Iowa Farmer Today
09-29-07
Soybean disease hits Iowa fields unusually hard
By Tim Hoskins, Iowa Farmer Today
Just a little more than a month ago, some Iowa farmers saw how sudden death
syndrome works in a greenhouse.
“One field that I saw had so much sudden death syndrome (SDS) that the leaves
died before they could fall off, something I have not seen except in the
greenhouse when inoculating plants with too much of the fungus,” says Daren
Mueller, Iowa State University Extension plant pathologist who has
researched SDS.
“I walked into some fields in Southeast Iowa where every single plant had SDS,”
he says.
“Anecdotally there was a drastic increase in both incidence (number of fields)
and severity (amount of disease in a particular field),” he says.
“There are still parts of the state, mostly northwest, that SDS is found but not at
crazy levels. However, all parts of the state had an increase in SDS this season.”
Many agronomists agree. In Southeast Iowa, they say it was the worst SDS
infection they have seen.
Allison Robertson, ISU plant pathologist, says symptoms of the disease
showed up in fields later than in 2006. That year, symptoms showed up in mid
July.
While environmental conditions are a key factor in determining the disease, the
exact conditions are not known.
“There has been a bunch of research on this, and I have yet to see a consistent
answer,” Mueller says.
He says soil moisture at two distinct times is the driving factor for disease
severity.
First, soil moisture early in the season will increase the amount of root rot, which
is the first phase of the disease. Heavy rains, much like the ones that peppered
Iowa this season, during the reproductive growth stages are critical for foliar
symptom development, Mueller says.
The fungus that causes SDS is a root rot pathogen that stays in the roots and
lower stem. It produces a toxin that moves to the leaflets, causing the
characteristic symptoms.
“Having soil moisture during the reproductive stages may increase the
translocation of that toxin,” he says.
Leonor Leandro, another ISU plant pathologist, is researching what
environmental conditions play in the development of SDS.
While farmers can’t control the environment, they can make some choices that
could prevent disease development.
The first way to manage the disease is to use SDS-resistant varieties, Leandro
says.
Mueller says generally more SDS resistance is bred into group 3 and 4 soybean
varieties. Also, there is SDS resistance in later varieties.
Farmers can check with the University of Illinois VIPS Web page:
web.aces.uiuc.edu/VIPS/, which has greenhouse and field ratings for SDS.
Other recommendations traditionally have included delay planting, tillage and
manage soybean cyst nematode (SCN).
Mueller says he disagrees with the delayed planting recommendation.
“There were many late-planted fields this season that got hammered with SDS,”
he says.
In addition, he says ISU Extension soybean specialist Palle Pedersen has
proven delay planting causes yield reduction by missing the ideal planting date.
Managing soybean cyst nematode will not prevent the disease, Mueller says, but
it may slow its spread.
He says tillage can effectively manage the disease, especially in fields that have
a hard pan that restricts the root growth. Hard pans and restrictive root growth is
one reason SDS is commonly seen at field edges first.