July 13, 2006:
The ancient Romans prayed to Rosbigus, the god of rust who could
protect their crops from (to them) mysterious diseases in harvested
wheat and in stored bread during winter months. Ceremonies were
given to allay this deity so the cereal crops—needed for the
daily sustenance of Rome—were productive and safe to eat.
Today, the huge multi-billion dollar United States soybean crop
is threatened by Asiatic Soybean Rust (ASR), a fungal disease caused
by Phakopsora pachirhizi. This pathogen can eliminate up
to 90 percent of soybean yield under severe epidemic environments.
While varieties vary in their tolerance, no commercial strain has
resistance to this rust.
The ASR fungus originally wrought havoc in Asia and Africa, hitting
more recently Brazil and the United States. Hurricane Ivan is attributed
with bringing ASR spores to Florida from South America or Africa
in the fall of 2004. By the end of 2004, soybean rust was identified
in about eight counties, mostly in the panhandle of northwestern
Florida. By the fall of 2005, the same rust was identified in about
10 states and in 138 counties.
As important as soybeans are to the US ag economy in terms of quantity—hence,
the threat of a catastrophic disease exposure—they are prominent
in organic production due to their distinctive quality. Because
of consumer concerns about Roundup Ready genetically modified soybeans
in food products, there has been increasing demand for organic soybeans
over the last decade. Because there has been no confirmed defense
against ASR that was permissible under the rules of the USDA’s
National Organic Program, the viability of organic beans seemed
to be in danger, especially in the southern states.
Better news in 2006
But wait. If you are an organic soybean farmer it’s way
too early to give up on the crop, given differences developing this
season. By early July, the number of counties with soybean rust
was only up to 24, all of them in the Deep South relatively close
to the Gulf of Mexico. Soybean rust, like soybeans, will overwinter
only in southern Florida and Texas. The overwintering host has been
shown to be kudzu (Pueraria spp.), the notorious runaway
forage vine that cascades over trees in the Southern summer but
only survives unfrozen on live leaves at the most southerly points
of the continental US.
More good news: AgraQuest, a venture capital group developing biocontrol
organisms for disease and pest management, has registered the first
organically approved biocontrol specifically formulated for rust.
In 2005, results both within and outside the United States shows
Ballad biocontrol fungicide significantly improves both yield and
protects against many non-rust diseases to boot.
Bacillus pumilus (BP) is a patented biocontrol bacteria
in the same genus as Bacillus thuringiensis (BT). Many
organic farmers are well aware of BT for its ability to control
caterpillar pests. These bacteria produce a resistant endospore,
which allows a biological agent to survive commercial storage and
distribution without decreasing insect or disease control in BT
or BP, respectively.
In addition to direct action to kill rust, B. pumilus
works by activating the soybean plant defense system as well as
both blocking leaf-to-rust-spore contact and through the action
of potent amino sugar antifungal antibiotics. This last action is
expressed by inhibiting fungal cross-wall formation, new cell development,
and destroying membranes, which leads to pathogen cell death. In
addition, the soybean rust action has been demonstrated on septoria
brown spot, frogeye leafspot (Cercospora sojina), soybean
powdery mildew (Microsphaera diffusa) and purple leaf blight
and seed stain (Cercospora kikuchii).
Controls pay for organic beans
But does the application pay? Average yield increases from two
to four applications is 17 percent. This would cost about $20 to
$40 approximately per acre for a seven-bushel bump on 45 bu/ac beans.
Whereas for conventional beans this might have a value of about
$40 an acre—being near a wash—in the case of $10 organic
beans, this would be a value of at least $70 dollars per acre on
the same yield potential. Wholesale prices reported from seven terminal
markets as July 11 ranged from $9.50 to $14.50 per bushel for certified
organic soybeans, according to The New
Farm Organic Price Index.
Ballad fungicide is used at two quarts per acre and applied at
14-day intervals starting at or slightly before or after start of
flowering at the R1 stage. For growth stage details, visit:
Trials in South Africa concluding in March 2005 showed Ballad applications
of two quarts per acre gave 95 percent control of soybean rust—not
statistically more significant than the synthetic chemical control
azoxystrobin. There were also two trials in 2005 in Argentina. In
the first trial, yield was increased five bushels per acre with
a 10-percent decrease in soybean rust. In the second, one application
at the R5 stage (just before full green-bean stage) reduced rust
by 96 percent and bumped yield by 17 bushels—two bushels per
acre more than the mixture of synthetic fungicides tested.
We are collaborating with Iowa State University, University of
Florida and Michigan State University to examine ASR controls for
organic farmers. Our Florida collaborators tested Ballad with three
applications at R1 that showed rust decreased about two-thirds.
Dr. Boyd Padgett of Louisiana State University showed Ballad bumped
the yield by seven bushels per acre—almost 50 percent over
non-treated by controlling Cercospora Leafspot in the absence of
rust in 2005.
We plan to look at Ballad closely in Pennsylvania this year and,
although we do not expect soybean rust, we will see how it (in addition
to lime sulfur and soybean oil treatments) does on other diseases
and get our feet wet on the product. Soybean rust was reported as
far north as North Carolina and Missouri last year, so you may want
to be particularly careful this year scouting your fields, particularly
below the Mason-Dixon line.
I don’t know if praying to the rust god will work for you,
but being prepared with information on ASR and its management will
definitely help put you ahead in the rust game.