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Disease Management

There are three basic management tactics that can play a role in reducing soybean rust epidemics: fungicides, genetic resistance, and cultural practices. At present, fungicides are the only highly effective tactic (Figure 16), but long-term management will probably depend more on resistance, in combination with fungicides and changes in cultural practices.

At present, the most effective means of managing soybean rust is the use of fungicides (Figure 1). However, to be effective, selecting the right fungicide and applying it at the right time are crucial. Several fungicides are registered in the US for soybean rust control, and most can be classified into three groups: chloronitriles, strobilurins, and triazoles (Figure 17). Chlorothalonil is the one chloronitrile fungicide registered for soybean rust control. Its protectant mode of action affects many biochemical pathways in the pathogen, but it is not taken up by the plant, not even by the cuticle. As a result, it is more subject to weathering than the strobilurins or the triazoles and complete coverage of the leaf surface is critical. To be effective, chlorothalonil may need to be reapplied several times if new growth or weathering occurs.


Fig. 16	Fig. 1	Fig. 17

Strobilurin fungicides are modeled after a natural antifungal compound, strobilurin, produced by certain mushrooms. Strobilurins (also known as QoI fungicides) inhibit mitochondrial respiration in the pathogen. Strobilurins are typically absorbed by the cuticle, and act as protectant fungicides (http://www.apsnet.org/education/AdvancedPlantPath/Topics/Strobilurin/top.htm). A protectant fungicide prevents infections from taking place, but it has little effect on disease development once infection has occurred (Figure 17). Therefore, to be effective, protectants like the strobilurins must be applied before infection occurs. Depending on the rate applied, strobilurins are effective for up to 2 weeks after an application, but they will not protect newly developing leaves. Strobilurins control a broad range of soybean pathogens.

Triazoles inhibit sterol production, which disrupts cell membrane function in the pathogen. Triazoles are absorbed and translocated upward in the plant. While they generally do not prevent infection, the triazoles can kill the fungus in the plant and prevent pustules and spores from forming (Figure 17). The extent to which these chemicals are translocated depends on the triazole, but all of them move up the plant into new growth to one degree or another. Still, systemicity of triazoles in plants is incomplete and does not approach the level of systemicity associated with certain herbicides or insecticides. Triazoles are effective for 3 or 4 weeks after application and give some protection to new growth. While highly effective against rust, the triazoles are not as effective as the strobilurins against other soybean pathogens. Some fungicide products (premixes) contain both a triazole and a strobilurin. The premixes provide protection against a broader range of pathogens and reduce the possibility of pathogens developing resistance to either product.

The number of applications required for disease control depends on the compounds used, when the rust epidemic starts, and the favorability of the weather conditions. Even with triazoles, which are effective for the longest period of time, two applications are often needed to control soybean rust. In some locations in Brazil, high levels of inoculum early in the season result in rust epidemics starting well before flowering, thus forcing growers to make as many as five fungicide applications in order to control the disease. Such early disease onset and the early need for fungicide application are unlikely in most of the US. However, rust could start as early as flowering (R1) and require an additional spray before harvest. It is generally felt that, once the plants reach the R6 growth stage (when seeds have filled the pod), most of the yield has been achieved and controlling rust beyond that point is not economical. One concern with multiple applications of the same fungicide is the development of fungicide resistant pathogen strains. While fungicide resistance in P. pachyrhizi has not been reported, other fungal pathogens may be affected and growers should try not to spray the same fungicide consecutively. Fungicide labels may restrict the number of times a particular compound or class of compounds can be applied within a season to reduce the possibility of resistance developing.

The key to effective control of soybean rust with fungicides is application timing. This is especially important in areas of the US where the soybean rust pathogen must be reintroduced each year. The introduction or reintroduction will probably occur at different times in different years or not at all in some years. All of the fungicides, even the systemic triazoles, are most effective when applied just before the rust epidemic starts in the field. From tests in South America, if disease incidence reaches 10% in the lower canopy before the first application, fungicides will not completely control soybean rust, and some yield loss will result if weather conditions are favorable. Such low levels of disease are difficult to detect so growers need an early warning system that predicts the onset of disease early enough so that they have time to apply fungicides to all of their fields. Application decisions, equipment, and technique can all greatly impact the level of rust control achieved.

At present, the most reliable early detection method is the use of “sentinel plots.” These are small plots (primarily soybean, but kudzu or another susceptible host also may be used) planted several weeks before the commercial crop, and often use early maturing cultivars. Both the early planting and the early maturity of the cultivars results in the sentinel plots flowering 1 to 3 weeks before the commercial crop. Since soybean rust usually develops after flowering, the disease can be observed in these sentinel plots a week or two before being found in adjacent commercial fields. This early warning gives growers in the area time to apply a protective fungicide treatment. Sentinel plots have been established throughout the soybean and dry bean production areas of the US. Information from these plots is uploaded weekly into a USDA website (www.sbrusa.net) where maps are generated showing rust activity in the country (Figure 18). This site also includes state and national commentaries, disease forecasts, and other pertinent information.

