Asian Soybean Rust

• Asian soybean rust (ASR) is a wind-borne fungal disease recently introduced into the southern and southeastern U.S. It is a very aggressive pathogen, with significant yield losses recorded in the absence of fungicide control.
• Asian soybean rust needs living tissue to survive. Survival on plant debris in the field has not been reported, and the disease is not seed-transmitted.
• Asian soybean rust will likely survive and overwinter on green tissue in the southern U.S., Mexico and the Caribbean. Rust spores will then be carried on wind currents into the main U.S. soybean growing regions each spring. This is similar to spread of corn and wheat rusts.
• Disease development is favored at temperatures between 68 to 75° F and leaf wetness periods greater than 6 hours. Rainfall plays a critical role in disease development and spread.
• Disease scouting to detect fungal lesions as early as possible is key to an effective control program. First disease symptoms generally appear in the lower canopy. Scouting should begin around the R1growth stage.
• Timely application of fungicides is needed for effective control. Applications should be initiated before the disease incidence exceeds 5%. Preventative and curative products are available. Fungicide rotation is necessary to prevent the development of fungicide resistance.
• Genetic resistance to ASR on a commercial level has not been identified. Pioneer is actively screening for resistance in research locations across the globe.

Asian soybean rust (ASR) can be caused by either of two fungal species, Phakopsora pachyrhizi or P. meibomiae. P. meibomiae has not been detected in the continental U.S., and is not known to be of economic importance to crop production. However, P. pachyrhizi, a species endemic to Asia, is a devastating disease. In untreated fields in Asia and South America, yield losses ranging from 10 to 80% have been reported due to premature defoliation, fewer seeds, lighter seeds and poor seed quality.

P. pachyrhizi is now present in most of the soybean growing areas of the world. It originated in NE Asia and was first reported on soybean in Africa in 1997. The first detection in the Americas was in Paraguay in 2001, and from there it spread rapidly to all soybean-growing areas of Brazil. It was first recorded in the northern hemisphere in 2004 in Colombia. Hurricane Ivan in September of 2004 was most likely responsible for the recent introduction into the U.S.

ASR is carried long distances by wind currents; however, field-to-field transmission through contaminated clothing is also common. The fungus is not seed-transmitted. Most of the knowledge about ASR was developed in subtropical and tropical areas of the world. As with any new disease, its epidemiology and resulting control strategies might change in the temperate growing areas of the U.S.

Asian soybean rust symptoms are most commonly found on the underside of the lower leaves, starting around the flowering stage. But under severe disease pressure, infection of leaves and even the cotyledons of early vegetative-stage soybeans has been reported. Initially, symptoms of ASR infection are difficult to detect, and can be mistaken for bacterial pustule, bacterial blight and Septoria brown spot (see section on scouting for details). The first symptoms of ASR infection are small, poorly defined discolorations on leaves in the middle to lower canopy (Figure 1).

Figure 1. Soybean leaf showing early ASR lesions.

Later, small brown-gray lesions appear predominately on the underside of the leaves. As the lesions mature, they increase in size, and their color changes to tan or reddish brown. Mature lesions are 2 to 5 millimeters² in size (for reference, 2 mm² = , 5 mm² = ), tan to dark brown in color, and polygonal in shape (Figure 2).

Figure 2. Soybean leaves with mature ASR lesions.

At the center of the mature lesions, uredinia are formed. Uredinia have the appearance of pustules (Figure 3) (see section on scouting).

Figure 3. Asian soybean rust pustules (magnified).

Inside these pustules, fungal spores are produced. Initially, the spores are released through a circular opening in the center of the pustule (Figure 4). On more mature lesions, the grayish-tan spores are released through irregular ruptures of the epidermis (Figures 5 and 6).

Figure 4. ASR lesion prior to spore release (magnified).

Figure 5. Sporulating early lesion of Asian soybean rust.

Figure 6. Sporulating mature lesion of Asian soybean rust.

There are two lesion types. Tan lesions, when mature, consist of small pustules (uredinia) surrounded by a slightly discolored necrotic area with masses of tan-grayish spores on the lower leaf surface (Figure 7).

