Soybean Rust Disease: Learn About Soybean Rust Control In Gardens
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There is a disease that has so terrorized the soybean growing community that at one point it was listed as a possible weapon of bioterrorism! Soybean rust disease was first discovered in the continental United States in late 2004, brought in on the heels of a Gulf Coast hurricane. Prior to its discovery here, it has been a scourge in the eastern hemisphere since the early 1900’s. Today, it is important for growers to identify what soybean rust is, soybean rust symptoms, and how to control soybean rust.
What is Soybean Rust?
Soybean rust disease is caused by one of two different fungi, Phakopsora pachyrhizi and Phakopsora meibomiae. P. meibomiae, also called the New World type of soybean rust, is a weaker pathogen that is found in small areas of the western hemisphere.
P. pachyrhizi, called Asian or Australasian soybean rust, on the other hand, is far more virulent. First reported in Japan in 1902, the disease was only found in tropical to semitropical regions of Asia and Australia. Today, however, it has spread rampantly and is now found in Hawaii, throughout Africa, and into most of South America.
Soybean Rust Symptoms
The symptoms of soybean rust are indistinguishable to the eye when caused by either of the two pathogens. The most common sign of soybean rust is a small lesion on the surface of a leaf. This lesion darkens and may be dark brown, reddish brown, to tan and gray-green. The injury may be angular to circular in shape, starting as small as a pin point.
The lesions often grow together killing off large areas of leaf tissue. Soybean rust is found first on the lower leaves at or near flowering but gradually lesions move into the middle and upper canopy of the plant.
Cone-shaped pustules filled with spores appear on the lower leaf surface. They first appear as small, raised blisters but as they mature, begin to produce light colored, powder spores which mound up out of the pustule. These tiny pustules are difficult to see with the eye, so a microscope will help to identify the disease at this stage.
These pustules may grow anywhere on the plant but are most commonly found on the undersides of leaves. Infected foliage may appear mosaic and leaves may yellow and drop.
The disease can’t overwinter in areas of freezing temps, but it can spread rapidly over very large areas via wind. The rapid development of the disease can decimate a soybean crop, causing defoliation and premature plant death. In countries where soybean rust has been established, crop losses run from between 10% to 80%, so it is imperative that growers learn all they can about soybean rust control.
How to Control Soybean Rust
Soybean rust disease thrives with temps of 46 to 82 degrees F. (8-27 C.) with lengthy periods of leaf wetness. Spore production continues for weeks, spewing vast numbers into the air where they are easily spread by wind. It survives the winter months on host plants such as kudzu or one of over 80 other hosts in the southern United States, making it a difficult disease to control.
The future of soybean rust control hinges on the development of disease resistant varieties. The development of such disease resistant cultivars is being worked on as we speak, but at the current juncture, available soybean varieties have little to no resistance.
So how do you manage soybean rust? Foliar fungicides are the tool of choice and only a few are labeled for use against soybean rust. Your local extension office can help you determine which fungicides might be useful.
Fungicides need to be applied upon early infection, however, quickly covering the entire canopy of the plant. The number of fungal applications needed is dependent upon how early in the season the disease is caught and weather conditions.
Soybean is one of the most important commercial crops around the world and in the United States. Asian soybean rust is the major disease that affects soybeans. It causes lesions on the leaves of soybean plants and eventually kills the plants. The disease has caused serious yield loss of soybeans.  In the areas where this disease is common, the yield losses can be up to 80%. In 2002, USDA reported 10-60% yield losses in South America and Africa. 
Soybean rust is caused by two types of fungi, Phakopsora pachyrhizi and Phakopsora meibomiae.  It affects several important commercial plants, however, most notable for soybeans. Asian Soybean Rust can infect and reproduce on 90 known plant species, 20 of which are found in the United States, such as, soybeans, dry beans, kidney beans, peas, leguminous forage crops such as trefoil and sweet clover and weeds such as kudzu. 
At the early stage of Asian Soybean Rust, it causes yellow mosaic discoloration on the upper surfaces of older foliage. At this stage, it is usually hard to identify since the symptoms are relatively small and poorly defined. 
Later as the disease continues to progress, the leaves will turn yellow and there will be lesions mostly on the undersides of the leaves and sometimes on petioles, stems or pods and premature defoliation can also be observed. 
