During the 2011 growing season, Extension soybean pathologists across the 12 North Central states, including Iowa, will be conducting a survey to identify the oomycete pathogens that cause damping off of soybeans. This will be the first region-wide work to identify which species are the biggest threat to soybean stand establishment for soybean growers.
As growers plant soybeans earlier into cooler and wetter soils, the risk of damping off by Pythium species increases, says Alison Robertson, an Iowa State University Extension plant pathologist. Later-planted beans are more susceptible to damping off caused by Phytophthora sojae when wet conditions occur soon after planting.
Information from the survey will be used to develop diagnostic tools for Pythium species and P. sojae that can be used in the field to improve the ability of farmers to select successful combinations of seed treatments and soybean varieties.
Locating soybean fields in Iowa where "damping off" is occurring
"I am looking for soybean fields that have stand establishment problems as a result of damping off," says Robertson. "I need to collect 50 diseased seedlings from the field and bring them back to the lab for processing. If you know of a field with damping off problems, please email me at firstname.lastname@example.org or a call me at 515-294-6708, so that we can co-ordinate sampling of the field.
* Symptoms of damping off. Seed rots, root-rots and hypocotyl-rots are primary symptoms of damping off, says Robertson. The soybean plants are easily pulled from the ground because of rotted roots. The stem of infected seedlings may be water soaked and the leaves grey-green (Figure 1). Scattered dead and seedlings are often visible on the ground (Figure 2).
* Causal organisms of damping off. Damping-off can be caused by oomycete pathogens such as Pythium and P. sojae, or fungi, such as Fusarium species and Rhizoctonia species.
Oomycetes, although similar in appearance to fungi, are actually more closely related to brown algae, she explains. Some of the most aggressive plant pathogens are oomycetes. They differ from fungi in their cell wall composition, their hyphae (threadlike filaments), have no cross-walls and they have diploid nuclei. They produce zoospores that have two flagella (tails) that enable them to swim in free water. Consequently, damping off caused by oomycete pathogens is a problem in areas of fields that have been flooded for a period of time.
Fungal pathogens produce spores that are dispersed by rain or wind. Damping off caused by Fusarium species is favored by cool, wet soils, while damping off caused by Rhizoctonia species usually occurs when soils are warm and damp.
More information on these diseases can be found in the e-Book Soybean Diseases, says Robertson.