Volume
2 Issue 1 May 17, 2005
Soybean Rust: If you’ve been growing
wheat,
you’ve been managing its ‘cousins’
Asian soybean rust (caused by
Phakopsora pachyrhizi) has made quite a name for
itself across U.S. soybean producing areas since November
of 2004. After being discovered attacking soybean and
other plant species in nine southern states, the disease
is now ‘larger than life’. Government agencies, land grand
universities, plant pathologists, and private industry are
currently involved in tracking its spread within the U.S.
Soybean production losses are common from countries
where the disease is known. Reported losses range up to
90%, depending on such things as climate and weather
conditions. U.S. and foreign scientists are cooperating
daily to enhance our understanding of the risks for
production related crop losses. Federal agencies and
extension plant pathologists recently met in St. Louis to
discuss strategies for disease management and the
associated risks of loss in soybean production and
quality.
Tall tales
The statures of two famous Minnesota residents, Paul
Bunyan and ‘Babe’ the blue ox, are considerably dwarfed by
some of the tall tales being told about soybean rust. In
hopes of dispelling some of the growing legend surrounding
this disease, read on for a simple comparison between
soybean rust and wheat diseases that are more familiar to
growers in the Red River Valley (RRV).
The fungal pathogens that cause wheat leaf rust (Figure
1), wheat stripe rust (Figure 2, 3),
and soybean rust (Figure 4; additional images online at
http://www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/sbrimagelibrary.htm)
are similar in several ways. Obvious similarities include
pathogen requirements for hours of leaf wetness, number of
days in a lifecycle (from spore to producing a spore) when
the environment is favorable for the pathogen, and
locations in the U.S. where the fungi can over-winter.
Other parallels can be drawn between rusts such as
temperature requirements for disease development, the
types of plant tissues infected, and our expectations for
identifying the diseases in Minnesota crops during the
average growing season.
Hours of leaf wetness needed for infection of
plants
- Wheat leaf rust - 6 to 8 hours
- Soybean rust - approx. 6 hours
Lifecycle duration: the average number of
days needed by a spore to; (1) infect a susceptible host
plant, (2) cause disease, and (3) produce more spores.
Lifecycle durations for wheat leaf and soybean rusts are
similar when environmental conditions promote disease
development.
- Wheat leaf rust - 7 to 10 days
- Soybean rust - 9 to 10 days
Over wintering inoculum: Rust diseases
require green plant tissues to survive the winter in the
U.S
- Wheat leaf rust
Spores generally arrive during late-spring or
early-summer in Minnesota each year via the ‘Puccinia’
pathway. The pathway consists of winds in the Red
River Valley that originate in Texas and Oklahoma.
Spores produced in the south on susceptible winter
wheat plants are deposited in the north central states
region by local precipitation. As of May 11, Oklahoma
plant pathologist Dr. Bob Hunger reports areas of
severe leaf rust disease in his state, depending on
drought conditions. The disease is widespread across
Texas at low to moderate levels, but drought and a
maturing crop are slowing inoculum production.
- Wheat stripe rust
Spore transport is identical to wheat leaf rust.
Oklahoma pathologists report yield reductions due to
stripe rust. Temperatures in the 80s and 90s are
expected to slow disease development. Texas reports
little stripe rust development due to drought and crop
growth stage.
- Soybean rust
During February of 2005, a green kudzu leaf was
confirmed to have rust in Florida. Since that time
four counties in Florida and one in Georgia have been
confirmed with diseased kudzu or volunteer soybean.
This presents compelling evidence that the fungus
survived in the continental US during the 2004-05
winter.
Temperatures that promote disease:
- Wheat leaf rust - approx. 60-70
F
- Wheat stripe rust - approx. 50-60
F
- Soybean rust - approx. 40-80
F
Types of wheat or soybean plant tissues
that are susceptible to infection:
- Wheat leaf rust - plant sheaths and leaves
- Wheat stripe rust - plant sheaths, leaves,
heads
- Soybean rust - plant stems, petioles,
leaves, and pods
Are the diseases expected to occur in Minnesota
in the 2005 growing season?
- Wheat leaf rust - Yes
- Wheat stripe rust - likely if the state has
an early growing season that’s cool
- Soybean rust - Inoculum sources in the
southern U.S. are establishing much more slowly than
expected. While we may see the disease in Minnesota
somewhere during 2005, it’s possible that it won’t
produce a statewide disease epidemic or even a
production problem. In an average growing year, it’s
likely that we’ll have scattered, localized areas with
the disease just as we do with Fusarium head blight
(FHB, scab). However, in certain years FHB has been a
substantial production problem, creating huge losses
in yield and quality for small grain producers. It
seems likely that epidemics of Asian soybean rust will
be sporadic across years, as well. Epidemics require
large numbers of spores in the agroecosystem as well
as an environment that promotes disease development.
How many other plant species are hosts for the
pathogens?
- Wheat leaf rust - Few
- Soybean rust - This pathogen has a wide
host range. We know that it can infect and survive on
more than 90 different plant species. Many hosts are
not cropped species, but are instead either naturally
occurring plants or weedy species. Kudzu, an
aggressive, large-leaved weedy species found in states
to our south has proven to be an excellent host for
the pathogen. The weed provides a means for the fungus
to produce huge numbers of spores, and kudzu doesn’t
seem any worse for wear. Not all plant hosts are as
susceptible to the disease as soybean and kudzu. In
fact, some bean species are known to have resistance
to soybean rust.
How many acres of susceptible crops are in the
U.S.?
- Wheat leaf rust - During the 2003 growing
season, approxmately 61.7 million acres of wheat were
planted in the U.S. Many of the varieties are
considered moderately resistant to resistant for wheat
leaf rust. While recent varietal releases have
excellent disease resistance, older varieties (those
released more than 4 or 5 years ago) are more
susceptible to the disease. Unfortunately for plant
breeders and producers, the fungal pathogen has an
ability to eventually overcome varietal resistance as
time passes.
- Soybean rust - During the 2003 growing
season, an estimated 73.4 million acres of soybean
were planted in the U.S. All commercial varieties are
susceptible to the disease, which increases the
probability that a spore will land on a susceptible
soybean plant with scores of susceptible plants
nearby. The only uncertainty is whether environmental
conditions will promote plant infection when the
spores are viable.
Soybean rust tracking efforts in Minnesota:
 Selected
locations in early planted commercial fields in Northwest
Minnesota that are identified to be at a greater risk for
disease will be monitored closely. The sites, known as
‘sentinel plots’ will be will be identified early in the
growing season by U of M regional and local extension
educators (see map). Working closely with
producer-cooperators, extension personnel will monitor
sites on a weekly basis. This level of monitoring will
continue until either the disease is confirmed in our
region or is identified in a state which serves as a
source of spores for wheat leaf rust such as Texas,
Oklahoma, and Kansas.
Wheat producers have learned to protect their crop from
rusts when disease is detected in the lower canopy.
Producers must stay just as informed regarding the spread
of soybean rust, weather conditions, and the potential for
crop loss. Timely information will be crucial in
implementing disease management strategies that will
preserve the economic sustainability of producing soybeans
in the RRV.
Charla Hollingsworth, Extension plant
pathologist
and
Carlyle Holen, IPM Specialist
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