Asian Soybean Rust produces two types of lesions. Lesions at the later stage will turn from gray to tan or reddish brown. Mature tan lesions consist of small pustules surrounded by discolored necrotic areas. Tan spores can be found at the necrotic areas on the underside of the leaf. In the case of reddish brown lesions, there are larger reddish brown necrotic areas with few pustules and visible spores on the underside of the leaf. A good way to distinguish Asian Soybean Rust from other diseases is to look at the pustules it produces. ASR pustules usually do not have the yellow halo which is typical of bacterial pustules. Besides, ASR pustules are raised and can be commonly found on the underside of the leaf which makes it different from the lesions caused by spot diseases.  
Soybean plants are one of ASR's most known hosts and are susceptible at any stage in the life cycle. However, symptoms are most commonly found during or after flowering.  ASR infection will reduce pod production and fill. 
Asian Soybean Rust (ASR) was first detected in Asia.  It has been found in many countries around the world since then. For example, Australia, China, Korea, India, Japan, Nepal, Taiwan, Thailand, the Philippines, Mozambique, Nigeria, Rwanda, Uganda, Zimbabwe, South Africa, Brazil, Argentina, and Paraguay. This disease was first detected in the Caribbean in Puerto Rico in 1976  and first reported in the continental United States in 2004. 
Asian Soybean Rust favors environments that are humid and warm. A continuous period of wetness on leaves will aid the growth of this disease since this situation is required for spores to germinate. Therefore, is most likely to appear under conditions of 60 to 85 °F (16 to 29 °C) and a relative humidity of 75% to 80%. Therefore, ASR is a more serious problem in tropical and subtropical areas in Asia, Africa, Australia and South America.  It is unable to survive the cold winters of northern habitats.
Soybean rust is spread by windblown spores and has caused significant crop losses in many soybean-growing regions of the world.  Windblown spores can travel for great distances and are released in cycles of seven days to two weeks. It is likely that ASR will survive on vast acreages of naturalized kudzu in the southern U.S. and thereby establish a permanent presence in the continental U.S. It is commonly believed that the disease was carried from Venezuela to the United States by Hurricane Ivan. [ citation needed ]
P. pachyrhizi is an obligate parasite, meaning that it must have live, green tissue to survive. For this reason ASR is something that will blow in every year, as cold winters will push it back. It can overwinter in southern states, so long as it has a living host.
ASR overwinters on live host legumes and sporulates the following spring. It cannot survive on dead tissue or crop residues. 
Additional hosts can serve as overwintering reservoirs for the pathogen and allow for build-up of inoculum, in those environs free from freezing temperatures. The pathogen is well adapted for long-distance dispersal, because spores can be readily carried long distances by the wind to new, rust-free regions. 
Overwintering sites of soybean rust are restricted to areas with very mild winters, such as the gulf coasts of Florida, the very southernmost areas of Texas, or in Mexico. Soybean rust will not survive over the winter in the North Central region because it can't live and reproduce without green living tissue. 
Spores of the soybean rust pathogen are transported readily by air currents and can be carried hundreds of miles in a few days. Weather conditions will determine when and where the spores travel from south to north.
Rust spores, called urediniospores, are able to penetrate the plant cells directly, rather than through natural openings or through wounds in the leaf tissue. Thus infection is relatively quick: about 9 to 10 days from initial infection to the next cycle of spore production.
Rust is a multi-cyclic disease. After the initial infection is established, the infection site can produce spores for 10 to 14 days. Abundant spore production occurs during wet leaf periods (in the form of rain or dew) of at least 8 hours and moderate temperatures of 60 to 80 °F (16 to 27 °C). 
The process Edit
The infection process starts when urediniospores germinate to produce a single germ tube that grows across the leaf surface, until an appressorium forms. Appressoria form over anticlinal walls or over the center of epidermal cells, but rarely over stomata. Penetration of epidermal cells is by direct penetration through the cuticle by an appressorial peg. When appressoria form over stomata, the hyphae penetrate one of the guard cells rather than entering the leaf through the stomatal opening. This rust and related species are unique in their ability to directly penetrate the epidermis most rust pathogens enter the leaf through stomatal openings and penetrate cells once inside the leaf. The direct penetration of the epidermal cells and the non-specific induction of appressoria in the infection process of P. pachyrhizi may aid in understanding the broad host range of the pathogen and may have consequences in the development of resistant cultivars. 
Uredinia can develop 5 to 8 days after infection by urediniospores. The first urediniospores can be produced as early as 9 days after infection, and spore production can continue for up to 3 weeks. Uredinia may develop for up to 4 weeks after a single inoculation, and secondary uredinia will arise on the margins of the initial infections for an additional 8 weeks. Thus, from an initial infection, there could be first generation pustules that maintain sporulation for up to 15 weeks. Even under dry conditions this extended sporulation capacity allows the pathogen to persist and remain a threat. If conditions for re-infection are sporadic throughout the season, significant inoculum potential still remains from the initial infection to reestablish an epidemic. Successful infection is dependent on the availability of moisture on plant surfaces. At least 6 hours of free moisture is needed for infection with maximum infections occurring with 10 to 12 hours of free moisture. Temperatures between 15 and 28 °C (59 and 82 °F) are ideal for infection. 
