Sustainable Soybean Production—Part III

Diseases can and do cause economic losses in Midsouthern soybeans. The below points outline the soybean farming tools that can/may be used to reduce or prevent these losses and enhance the sustainability of soybean production in the region.

Until the early 2000's, many diseases could be managed only with resistant varieties or with cultural practices that were marginally effective as soybean farming tools. During the last decade, the use of resistant varieties has been complemented by the development and use of fungicides that are effective against many of the pathogens that cause soybean diseases.

Click here for a summary of major midsouthern soybean diseases and soybean farming tools that can be used for their prevention/control.

Several important diseases (e.g. stem canker, sudden death syndrome [SDS], Phytophthora root rot [PRR], seed and seedling diseases, charcoal rot) of soybeans have no curative control; i.e., these diseases may be prevented but not cured once present.

• Stem canker, SDS, and PRR can be prevented or avoided by using less-susceptible or resistant varieties. Development of varieties with resistance to stem canker is an example of a major development in using genetic resistance to effectively manage a devastating soybean disease.


• Seed and seedling diseases (caused by numerous fungi including but not limited to Cercospora, Fusarium, Phomopsis, Pythium, Phytophthora) can be effectively prevented by using seed treatments.


• There are no resistant varieties or fungicides for charcoal rot management. Additionally, it is likely that the majority of germinating seed are infected with the causal agent Macrophomina phaseolus. Thus, it is considered a major problematic disease.


 

An array of foliar fungicides is available for application to prevent several prominent soybean diseases. Preventive fungicides (e.g. strobilurins such as azoxystrobin [Quadris] or pyraclostrobin [Headline]) are most effective when applied prior to or at the earliest appearance of a disease.

Soybean rust can be managed with applications of preventive and curative (e.g. triazole, such as flutriafol [Topguard] or tetraconazole [Domark]) foliar fungicides timed according to occurrence of rust in sentinel plots. Additionally and based on previous years’ experience, rust may be avoided in the Midsouth by planting early-maturing varieties early so that R6 or full seed stage is reached before about August 1. This is an example of a cultural practice that is an effective soybean farming tool to use against a potentially problematic disease.

Soybean breeders, geneticists, and pathologists in the Midsouth are currently working to identify and/or provide genetic sources of resistance to Phomopsis seed decay (PSD) and charcoal rot. Results so far are promising. Resistance to PSD has been identified in soybean plant introductions (PIs), and a line with moderate resistance to charcoal rot has been released. These accessions can be used to develop varieties with resistance to these two diseases.

Presently, there are no transgenic disease management traits in soybeans. However, disease management through molecular genetic approaches may be forthcoming as an addition to the soybean farming tools available for disease prevention and/or control.

From the above discussion, the sustainability of Midsouth soybean production from a disease management standpoint rests on the following points.

• Soybean breeders should (and presumably will) continue to develop varieties that are either resistant to or tolerant of the damaging effects of the prevalent diseases that can plaque susceptible soybean varieties in the Midsouth. This should involve the continually-needed screening of potential varieties in disease nurseries to ensure these new varieties have the disease resistance necessary to withstand infection from prevalent pathogens in the Midsouth. Also, the process of new variety development must ensure that already-incorporated pathogen resistance (e.g. resistance to stem canker) is carried forward into the new genotypes.


• Charcoal rot and PSD, two diseases that are considered major problems, must receive increased research attention to ensure that the needed resources are available to continue the screening of germplasm and incorporation of identified resistance genes into new varieties. These two diseases should receive priority research attention since they are associated with large economic losses.


• For fungal diseases where host-plant resistance has not been identified and fungicides are the only soybean farming tools available for their management, current effective fungicides and those that may be forthcoming should be used in an integrated management approach to mitigate development of resistance in fungal populations and avoid secondary environmental impacts.


• For fungal diseases where resistant varieties are available, they should be used.  This is even more important for future sustainability since frogeye leaf spot resistance to strobilurin fungicides has been identified in Illinois, Kentucky, Missouri, and Tennessee.  The Fungicide Resistance Action Committee classifies these fungicides as being at a high risk for having resistance develop in targeted fungi.


• Soybean rust, though devastating if allowed to infect soybeans, should not be a major disease threat to Midsouth soybean production sustainability if fungicide applications, when needed, are timed according to sentinel plot data (this assumes that the sentinel plot system will remain in place). Furthermore, planting early-maturing varieties early is a proven effective management tool to avoid infestations. Resistant varieties should be forthcoming since genetic resistance has been identified in germplasm/PIs.


 

larryheatherly@bellsouth.net