Fig. 18

Besides the sentinel plot findings, extension specialists in each state also include state-specific commentary on soybean rust and the need for control measures. In addition, rust information from all of the state plant diagnostic clinics is networked together, and new finds of soybean rust are included on the site. Information from this USDA website can be used by growers and scientists to see where rust is active and to determine if their area is threatened by the disease. In addition to the USDA website, information is also available on many state Cooperative Extension Service’s websites and on several agricultural industry websites. Information about soybean rust in Argentina can be found at http://www.sinavimo.gov.ar/ and in Brazil at http://www.cnpso.embrapa.br/alerta/. A partial list of websites can be found in the Selected References section of this lesson.

Several experimental early warning and disease forecasting systems are under development. These models relate a variety of weather, crop and disease conditions to spore movement, spore deposition, and infection. Some of the factors included in these models are sources of inoculum, wind direction and speed, temperature, humidity, leaf wetness, sunlight intensity, and crop developmental stage. These models are currently being used to indicate where and when scouting efforts should be intensified.

Another method of early detection of soybean rust is spore trapping, in which two strategies are being assessed. One traps windblown spores on glass slides coated with petroleum jelly (Figure 19). The spores are examined microscopically, and the presence and number of soybean rust-like spores noted. At this time, microscopic examination can only identify spores that resemble the soybean rust pathogen because it is not currently possible to identify P. pachyrhizi with certainty by simply examining the urediniospores. More conclusive identification of urediniospores of P. pachyrhizi is being developed by using labeled antibodies and polymerase chain reaction (PCR) protocols.


Fig. 19	Fig. 20

The other spore trapping approach involves collecting and filtering rainwater and then uses PCR to determine the presence of P. pachyrhizi on the filters (Figure 20). It is thought that long-distance spread of urediniospores occurs when storms pick up the spores and then deposit them in rainwater at distant locations. Because this technique uses species-specific molecular markers, positive findings are thought to be more reliable. In 2005 and 2006, both air and rain sampling found P. pachyrhizi or P. pachyrhizi-like spores over a wide area, far from where soybean rust was active. While neither approach can determine if the spores arrived alive, they do indicate that this pathogen has the potential to spread widely and quickly.

Genetic Resistance

Soybean plants respond to infection by P. pachyrhizi by producing either tan, red-brown, or no lesions at all. Tan lesions produce many pustules with many spores (Figure 10). Red-brown lesions produce a few pustules with limited spore production, and no pustules or spores are produced where no lesions are formed (Figure 11). It is thought that these responses represent susceptible, moderate, or highly resistant reactions, respectively. High levels of resistance are usually associated with one or a few dominant genes. There are four known dominant genes for resistance to soybean rust, Rpp1 through Rpp4. While these dominant genes confer high levels of resistance and are relatively easy to incorporate into new soybean cultivars, they are not effective against all races of P. pachyrhizi. Deployment of varieties with new resistance genes is usually followed in a few years by the emergence of races of P. pachyrhizi that are virulent on them. This high degree of variability in the soybean rust pathogen is common in many rusts [see wheat stem rust] and requires the frequent discovery and incorporation of new sources of resistance. Currently, isolates of P. pachyrhizi exist that are virulent on each of the four known genes for resistance.


Fig. 10	Fig. 11

Another approach is the use of moderate resistance. Moderate resistance is usually conferred by a number of genes, each contributing a little to the overall resistance of the cultivar. This type of resistance often is effective against all races of a pathogen, but it is more difficult to incorporate into cultivars and does allow some disease and yield loss. Moderately resistant cultivars have been developed in Asia, but adapted varieties with this type of resistance are not yet available in the US or South America. Ultimately, moderate resistance may be used in combination with cultural practices and fungicides when needed.

Cultural practices

There are several cultural practices that may help manage soybean rust. In most areas of the US where rust must be introduced each year for an epidemic to occur, changing planting and harvest dates may avoid disease. Planting early with an early maturing cultivar may avoid the rust until the crop has either been harvested or is so far along that the disease will have little impact on yield. Planting dates may also be delayed so that the vulnerable reproductive period occurs during dry conditions that do not favor rust. In areas where the weather is marginal for rust development, wider row spacing along with lower plant populations may hasten canopy drying, thus reducing the dew period enough to prevent or at least slow disease development. It may also allow better fungicide penetration into the canopy, increasing the effectiveness of chemical control. Research is needed to confirm this. However, because the more open canopy provides less weed suppression, weed problems may be more severe with this strategy, and this method is unlikely to affect rust significantly if weather conditions are very favorable for the disease. Adjusting soil fertility, particularly potassium and phosphorus levels, may help increase disease resistance, but there is little research in this area yet. While it is unlikely that cultural control measures alone will be enough to control soybean rust, they may increase the effectiveness of host resistance or fungicide applications.

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