Wallace's Farmer, IA
04-16-07
Manage Soybean Diseases with Planting Date
Rod Swoboda rswoboda@farmprogress.com
Unlike the weather in recent years, the spring of 2007 has been wet. There is a
lot of moisture in the soil. Soybean diseases are affected by planting dates and
planting conditions. "Early planting as well as delayed planting may increase,
reduce or not affect a soybean disease, depending on when the disease infects
the soybeans and what the soil conditions are after planting," says X.B. Yang, an
Iowa State University plant pathologist.
* Reduce risk by later planting. For diseases in which infection occurs at the
seedling stage, planting dates directly affect disease risk. Sudden death
syndrome caused by Fusarium solani and seedling blight caused by Pythium
require cold soil temperatures when soybeans are in the seedling stage.
"Therefore, soybeans will have a higher risk of being infected by these two
diseases if planted early in cold, wet soils," says Yang. "If these diseases were
severe in the past, you can delay your planting until the soil warms up and that
will reduce the disease risk significantly."
In Iowa, Yang says severe SDS is more likely to be found in early-planted
soybean fields than in late-planted fields. Be aware that the fungi will not cause
much damage if soil moisture is not excessive. When spring conditions are not
unusually wet, diseases may not be a concern.
* Reduce risk by early planting. On the other hand, there are other diseases that
can be reduced by early planting of soybeans. "Contrary to Pythium damping off
and SDS, seedling blight caused by Rhizoctonia and Phytophthora may be
reduced by early planting," says Yang.
"That's because optimum conditions for infections by the two fungi are created by
warm soil temperatures," he explains. "If planted early, soybeans may grow out
of the susceptible seedling stage and escape damping off."
* No effect on planting date. Some diseases, such as bacterial blight, brown spot
and stem canker, are not affected by planting date, says Yang, because
infections of these diseases don't have critical stages.
However, there are indirect effects
Planting date also indirectly affects occurrence of white mold, pod and stem
blight and brown stem rot. Infections of the first two diseases do not occur in
seedling stages, he says. But there is a window of time during which soybeans
are susceptible to these pathogens.
Soybeans planted at different times in the spring will reach a disease susceptible
stage at different times in the summer—some early, some later. A planting that
has a window or susceptible stages that overlap with disease favorable weather
conditions will have higher risk of developing disease than a planting whose
window misses the disease-favorable weather.
For example, in 1996, more white mold was observed in late-planted soybeans
because the cool, wet weather came late in the growing season. Similarly, the
susceptible growth stage for pod and stem blight is in the pod-setting stage, and
favorable weather during this growth stage affects the level of this disease.
Should you use a seed treatment?
The spring of 2007 appears that it isn't going to be a dry one. If weather forecasts
favor wet weather, use of seed treatments with fungicide in fields that have had
problems with seedling diseases in the past could be beneficial. "If you
experienced damping off by Phytophthora, Pythium or Rhizoctonia in the past for
a particular field that is to be planted this year, you should consider the use of
fungicide-treated seed," says Yang.
Research has shown that early planting increases the possibility of achieving
maximum yield and that the level of success decreases as planting is delayed.
"Because of narrow planting windows to achieve high yield, especially in northern
Iowa, you should not hold back the planting date just for fear of diseases," he
says. "Knowing your fields is the key."
For instance, if you farm a large acreage and have fields that have disease
problems, arranging a planting order for them will work to reduce the disease
risk. "If you have six fields, for example, and one of them had SDS disease in the
past, you can reduce its risk by designing a planting schedule whereby you
would plant the problematic field last and choose a soybean variety that has
tolerance to SDS," says Yang.