Soybean Rust

Soybean Rust: If you’ve been growing wheat you’ve been managing its ‘cousin’

By Charla Hollingsworth
U of M extension plant pathologist

Asian soybean rust (caused by Phakopsora pachyrhizi) has made quite a name for itself across U.S. soybean producing areas since November, 2004. After being discovered on soybean and other plant species in nine southern states, the disease is now “larger than life” and will be monitored closely during the 2005 growing season. 

To understand soybean rust better, it might help to draw comparisons to wheat leaf rust (puccinia recondita f. sp. tritici). Let’s look at the two fungal pathogens, how they are similar and how they differ.

Temperatures that promote pathogen infection of plants
Wheat leaf rust: between 59 to 73^o F
Soybean rust: between 59 to 82^o F, however, optimum infection temperatures are closer to the warmer end of the range rather than the cooler.

Hours of leaf wetness needed for plant infection
Wheat leaf rust: 6 to 8 hours
Soybean rust: approximately 6 hours

Lifecycle duration: includes average number of days, under favorable disease conditions, needed by a spore to

1. infect a susceptible host plant,
2. cause disease,
3. produce more spores. 

Wheat leaf rust: 7 to 10 days
Soybean rust: 9 to 10 days

Plant tissues susceptible to infection
Wheat leaf rust: wheat plant sheaths and leaves
Soybean rust: soybean plant stems, petioles, leaves, and pods

Over-wintering inoculum sources in the U.S.
Wheat leaf rust: Spores blow in with southerly winds during the summer, produced on susceptible winter wheat plants grown in southern states.
Soybean rust: Spores also spread by southerly winds during the summer, produced on susceptible broadleaf plants grown in southern states.

Plant species that are hosts for the rust pathogen
Wheat leaf rust: Few
Soybean rust: Many – this pathogen has a wide host range; we know that it can infect and survive on more than 90 different plant species, including cultivated crops (yellow sweet clover, lima bean, kidney bean, green beans, garden peas) and many non-cropped species (wild plants and weeds) including kudzu, an aggressive, large-leaf weed found in states to our south that has proven to be an excellent host for the pathogen.

Susceptible varieties
Wheat leaf rust: Many of the spring wheat varieties grown are considered moderately resistant to resistant to wheat leaf rust.  While recent varietal releases have excellent disease resistance, some older varieties (those released more than 4 or 5 years ago) are becoming 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: All current varieties can be considered susceptible to infection, to a varying degree.  The only uncertainty is if environmental conditions will support plant infection and disease development.

Control measures
Wheat leaf rust: Strobilurin fungicides (i.e. Quadris, Headline) triazole fungicides (i.e. Tilt, Bumper, Folicure) and strobilurin-triazole combinations (Stratego, Quilt) are available for control of leaf rust and other wheat diseases. Strobilurin fungicides are “preventative,” used as protectants to prevent sporulation, before the disease shows up. Triazole fungicides are “curative,” best when rust is present at the time of spraying. Application timing for both types of fungicides is critical for treatment success. When it comes to managing wheat rust, strobilurins and triazoles are generally applied once, after flag leaf emergence (Feekes 10) if the disease is established in the under-canopy and the weather promotes further disease development (wet and warm).  Application rates vary by product, but are generally less than what’s required for soybean rust control.

Soybean rust: A number of fungicides have been approved with either full or limited label approval for control of soybean rust in soybeans this season.  As with wheat rust, products available include strobilurin (applied to help prevent disease) and triazole  (applied to help “cure” the disease if applications are made early enough in the disease cycle) . There is also chlorothalonil (Bravo, Echo) another type of fungicide that works preventatively. Application timing is critical, just as it is with wheat rust. Most data from Brazil suggests that maximum benefit occurs when fungicides are applied between beginning flowering (R1) through full pod (R6), and that applications made before R1 or after R6 may not be economically beneficial. Application rates vary, but will likely be more than what is used for control of wheat rust.

Disease Outlook for 2005
Wheat leaf rust: With early-season reports of inoculum increasing south, it’s very possible we’ll see wheat leaf rust this season. Monitor the disease closely and be ready to take treatment measures.

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 host plant eco-system as well as an environment that promotes disease development.  With this said, it is prudent to prepare for the worst case scenario, such as a statewide epidemic, and be pleasantly relieved if it doesn’t occur, than to not be prepared.

Comparisons between wheat leaf rust (caused by Puccinia recondita f. sp. tritici) and Asian soybean rust (Phakopsora pachyrhizi)

	Rust
	Wheat Leaf	Asian Soybean
Infection temperatures	59 – 73F	59 – 82F
Leaf wetness minimum	6 – 8 hrs	6 hrs
Lifecycle interval	7 – 10 days	9 – 10 days
Plant tissues infected	sheaths and leaves	stems, petioles, leaves, pods
Number of known plant hosts	Few	Many
Plant genetic resistance	Varies by variety	All varieties susceptible to varying degree
Control	Both preventative and curative fungicides available. Application timing is critical: apply most products between early flag leaf to heading (Feekes 8-10.5)	Both preventative and curative fungicides available. Application timing critical: apply most products between early flowering (R1) through full pod (R6)

Asian Rust Information Online

NDSU -- www.ag.ndsu.nodak.edu/extplantpath (click on “Soybean Rust Information”)

SDSU -- http://plantsci.sdstate.edu/planthealth (Click on “Soybean Rust”)

UM -- www.soybeans.umn.edu/crop/diseases/soybeanrust.htm

OTHER -- www.oardc.ohio-state.edu/soyrust (excellent info on using foliar fungicides to manage soybean rust)