Disease control options for ASR are limited. Rust descends in clouds of spores across the countryside. Cultural practices such as row spacing and crop rotations have little effect. Resistant cultivars do exist,   carrying what are called Rpp genes.   When weather and disease infection conditions are favorable, the occurrence of ASR can be widespread. Thus, remedial control measures—using fungicides as protective sprays—are the main effective disease control method.   
Synthetic fungicides are the primary disease control option for protection against Asian soybean rust. The cost of spraying is estimated to be about $15 to $20 per acre however, two or three sprays may be needed over the course of the growing season. These are significant additional production costs for soybean growers. 
Fungicide screening trials to determine disease control efficacy have been field conducted in South America and South Africa. These reports are available on the Web through USDA's Integrated Pest Management Information Centers.  These research trials form the basis for fungicidal recommendations in the U.S.
Recent research from Washington State University indicates that the herbicide Glyphosate may be effective in dealing with the fungus. [ citation needed ]
Rust-resistant varieties of soybeans are currently in development by both public universities and private industry.  
In some regions, the selection of winter cover crops and forage legumes may be effected, since they can serve as host plants.  Resistance genes (Rpps) have been identified   and host resistance is expected to be an effective, long-term solution for soybean rust in the future.   Until resistant commercial varieties are in place, the management of rust depends on judicious use of fungicides.   
When untreated, soybean rust causes yield losses due to premature defoliation, fewer seeds per pod and decreased number of filled pods per plant. 
Soybean Rust Symptoms - How To Control Soybean Rust In The Garden - garden
Soybeans in New Jersey: In 2003, New Jersey farmers harvested 88,000 acres of soybeans yielding 2.99 million bushels. Cash receipts totalled $16,071,000.
Prevention: The key to managing soybean rust is preventing it. Yield losses from soybean rust can be managed with the proper use of fungicides. Fungicides currently available for soybean rust control include: The fungicides azoxystrobin (Quadris ®), boscalid (Pristine ®), chlorothalonil (Bravo Weather Stik ®, Echo 720 ®, Echo 90DF ®, EQUUS 720 SST ®, EQUUS DF ®) and pyraclostrobin (Headline ®).
Additionally, growers in New Jersey may treat soybeans with the following fungicides under a soybean rust section 18 emergency exemption: propiconazole (Tilt ®, Propimax EC ®, Bumper ®), myclobutanil (Laredo EC ®, Laredo EW ®) tebuconazole ( Domark 230 ME ®, Folicur 3.6F ®, Orius 3.6F ®) azoxystrobin & propiconazole (Quilt ®) propiconazole + trifloxystrobin (Stratego ®) and propiconazole + tebuconazole (Headline SBR ®).
Growers may apply a maximum of three applications of approved section 18 products collectively under the soybean rust section 18 exemption however, no more than two applications may be made with any given active ingredient. Growers should follow label directions and Rutgers Cooperative Research and Extension recommendations on the use and proper timing of fungicide applications. For more information, contact your county Extension office: www.rcre.rutgers.edu/county/
Crop Insurance Information:
Rutgers Extension Soybean Rust Page
Crop Insurance Information:
Other Helpful Links:
Northeast Integrated Pest Management Center:
For more information, please contact the Division of Plant Industry, New Jersey Department of Agriculture at (609) 292-5441.
phakopsora pachyrhizi uredospore germination: Control (A), Roasted coffee oil 1% (B), Serenade ® (C), Pyraclostrobin + epoxiconazole (D), Asian soybean rust on detached leaves: Control (E), Serenade ® (F), AP 3 (G), Crude oil 1% (H), Roasted coffee oil 2% (I), Pyraclostrobin + epoxiconazole (J).
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The pathogen causing Asian soybean rust has been reported to survive mild winter conditions. P. pachyrizihas been shown to overwinter in kudzu in protected areas along the Gulf coast and in Mexico. Spores produced by P. pachyrizi are airborne and can be spread to nearby soybeans or longer distances by winds and storm systems. Soybean plants can be infected at all growth stages. Spores deposited on plant surface require the presence of free moisture for 6-7 hours and temperatures ranging between 65 and 80˚F (18-26.5˚C) to start infection, what are generally thought to be cooler conditions. About 9-10 days after initial infection, resulting spore producing structures can start producing spores for secondary infections. Leaf moisture or frequent rains are needed to support widespread infections.
