Soybeans Vulnerable To Dry Weather Diseases This Season

Nebraska plant pathologists say to watch for charcoal rot and soybean cyst nematode.

Published on: Mar 8, 2013

While drought slowed down or prevented many soybean diseases in 2012, it favored others. Loren Giesler, UNL plant pathologist, says that among the problems to watch for in 2013 are Fusarium wilt and root rot and charcoal rot. Additionally, soybean cyst nematode, a root parasite, is more apt in dry years to cut into yields.

Charcoal rot plays no favorites. It can affect corn and sorghum in addition to soybeans. It is caused by a fungus that survives persistently in soils, Giesler says. Infection generally occurs at soybean emergence and early seedling stages. But the infection lays dormant until plants are stressed by drought and heat in the reproductive stages, from flowering through mature pod formation.

WESTERWARD HO: University of Nebraska-Lincoln map shows how soybean cyst nematode has spread in Nebraska.
WESTERWARD HO: University of Nebraska-Lincoln map shows how soybean cyst nematode has spread in Nebraska.

At these stages, infected soybeans can produce flat pods, be stunted and have reduced vigor. As the disease progresses, leaves will wilt and turn brown as the plant dies. "When plants are split open, a light gray to silver discoloration will be visible in the tap root and lower stem. Black specks will be visible in the tissue of the stem and tap root."

Giesler's suggestions: "Plant varieties that are not highly susceptible to charcoal rot. You may need to plant short-season varieties that mature before drought stress. In fields with a history of charcoal rot, rotate out of soybeans for two years to reduce the pathogen."

Due to dry conditions, you may see more Fusarium diseases this year, in both seedling soybeans and later as wilt around the seed-fill stage. Several species of the soilborne fungus can affect soybean roots and cause root rot and the late-season Fusarium wilt.

Giesler says the fungi can affect plants at any stage but infection is more likely to occur when plants are weakened by drought and other stresses.

~~~PAGE_BREAK_HERE~~~One species of the Fusarium fungus can lead to sudden death syndrome, but Giesler says he doesn't expect SDS to be a problem in 2013 since the disease requires more moisture to develop.

There is limited Fusarium resistance available in soybean varieties, he adds. "Check with you seed dealer about their varieties," he says

Seed treatments have some activity against the disease, he says.

SCN continues spreading across Nebraska, with confirmation in 54 counties of eastern and central Nebraska, regions that constitute 90% of the state's soybean production. SCN was originally found along Missouri River counties but has been identified as far west as Red Willow County the past few years.

"Based on the frequency and level of infestations in some of these counties, it would appear SCN has been present in those areas for a number of years and only recently identified," Giesler says.

SCN is difficult to identify based on outward symptoms. The first symptom, according to Geisler, is a healthy-looking soybean field that does not meet yield expectations. Yields losses can be 30% or more.

"In 2012, there were fields that yielded extremely low vs. soybeans across the fence in adjacent fields. Anytime there is moisture stress, SCN becomes more damaging, he says.

Giesler says there are a large number of varieties on the market with some resistance to SCN.

He stresses the importance of taking soil samples to identify if SCN is presents. He recommends obtaining sample bags from your local UNL Extension educator or call the UNL Department of Plant Pathology, at 402-472-2559, which can send you a free sample bag. The Nebraska Soybean Board has provided funds to support SCN sampling.