Sunday, May 18, 2014

Thoughts on Early Season Soybean Damping Off from a Diagnostic View

Yes, soybeans can be vulnerable to pythium and Phytophthora roots rots (and even Rhizoctonia root rot).  Pythium and Phytophthora spp. belong to the group of fungal-like organisms called the oomycetes, which are also known as water molds.  Moisture has to be present for zoospores (spores) to infect roots. Most pythium species can infect corn and soybeans (as well as other crops), but as research continues, they are finding that some pythium species may prefer to infect corn; whereas other pythium species may be more likely to infect soybeans.  Hystorically, pythium species are more likely infect early because they favor cooler temperatures (50 to 60 F); however, further research has shown that some pythium species may infect at warmer temperatures.  Phytophthora spp.prefer temperatures of 77 to 86 F and have a very, wide host range.  Phytophtora root rot can attack/kill soybeans at all growth stages.

Both pythium and Phytophthora root rot pathogens can cause cause a brown rot to occur on soybean roots or hypocotyls.  In my opinion, it is very, difficult to make the distinction between pythium and Phytophthora root rot within the field, when soybeans are infected at early growth stages.  This is when your state's Plant Clinic may come in handy as they have "many diagnostic tricks up their sleeve", such as examining oospores (spores) within roots via a microscope (pythium), examining lemon shaped sporangia growing on roots or hypocotyls (Phytophthora), or use a form of Enzyme-Linked Immunosorbent Assay (ELISA) , a technique used to detect antibodies or infectious agents in a sample (Phytophthora).

It looks like a root rot or damping off on soybean.  Have these plants been infected by pythium or Phytophthora spp.? - Pictures taken at the U of I Plant Clinic



Lemon shaped sporangia (fruiting structures that contain spores) of Phytophthora - Picture taken at the U of I Plant Clinic
Oospore (spore) of Pythium found embedded within roots - Picture taken at the U of I Plant Clinic
If you have soybean damping off or root rot early in the season in your field, why should you care if it is infected by pythium or Phytophthora?  The answer is management.

Both pythium and Phytophthora spp. are more likely to infect plants that lack vigor or that are planted in a poor location. Further investigation of plant stress may lead to clues why infection may have occurred. Since both of these pathogens require moisture for infection, it makes sense that flooding, lower lying areas of the field as well as clay soils that have a higher water holding capacity may be more at risk for pythium and Phytophthora infection.  Therefore, improving or the addition of tile drainage could offer some aid in some situations.  

Fungicide seed treatments can offer control early in the season, but keep in mind that not all of them specifically protect or are registered to be used against the oomycete fungi like pythium and Phytophthora.  The reason I say "early season control" is because these fungicide seed treatments do not last forever.  These seed treatments are located on the seed and may not offer protection to the soybean's roots or hypocotyls. There is always the chance of resistance to fungicide seed treatments, but has not yet been documented in "our neck of the woods."

There is no resistance available within plants for pythium; however there is resistance available within soybean for use again Phytophora.     
1.) root resistance (several genes) - This is expressed in the root, almost a complete resistance, qualitatively inherited and harder to breed

2.) partial resistance (several genes) - This is expressed in the roots after the cotyledons and true leaves are exposed and is said to reduce colonization of the roots as well as slow the expansion of lesions, qualitatively inherited and harder to breed

3.) R- gene resistance (single gene) - "R-gene mediated resistance has been described for 14 Rps genes and most have been mapped on the soybean genome. Commercially, only six genes, Rps1a, Rps1b, Rps1c, Rps1k, Rps3a and Rps6, have been deployed and one more, Rps8, is in commercial development."  They are expressed in the seed early in germination (effective early and throughout the growing season), and are measured in lab/greenhouse assays by inoculating hypocotyls of young seedlings to see if there is a hypersensitive response.  R-gene resistance is the most common, because it is the easiest to breed.  .Not all Phytophthora races will be controlled by R-gene resistance.

R-gene and partial resistance are most commonly used today in soybeans.

What can you do?
1.) If you have a soybean root rot problem, determine if it is pythium or Phytophthora by contacting your agronomist or utilizing a Plant Clinic, so you can properly manage the problem.

2.) Keep track of what fungicide seed treatments that are on your seed.  Are they the right seed treatments for use against oomycetes (water molds)? Are they they same type of fungicide seed treatments that you are using on corn?   Could you be setting up a situation for resistance to occur?

For additional information on seed treatments, you can go to: 
What is on your seed?
Specific Activity of Soybean Seed Fungicides

3.) If you have a Phytophthora problem, what type of resistance are you utilizing? At Burrus, you can find this information in the Production Selection Guide .  If you have questions, you can contact an agronomist.  Do you have a race of Phytophthora that is not controlled by single gene resistance?

For more information on Phytophtora root rot in soybeans, you can refer to the following:   http://www.apsnet.org/edcenter/intropp/lessons/fungi/Oomycetes/Pages/PhytophthoraSojae.aspx



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