In getting ready for soybean planting in 2011, there are many factors to consider. In my experience, preparing for what the weather will likely be is probably the greatest challenge of all, as each year is different. Different growing conditions trigger different pathogens that affect germination and early growth of soybeans.
Phytophthora root and stem rot, caused by Phytophthora sojae, is a very common and well-known disease that can affect the plant at any growth stage from seedling to maturity.
When soybeans are seedlings, phytophthora will infect the plant causing “damping off,” or seedling blight. Symptoms include soft wet rot of the stem or hypocotyl, also described as water-soaked, wilting and root rot.
Conditions that best favor this disease are saturated soils, particularly in heavy clay and poorly drained soils, as the pathogen needs water to infect the plant. A week of wet, rainy weather followed by a week of dry weather is ideal for a severe disease outbreak. Along with wet soil conditions, phytophthora prefers warm soil temperatures of 70 to 77 degrees F for optimum infection.
Another ‘damping off’ disease
Pythium is another pathogen that causes damping off of soybean seedlings. Infection by this pathogen results in similar symptoms to damping off caused by phytophthora. The only way to tell the difference between symptoms caused by the two pathogens is to send the sample to a lab for proper diagnosis. Pythium also likes saturated soils, but prefers soil temperatures of 50 to 59 degrees F.
If beans are planted early into cold, wet soils, pythium is the most likely candidate to infect soybeans, more so than phytophthora. A decade ago phytophthora was the No. 1 seedling disease, but as growers have started to plant beans earlier, pythium is now our No. 1 seedling disease. Both diseases in total attribute to 55% to 60% of the Iowa soybean seedling diseases each year.
Seedling diseases may also be caused by two fungal species that are favored by drier soil conditions: rhizoctonia root rot (Rhizoctonia solani) and fusarium root rot (Fusarium oxysporum and Fusarium solani). Of the two diseases, rhizoctonia is a more common and serious pathogen causing root rot in Iowa than fusarium.
Soybeans affected with rhizoctonia occur as single plants or as patches in a field, often in the sloped areas. Rhizoctonia has very distinct identifiable symptoms to help diagnose it.
Symptoms include red (dark brown to brick red in color) sunken lesions on the plant stem at the soil level and poor lateral root development.
Reddish lesions are also symptoms of infection by fusarium. Stem lesions of rhizoctonia root rot usually occur at the soil line, while fusarium root rot lesions start at the lower portion of the root system.
So what can a grower do?
To help manage these diseases, ISU Extension plant pathologists recommend a combination of practices be used.
For phytophthora, select resistant soybean cultivars, although you need to realize that many races of this disease may occur in your field, and the resistant cultivar may not be effective against all races. Planting resistant bean varieties is the primary method to control Phytophthora. Continuous soybeans may increase severity of this pathogen. No-till fields can increase the severity of this disease, as the fields can be wetter.
For pythium management, avoid planting soybeans into cold wet soils, as this pathogen infects at low temperatures. No-till practices increase the chances of pythium infection, as soil temperature is more likely to be lower and soil moisture higher.
If you want to use tillage to help manage these diseases by improving the soil drainage, use conservation tillage (leaving 30% or more crop residue on the soil surface) to maintain soil quality.
Plant high-quality seed without broken seed coats and avoid carryover seed. Because phytophthora and pythium pathogens can survive in soil for many years, rotating soybeans with corn isn’t an effective control measure. Be sure to plant seed at proper depth, and use seedbed preparation to help reduce risk of disease.
Fungicide seed treatments
For rhizoctonia and fusarium, there are limited control measures besides chemical (fungicide seed treatment) control. There are no varieties that are being developed for resistance.
One of the most important management strategies is to reduce the stress a plant incurs as a seedling. Be sure not to use a herbicide that will cause root injury, and select varieties that have soybean cyst nematode resistance and tolerance to iron chlorosis if your field has that concern.
Not all fungicide seed treatments are equally effective against all seedling diseases. The active ingredients metalaxyl or mefenoxam (for example, Apron formulations) will provide protection against phytophthora and pythium.
For rhizoctonia and fusarium, products that contain the active ingredients azoxystrobin, trifloxystrobin, fludioxonil, PCNB, thiram and thiabendazaole should be effective against these pathogens. Most commercial products have a combination of these active ingredients to help manage these seedling diseases.
Sudden death syndrome, or SDS, is a disease that also infects the soybean plant when it’s young. Infection can occur within days of planting, and is most severe in wet and low temperature soils. There is no fungicide treatment to control SDS.
Basol is the ISU Extension field agronomist at Nashua in northeast Iowa.
DAMPING OFF: This withered plant is an example of “damping off,” or seedling blight, where the dead soybean seedlings are visible on the ground. Before turning brown, the leaves will be gray-green in color.