New Test for Soybean Sudden Death Syndrome

A simple, cheap laboratory test developed at Southern Illinois University can detect Sudden Death Syndrome (SDS), a costly fungal disease caused by Fusarium solani f.sp. glycine.  Once commercialized, its use will help breeders produce SDS-resistant soybean varieties much faster than they can now.  “You can do a reliable assay in the greenhouse in a plastic cup and four weeks later, you’ll see the result,” said David Lightfoot, a biotechnologist in SIU’s College of Agricultural Sciences who developed the procedure.  “The seedlings develop the leaf symptoms and the root rot or they don’t.  It works every time, and the labor cost is very low - about $1 per assay.”

Not so long ago, breeding SDS-resistant soybean varieties was pretty much hit or miss.  Breeders would plant a bunch of beans in multiple fields and hope the disease would show up somewhere by the end of the growing season.  If it did, they could make breeding crosses with the survivors and repeat the process during the next season.  But sometimes the disease didn’t appear, and sometimes when it did, it didn’t behave normally, disappearing in mid-season.  In addition, SDS resistance is not the only part of the equation.  Breeders also must incorporate other traits, such as yield and adaptability, when developing new lines.  Lightfoot and colleagues figured starting out with disease-resistant breeding lines would cut the time it took breeders to produce marketable new varieties.  In 1994, they began working on an assay that could quickly pinpoint seedlings that could withstand SDS.

Lightfoot said “People had done two things wrong in the past in attempts to develop an assay.  They had grown the fungus on rich media, so it wasn’t hungry which could lead to false negatives.  The other mistake was to just jab the plant with the fungus or put it in large quantities in the ground, making the disease inevitable, leading to false positives.  The amount of fungus and its aggressiveness are critical parameters in SDS - much more so than in other diseases.”  Lightfoot grows his fungus on a mix of cornmeal, sand, mineral salts and agar.  That’s enough to keep it alive but not “fat, lazy or happy,” Lightfoot said.  Graduate students, armed with spades and pails of the mix, then dig that mix into piles of soil, turning it until the fungus-laden growing medium is distributed evenly throughout.  Vulnerable seedlings planted in that infected soil will contract the disease while seedlings with resistance potential won’t.  (SIU, 1/7/08).