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8/4/2008 - 8/10/2008

Midsummer Scouting for Downy Mildew and other Soybean Diseases

By XB Yang, Department of Plant Pathology and John Kennicker, Iowa State University Extension

Summer soybean disease scouting is revealing some interesting finds due to several years of unusual weather. This is the third year in a row that Iowa has had a cool summer. The summer of 2006 was cool, but not wet; this year and last year, cool and wet.

Downy mildew, usually a later season disease, showed up the end of July and with more incidence than normal in many sentinel plots in Iowa, according to John Kennicker ISU Extension sentinel plot agronomist. Early downy mildew is only one of the unique 2008 soybean disease events. 

Downy mildew is a fungal disease prevalent in a cool, wet season. Infected soybean leaves have regular shape, small lesions defined by a few cells. The lesions are pale or light yellow in color on the upper surface of the leaves. On the underside of the infected leaves, the lesions are grey in color with turf like mycelium which can be seen with the bare eye. The lesions are found in the upper plant because the fungal spores are airborne. Defoliation can occur when the disease level is high.

downy mildew on soybean plant

Downy mildew. X. Li - 2008

This disease usually occurs later in the fall in Iowa, and previously, no yield reduction due to downy mildew has been documented in Iowa. But downy mildew does affect seed quality. Affected soybean seeds have white mycelium on the seed coat which can be mistaken as white mold.

High incidence of this disease is now occurring in Iowa, with some fields over 80 percent, according to Kennicker’s reports. Soybean seed growers that find high levels of this disease should consider fungicide use to control the disease. Fungicides that have already been applied to the soybeans should sufficiently manage this disease.

Sudden death syndrome has shown up since the first of August in both research plot and production fields in central Iowa, even though we had a late planting season. As usual, the disease was prevalent in fields planted before the third week of May. SDS may also show up in early September, with less intensive symptoms, in fields planted after the third week of May since the planting season was cool and wet. We expect to see more SDS in Eastern and central Iowa than western Iowa this year.

Preventative measures are a key to reducing the risk of SDS, and scouting helps identify the need for preventative measures. This is a good time to scout for SDS, especially in fields where SDS has not been found previously. Pay attention to early planted soybean fields. Symptoms of this disease are characterized by inter-venial necrosis. A major management measure is to use tolerant varieties and such varieties are available in most seed companies. Other management measures are discussed in an article on the Soybean Extension and Research Web site.

White mold infects soybean plants during flowering time under a closed soybean canopy.  Although the summer is cool and wet, the risk of white mold is lower when beans are planted later and have wide open row space during flowering, which is the case for the majority of Iowa soybean fields this summer. However, if a field which had white mold previously was planted earlier and has a dense canopy at flowering, the disease risk will be high. Beans planted late with narrow row or drilled soybean have a higher possibility of having white mold infection, but the severity may be less depending on the density of canopy.

In a summer like this year, producers should watch fields with either of the following conditions: 1) previously had white mold, and 2) have a densely closed canopy during flowering. The denser the canopy is during the flowering time, the higher the risk is in fields with a history of white mold. During scouting, pay close attention to fields with dense canopy and check for white mold mushrooms.

We often have reports of this disease late in August. For more details on white mold refer to July 14 ICM article, White Mold Control in a Flood Year.

Other diseases. Brown spot, bacterial blight, and Cercrospore leaf blight continue to be prevalent in many areas; frogeye leaf spot is being detected, but at lower incidence, according to extension field agronomist and sentinel plot reports. These diseases will likely be problems this fall if weather continues to be wet as the crop season progresses.



XB Yang is a professor of plant pathology with research and extension responsibilities in crop diseases. John Kennicker is an extension field agronomist with sentinel plot responsibilities.

Soybean Rust and Late Planted Soybeans

By Daren Mueller, Department of Plant Pathology
Iowa producers are asking if soybean rust will affect the late planted soybean crop. Taking a look at current U.S. locations of soybean rust and understanding the movement of the rust, helps answer the question for Iowa producers.

