Fall soybean disease scouting

In Iowa, root and stem diseases are important yield robbers. Infestations by these diseases are more evident in September. The Iowa State University Plant Disease Clinic receives more soybean disease samples in late August and September than at any other time. Later summer and early fall is a good time for scouting for diseases. This information will be useful for disease management of next year's soybean crop. This article describes the diseases that we have seen in the clinic, observed in the field, or received reports of from Iowa State University Extension staff.

Brown stem rot (BSR)

Cool temperatures and good soil moisture early this summer were ideal for the brown stem pathogen to attack soybeans. It is likely for us to see more BSR this year than in the last few years. The fungus infects soybean in early plant growth stages and symptoms appear late in the season. One typical symptom is internal stem browning. Some strains of BSR fungus also can cause defoliation in late summer. These strains produce toxins that lead to interveinal necrosis in which tissue between the veins turns brown and dries rapidly, while tissue adjacent to the veins remains green a few days longer.

Don't be surprised if you find BSR because this fungus is present in at least 95 percent of Iowa soybean fields. The important issue is disease severity. Severe disease causes premature yellowing, sometimes followed by lodging and defoliation. Incidence of BSR normally is higher in reduced tillage fields than in conventional tillage fields because BSR fungus survives in soybean residues. Therefore, no-till fields should receive more scouting attention.



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Soybean stem with brown stem rot.

White mold

July weather conditions were good for white mold mushroom production. After disappearing for years, white mold has shown up again in some areas. Iowa State University Extension field crop specialists Virgil Schmidt in southeast Iowa and John Holmes in central Iowa reported finding this disease.

Because most Iowa soybean fields were not planted late this spring, soybean did not have a dense canopy during flowering, which is critical to disease infection. Therefore, this disease is unlikely to be prevalent this fall in Iowa, as indicated by Holmes' observations. My communications with other Iowa State University Extension field crop specialists indicated that this disease has not been observed in most Iowa regions. In regions that received a lot of rain in July, check fields that were planted early and have dense canopy. Fields that had disease before are more likely to have white mold than other fields.

In most cases, white mold damage should be minimal because white mold fungus could not build up its population in the past few dry summers. If you do find fields with high incidence, do not save beans from these fields if you plan to use your own seed. Combines cannot separate white mold sclerotia (a fungal structure that looks like mice droppings) from beans. Using contaminated seeds will spread the pathogen from one field to another. If you find white mold in any fields, take notes and the information you gather this season will be useful to manage the disease in your next soybean crops.



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White mold on soybean stem.

Sudden death syndrome (SDS)

Holmes also reported the occurrence of sudden death syndrome in his region. Although late planting this season reduced the risk of sudden death syndrome greatly, there would be some SDS this year as excessive rains in spring and early summer were good for SDS. Please note that some BSR strains can cause SDS-like interveinal necrosis on leaves. Foliar symptoms of BSR can be misidentified as sudden death syndrome. When you have doubts, check roots of diseased plants. Plants with SDS have root rot while BSR does not cause root rot.

Finally, Phytophthora stem rot has been a major disease problem this season as samples continue to arrive at the Iowa State University Plant Disease Clinic. Damage as large as 40 acres in area has been reported. Unlike in most of years, we saw more stem rot problems than seedling damping-off this year.

This article originally appeared on pages 153-154 of the IC-490(21) -- September 15, 2003 issue.

Updated 09/14/2003 - 1:00pm