Scouting and Threshold
Sentinel plots have been set up throughout the US to monitor the movement of Asian soybean rust. Check the USDA ipmPIPE soybean rust map to follow the progression of the disease in a given year. Scouting for soybean rust is critical during the R1 (first flower) to R5 (beginning seed formation) growth stages. To scout for soybean rust, walk the field in a 'W' or 'Z' pattern and check the plants as you move through the field. As symptoms appear first on lower and mid-canopy, check the leaves in these zones for soybean rust symptoms and signs. It is important to identify soybean rust correctly since it may affect the management decision. A guide to differentiate soybean rust from other similar-looking diseases can be found atUSDA website. If soybean rust is suspected in a field, collect samples and send them to your state diagnostic clinic for a confirmation. Select leaves, stems or pods that show the symptoms of Asian soybean rust. Arrange the leaves between layers of cardboard and dry paper towels to keep them flat (i.e. a sandwich consisting of cardboard, paper towel, leaves, paper towel, cardboard). Place this arrangement within a self-sealing plastic bag to retain moisture. DO NOT add water to the bag, Care should be taken that the outer surface of the plastic bag is not contaminated by Asian soybean rust spores. Include a detailed information of collection (e.g. location, date, host plant) when sending the samples.
Host Plant Resistance
Currently no commercial soybean cultivar resistant to Asian soybean rust is available. Gene conditioning resistance against this pathogen has been reported and breeding programs to incorporate this gene to commercial cultivar are underway.
In absence of resistant soybean cultivars, application of foliar fungicide is critical in managing Asian soybean rust. Generally fungicide should not be applied until the risk of infection is high and the crop is at the susceptible stages of R1-R6. A forecasting system provided by theUSDA ipmPIPE can be used to guide spray decisions.
Asian soybean rust developing in a soybean field.
Asian soybean rust is caused by the fungus Phakopsora pachyrhizi. The soybean rust pathogen has been moving progressively westward from its center of origin in China and has now reach North America. There is much speculation and apprehension on how soybean rust will develop and impact soybean production in the U.S. The reality of the situation is that soybean rust has been a devastating disease in many parts of the world. Thus, it is prudent that we respect its potential to reduce yield, and be prepared to manage it each season.
Monitoring rust with sentinel plots
Soybean rust is monitored each season in a national sentinel plot monitoring effort. The main goal of the sentinel plots is to detect soybean rust at low levels and provide early warning for soybean producers. Over 30 states have joined this sentinel plot network that utilizes trained observers to look for signs and symptoms of rust on soybean, dry beans and kudzu. Most Wisconsin soybean growers will have a sentinel plot within their county or will border a county with a sentinel plot. The plots are monitored through the combined efforts of UW county extension agents, agriculture research station staff, and UW-Madison campus research staff. The results of the sentinel plot observations are posted on the USDA Pest Information Platform for Extension and Education (PIPE) website.
Will soybean rust occur in Wisconsin this year?
Rust spores are light and easily disseminated by winds over large geographical areas. The pathway shown here is similar for that of other rust diseases found on grains, and serves as a model for likely soybean rust movement from south to north.
There are three key factors in determining the risk of soybean rust movement into northern soybean production regions:
- the occurrence of soybean rust during the spring and early summer in the Gulf coast areas. This determines the amount of spores available to blow northward.
- the July-August climate conditions, which establish where in the U.S. conditions are favorable for soybean rust development
- the northward movement of soybean rust spores in weather systems and by “green-bridging”.
The soybean rust pathogen has not been shown to be moved with soybean seed. Thus, growers should not be reluctant to purchase seed grown in regions where soybean rust developed the previous growing season.
Scouting for Rust
Accurate and timely diagnosis of soybean rust is critical to achieve control of soybean rust, especially if fungicides are involved in the management plan. If national monitoring efforts indicate that rust spores are moving northward, soybean growers and crop advisors should plan to scout frequently for rust. Scout these areas first:
- Early-planted fields
- Early-maturing varieties
- Low-lying or protected fields with prolonged dew periods
- Fields with early canopy closure
Check the lower leaves in the lower canopy
Leaves from the lower canopy will show symptoms first. Environmental conditions in the lower canopy favor spore germination and older leaves have likely been exposed to rust spores longer. Select leaves from the main stem only. Leaves from the lateral branches are less mature, even if picked from the same height.
Inspect leaves on site to avoid disseminating spores. Use a 10X or 20X hand lens to view symptoms. Rust is most likely to appear on the areas of the leaves that stay wetter longer: the bottom of a leaflet and close to major veins. Backlighting may enhance the viewing of early symptoms.