Figure 7. Tan ASR lesion type (magnified)¹.

Reddish brown lesions have a larger reddish brown necrotic area, with a limited number of pustules and few visible spores on the lower leaf surface (Figure 8).

Figure 8. Reddish-brown ASR lesion type (magnified)¹.

As ASR severity increases, premature defoliation and early maturation are common. Lesions can also be found on petioles, pods and stems, but are most common on leaves.

Hosts

In addition to soybeans, the Asian soybean rust fungus is able to infect over 30 legumes such as lima and butter beans, green beans, kidney beans, cowpeas, pigeon peas, yam bean and jicama. Kudzu is also a suitable host.

Survival

ASR requires living host tissue to infect, grow, and sporulate. Spores on crop debris left in fields after harvest will not serve as initial inoculum in the following growing season. The survival of the fungus is therefore restricted to areas where green host tissue is available. Across the continental U.S., the fungus is most likely to survive in certain southern and southeast states on kudzu and volunteer soybeans. The area of survival will most likely be determined by the frequency of freezing temperatures across the southern U.S. The Caribbean islands and Mexico also serve as locations for year-round survival.

Spread

Starting from the area where the fungus survived during the winter months, infection of soybeans in the other growing areas of the U.S. will depend on air currents blowing over the areas of survival in a northerly direction. These air currents pick up spores and deposit them along their path on host tissue. If conditions are favorable at the point of deposition, infection will occur and new spores will be produced. This process will repeat itself many times during a growing season. Experts expect the northerly pattern of spread of ASR to mimic that of common and southern rust of corn (Figure 9).

Environmental Conditions

Temperatures between 59 to 86° F. are ideal for fungal infection. In addition, a minimum of 6 hours of leaf wetness is required for infection, with maximum infection occurring with 10 to 12 hours of free moisture. Uredinia (which contain spores) can develop 5 to 8 days after infection. The first spores can be produced as early as 9 days after infection, and spore production can continue for up to 3 weeks. High relative humidities (75 to 80%) are conducive for disease development and sporulation.

Figure 9. Likely pattern of northern spread of ASR from southern areas during the growing season.

Asian soybean rust can only be managed by scouting and timely fungicide applications. No soybean varieties with resistance or tolerance to ASR are available.

Scouting
Early disease detection is the basis of a successful control program. Scouting should be initiated when:

• ASR has been reported in your area
• Anytime after flowering
• When large weather systems from southerly directions have moved through the area.

After initial discovery of ASR in your area, scouting should be done on a weekly basis. If ASR is suspected, laboratory confirmation may be needed for positive ID.

Field Scouting Guidelines

• Examine areas conducive to ASR development that have a high likelihood of showing the symptoms first. These are areas that promote leaf wetness and high relative humidity such as:
  
  - Low lying areas
  - Wet spots
  - Areas along tree lines
• In general, evaluate 20 locations in a field examining five plants per location (scouting patterns may vary depending on the purpose).
• Look in the lower soybean canopy
• Know your other diseases (bacterial blight, bacterial pustule, brown spot)
• Use a 20X handheld lens to greatly enhance your ability to correctly identify ASR lesions.

Sampling Guidelines

• Collect 20 leaflets, flatten and place between paper towels
• Place in sample bags and clearly mark pertinent information: date, location, name, phone number, etc.

It is important to remember that initial lesions of ASR can be misdiagnosed as bacterial pustule, bacterial blight, and Septoria brown spot. The best way to differentiate these diseases is the characteristic spore production of ASR. Spores are round to oblong, hyaline to grayish color, and protrude through the blister profusely, first through the circular opening in the blister, and later through the irregular ruptured blister cover. The best opportunity to observe sporulation on the leaves is during the early morning hours, or after prolonged leaf wetness periods.

More tips for differentiating Asian soybean rust from other common soybean leaf diseases are shown in Table 1. In general, Septoria brown spot and ASR start in the lower canopy, but Septoria brown spot doesn't have pustules. Both bacterial diseases start in the upper canopy. Bacterial pustule produces pustules, but does not produce spores.

