed
they can detect the viruses in infected geranium. The new tests will help
the U.S. potted plant industry since both exported and imported geraniums
have to be virus free.
U.S. National
Arboretum, Beltsville, MD
Ramon L. Jordan, (301)504-5670
A fly new to science was discovered infesting the flower buds of a
native azalea, called pinxterbloom. This valuable ornamental
(Rhododendron periclymenoides) develops pink blooms in
late spring in the Washington, DC, area. An ARS scientist named the tiny
new gall midge (Dasineura praecox) and characterized it
anatomically and biologically so it can be readily identified and
controlled. In spring, the fly larvae live among flower parts in the
newly burgeoning azalea buds. Their salivary secretions cause the buds to
remain closed and provide food for the larvae, instead of developing fully
into flowers. Around the end of April, when uninfested flowers are
beginning to open, fully developed larvae crawl out of aborted buds, drop
to the ground, burrow into the soil and form cocoons. They remain there
until September when they change into short-lived flies that lay eggs
inside the next season's buds. Larvae develop into eggs but remain
encased until the following February when they hatch and begin to feed.
Scientists do not know whether the new pest is native and has gone
undiscovered or a new accidentally introduced fly from abroad. Their
information will be used by scientists working to control azalea pests and
to plant inspectors at ports of entry charged with identifying alien
insect pests.
Systematic
Entomology
Laboratory, Washington, DC
Raymond Gagne, (202) 382-1796
Common barberry is making a comeback in the north-central states--which
is bad news for crops. That's because common barberry,
Berberis vulgaris, is an alternate host for wheat stem
rust, and its presence puts vulnerable crops like wheat, oats and barley
at greater risk of rust infection. Barberry bushes were identified as the
primary source of new stem rust races before a nationwide eradication
program was initiated to remove them. More than 100 million barberry
bushes were destroyed between 1918 and 1975. However, it is re-emerging
in wooded areas of Wisconsin and Minnesota. Birds spread barberry seeds
by eating berries that grow on the bushes. Barberry growing at or near
the edge of wooded areas are more likely to be visited by birds. ARS
researchers are keeping tabs on new barberry growth, and are continuing
efforts to breed wheat varieties with increased rust resistance. A
resurgence in barberry populations could complicate efforts to develop
rust resistant varieties of wheat, oats and barley. Berberis
vulgaris has not been used as an ornamental shrub since
eradication began. Nationally, the horticulture industry cooperated by
removing the bush from ornamental stocks.
Cereal Rust Laboratory, St.
Paul, MN
Dave Long, (612) 625-1284
A new test detects and identifies in only one to two days the agent
that causes chrysanthemum stunt, an economically important disease
worldwide. Such a fast technique can be used for large scale
screening of the disease in quarantine procedures and plant certification.
Caused by the chrysanthemum stunt viroid (CSVd), the disease now can take
two to six months to detect by screening plants biologically. ARS
scientists developed a test relying on a rapid, sensitive molecular
technique called polymerase chain reaction. An analysis is completed from
a billion copies of a fragment of DNA from sap or nucleic acid extracts of
infected dried or fresh leaves. CSVd can also be detected from infected
tissue by another molecular technique called tissue blot hybridization.
Thin cross sections of chrysanthemum leaf petioles and stem internodes are
pressed against a nylon membrane. The membrane is hybridized with a
non-radioactive probe for CSVd and then the hybridized products are
visualized on film. PCR and tissue blot hybridization eliminate the need
to use radioactive material.
National Germplasm
Resources Laboratory, Beltsville, MD
Ahmed Hadidi, (301) 504-6460
A new, accurate and sensitive test confirms the connection between
superficial scald, a disease that attacks stored apples, and levels of
farnesene, a compound found in apple peels. Farnesene breaks down
into trienol, which is toxic to the fruit. Stored Granny Smith apples
easily succumb to scald, which blackens the fruit and makes it
unmarketable, but Gala apples aren't as susceptible. Using the new tests
on fruit stored for up to four months, ARS researchers showed levels of
farnesene and trienol were 20 times higher in Granny Smith than in Gala.
Another finding: Decreasing oxygen levels could increase apple shelf life.
That's because when oxygen levels in storage go up, both farnesene and
trienol increase, but boosts in trienol are more closely correlated with
oxygen concentration. Stored under 100 percent oxygen, Granny Smith fruit
were completely covered with scald after three months.
Horticultural
Crops Quality Laboratory, Beltsville, MD
Bruce D. Whitaker, (301) 504-6128
Researchers have pinpointed the genes that help some corn plants
naturally fend off Phaeosphaeria mayis, a crop-damaging
fungus found mainly in Brazil. Although limited outbreaks of the
fungus have occurred in Florida the past four years, P. mayis
has never gained a solid foothold in the United States. But there is a
potential problem: Iowa Stiff Stalk Synthetic, a favored breeding
population for domestic corn hybrids, has some "family members" that are
vulnerable to P. mayis. By knowing which genes provide
resistance, plant breeders now can ensure new corn hybrids aren't
vulnerable. Also, varieties now in development can be checked for
possible susceptibility. ARS researchers were first to identify the
fungus in the United States. P. mayis can slash yields
and leave tell-tale gray rings on plant leaves.
Plant
Science Research, Raleigh, NC
Martin L. Carson, (919) 515-3516
One hot spring day can mean the difference between a weed "sweet dream"
and a nightmare. A two-year study of several weed species from Ohio
to Colorado and Missouri to Minnesota showed that each species has a
different weather trigger that prevents seeds from sprouting. Scientists
found this trigger an important reason why weed sprouting varies each
spring, typically from one- to 100-percent of the buried seed. For
example, if the top inch of soil warmed to about 90 degrees F for only a
single day in April, giant foxtail seeds went into dormancy, refusing to
sprout for the rest of the season. Dry soil in the spring induces a
similar all-season dormancy for pigweed seed, which is why pigweed was a
major problem in areas that had a wet spring this year, such as west
central Minnesota. The discovery also helps the ARS weed simulation
computer model, called WeedCast, to more accurately forecast each weed
season. The forecasts can be used with the WeedSim model that helps
farmers reduce herbicide use by advising them when it's not needed.
Soil Management Research,
Morris, MN
Frank Forcella, (320) 589-3411
Last updated: October 22, 1996
Return to: Quarterly Report
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