Initial symptoms of soybean rust include small, gray spots, particularly on the undersides of leaves and along leaf veins. These can be visible on both sides of the leaf, and also on petioles, stems and pods. The spots increase in size over time and change color from gray, to tan, reddish-brown or black. It is important to recognize that these symptoms are not exclusive to rust. Other diseases of soybean including brown spot, bacterial blight, and particularly downy mildew can easily be confused with soybean rust. If symptoms are observed, the next step is to verify the cause. See Common Soybean Leaf Diseases and Asian Soybean Rust (pdf) for a good overview of look-alike diseases.
The next step is to look for signs (sporulation) of the pathogen
If symptoms are observed, that is a signal to look for signs of the pathogen. In the case of rust, look for evidence of sporulation. Tan lesions mature to form pimple-like structures called pustules on the lower leaf surface. Active pustules contain the powdery tan spores of the rust fungus.
In the early stages of infection the emerging pustules look like miniature volcanoes topped with a pore.
In the early stages of infection the emerging pustules look like miniature volcanoes topped with a pore. There is no yellow halo surrounding the pustule. Later, the pustules “burst” releasing masses of spores. Use a hand lens to inspect the lower sides of symptomatic leaves for pustules that contain spores. These signs are diagnostic of rust.
It is absolutely necessary to observe sporulation in order to diagnose rust. Symptoms are a signal to look for signs, and signs are the only definitive means to diagnose soybean rust. Symptoms are not.
Printable online diagnostic guides
Soybean Rust – Wisconsin Pest Alert (pdf)
Identifying Soybean Rust – ID card (pdf) with excellent photos, including “look-alike” diseases, prepared by the USDA, U.S. land grant universities, and the Ontario Ministry of Agriculture and Food
Soybean Rust – United Soybean Board (pdf)
Common Soybean Leaf Diseases and Asian Soybean Rust (pdf) – Iowa State University
Soybean rust (SBR), caused by Phakopsora pachyrhizi, has become established in Africa since the first report in Uganda in 1996 (2). The urediniospores, as windborne propagules, have infested new regions of Africa, initiating SBR in many countries, including Ghana and Democratic Republic of the Congo in 2007 (4) and Tanzania in 2014 (3). No refereed reports have been published about rust in Malawi, but some people have indicated that soybean rust may have been observed as early as 2008. Typical symptoms and signs of SBR, including leaf yellowing and tan, sporulating uredinia, were observed on soybean in May 2014 during field surveys in the major soybean-growing areas of Malawi, including the central (Dowa, Mchinji, and Kasungu) and southern (Thyolo) regions in nine out of 12 sites surveyed. When microscopically examined, urediniospores were elliptical, echinulate, and hyaline to pale yellowish brown. Leaves exhibiting sporuliferous uredinia were sent by APHIS permit to the University of Illinois. To confirm the pathogen, symptomatic soybean leaf tissue of approximately 1 cm 2 was excised from each of the samples, and DNA was extracted using the FastDNA Spin Kit (MP Biomedicals, Solon, OH), with further purification using the MicroElute DNA Clean-up Kit (Omega Bio-Tek, Norcross, GA). The resulting DNA was analyzed by quantitative PCR using published Taqman assays for P. pachyrhizi and P. meibomiae, with a multiplexed exogenous internal control reaction to validate negative results (1). P. pachyrhizi DNA was detected in excess of 180,000 genome equivalents/cm 2 in all samples, indicating a substantial infection. P. meibomiae DNA was determined to be absent from all samples, within the limit of quantification of
2 pg DNA/cm 2 . Urediniospores dislodged from three leaves and inoculated onto susceptible soybean cultivar Williams 82 produced tan lesions after 2 weeks of incubation in a detached-leaf assay. This is the first confirmed report of P. pachyrhizi causing rust on soybean in Malawi, putting at risk 14,000 ha currently under soybean production. The reports of soybean rust in Malawi and adjoining countries will alter soybean production practices and research interests. In some cases, foliar application of fungicides has increased and planting dates have been changed to avoid conditions that are most conducive for rust development. Efforts to understand the virulence and genetic diversity of the pathogen in the region are needed in order to develop and deploy resistant cultivars. References: (1) J. S. Haudenshield and G. L. Hartman. Plant Dis. 95:343, 2011. (2) R. Kawuki, et al. Afr. Crop Sci. J. 11:301, 2003. (3) H. M. Murithi et al. Plant Dis. 98:1586, 2014. (4) P. S. Ojiambo et al. Plant Dis. 91:1204, 2007.