Table 1. Characteristics differentiating Asian soybean rust from other soybean leaf diseases.

If Asian soybean rust is confirmed, fungicide treatment is required for control. This is unlike some other diseases, for which control is based on disease levels or thresholds.

Plastic Bag Method: Immature, non-sporulating lesions of ASR are hard to identify. An easy method has been developed to help ensure correct diagnosis without the need for repeated trips to the field. Simply harvest leaves with suspected ASR lesions and put them in a sealed plastic bag containing moist paper (Figure 10). The plastic bag should then be stored at room temperature and shielded from direct sunlight. Observe leaves daily for spore production. As soon as spores are visible and correctly identified, appropriate control measures should be initiated.

Figure 10. Soybean leaves in a sealed plastic bag with a moist paper towel. This procedure can hasten the diagnosis of ASR if plants are infected but lesions are immature.

Chemical Control

Fungicides have been very effective for control of ASR when applied early with good spray coverage (Figure 11 and 12). Under severe infection conditions (untreated check variety = 100% severity), two applications of a systemic fungicide lowered disease severity to a level of 10%.

Figure 11. Effect of three fungicide treatments on percent disease in soybeans under severe ASR infection conditions. Source: DuPont Crop Protection, 2004.

Fungicide application needs to be started as soon as fungal lesions are detected in the lower canopy of the soybean plant. If disease severity exceeds 5%, fungicide efficacy is reduced. Penetration of the fungicide into the lower canopy is important, since this is where the disease initially occurs. Up to three applications have been necessary in high-risk areas of Brazil; however, 1-2 applications probably will be sufficient in the central U.S. More detailed guidelines in regard to application technology will be available in the near future.

Fungicides will be classified as either preventative or curative. Preventative fungicides will protect against infection, but are not effective once the fungus has penetrated the leaf. Curative fungicides give protection against infection, but because they are systemic (i.e. they are absorbed into and move within the plant), they also control the fungus in the leaf tissue. The USDA/ARS has conducted fungicide efficacy trials in 2003 and 2004 in Paraguay and Zimbabwe. The results can be accessed at the following website: http://www.ipmcenters.org/soybeanrust/

In order to avoid the buildup of fungicide resistance, rotation of fungicides is recommended.

Soybean Variety Resistance

Pioneer began research work on Asian soybean rust resistance in the Philippines in 1995. Efforts were expanded to include Brazil in 2002 and India in 2003. Each of these locations is important because of different local races. Specific (single gene) resistance for ASR has been identified. However, incorporation of these genes into soybean germplasm has not led to commercial products because the fungus quickly overcame this type of resistance.

Current resistance screening efforts are aimed at identifying partial (rate-reducing) resistance and tolerance in the germplasm. Varieties that have the rate-reducing resistance show longer latent periods and/or reduced sporulation. Varieties that show the tolerance trait in general have less yield loss from ASR. Rust development rates and estimates of rust severity on foliage were less correlated with yield loss in tolerant materials.

Figure 12. Soybean field with untreated and treated strips under severe Asian soybean rust infection.

Pioneer is using the latest breeding technologies such as genomics in its efforts to develop a genetic solution to counter the ASR problem, and is also working on generating new resistance sources. Those efforts are likely to provide help in the long term, but for now, the only effective management practice for ASR is timely application of fungicides.

• USDA-APHIS Asian soybean rust website:
• APS (American Phytopathological Society)
• North Central Region Pest Management Center
• United Soybean Board website:
• Butzen, S., A. Marcon, B. McInnes, and W. Schuh. 2005.
  Asian Soybean Rust: Fungicide Application Technology.
  Crop Insights Vol. 5 no. 1. Pioneer Hi-Bred. Johnston, IA
• Butzen, S., F. Beudot, and B. McInnes, 2005.
  Asian Soybean Rust: Fungicides.
  Crop Insights Vol. 5 no. 1., Pioneer Hi-Bred. Johnston, Iowa.

¹Figures 7 and 8 courtesy of Glen Hartman, USDA-ARS.

Other images courtesy of Elcio Alves, Pioneer Sementes, Itumbiara, Brazil.