Where is soybean rust currently?
Soybean rust is reported in five states - Florida, Georgia, Alabama, Louisiana and Texas, with the majority of the finds in Florida. Outside of Florida, soybean rust only has been reported in seven counties, the furthest north and west being in a kudzu patch in eastern Texas.  

How fast is soybean rust moving?
Compared to 2007, soybean rust is moving much slower in the southern states, namely Louisiana and Texas. On April 19th, a trace amount of soybean rust was found on the new growth of kudzu at a kudzu sentinel site in Polk County, Texas in the eastern part of that state.

Since that find, the weather turned dry throughout the southern half of Texas, and soybean rust has not moved out of that kudzu patch. In fact, drier than normal conditions continue to plague much of the southeastern U.S. (see Figure), which has slowed the movement of soybean rust, much like 2005 and 2006.

Uunited States July 2008 drought map


So will the late planted soybean crop in Iowa be at risk?
Right now, the chances of soybean rust moving into the Midwest anytime soon appear to be low. Movement of soybean rust in late September and October has been the trend the past few years, and this year appears to be no different.

According to Palle Pedersen, late planted soybeans will reach growth stage R6 (full seed) September 10 - 20. We will continue to monitor movement of soybean rust to be sure these late-season crops will not be affected, but for now there is not much movement to monitor.

soybean rust

Soybean rust - Mueller. 2008


Monitor weekly soybean rust updates
The Iowa State University Soybean Rust Web site has been overhauled. The new version highlights weekly messages written throughout the growing season that provide up-to-date information on soybean rust.

Individuals can sign up to have these reports e-mailed to them. The website also provides the basics and management of soybean rust, as well as fast facts about soybean rust.



Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative.

All Things Considered, It Could Be Worse!

by Rich Pope, Department of Plant Pathology

Corn and soybean condition has remained remarkably good in general as we enter August. Favorable weather in the early summer certainly has not made up for the early season planting problems that plagued parts of Iowa, but it has allowed crops to recover. The last week of July was warm and allowed vegetative staged crops to make up some lost degree day ground. 

degree day accumulations map of Iowa August 3, 2008

Our outlook for August is that if we can maintain occasional rains and stay warm but away from triple digit heat, especially with night temperatures in the 60s, a good crop should be possible.


Rich Pope is an extension program specialist with responsibilities in the Corn and Soybean Initiative.

Know Your Spots as Foliar Diseases Show up in Corn

By Alison Robertson, Department of Plant Pathology
Several foliar diseases are being reported in corn at this time, and it is important to be able to differentiate between the various leaf spots because management options will vary. Some leaf spots are caused by bacteria and therefore cannot be managed with a foliar fungicide.  Development of other leaf spots will slow considerably in the extreme hot temperatures we are currently having. However, there are some leaf spots that favor these hot, humid conditions and, depending on disease pressure and hybrid susceptibility, management with a foliar fungicide may be warranted.


Common rust
There have been numerous reports of common rust across Iowa.  While some reports have indicated higher common rust pressure than previous years, most reports indicate that disease pressure is “as per usual”. 

Common rust development is favored by moderate temperatures thus the current hot temperatures should slow common rust development down significantly.


Gray leaf spot
A few gray leaf spot (GLS) lesions can now be found in the lower part of the crop canopy.  Lesions of GLS are light, tan and rectangular in shape because the lesion’s width is limited by the leaf veins (Photo 1).  The lesions expand lengthways (1/4 inch to 2 inches) and become gray in color.

gray leaf spot

Photo 1. Rectangular shaped- lesions characteristic of early gray leaf spot.
Most reports received this growing season specify that GLS lesions can be found on the fifth or sixth leaf below the ear leaf, although there are some reports of GLS occurring as high as the third leaf below the ear leaf.  Hot (75 to 95 degree Fahrenheit), very humid (90 percent humidity for 12 hours or more) conditions favor GLS development. 

Current disease management recommendations advise if a susceptible hybrid is being grown, GLS can be found on the third leaf below the ear leaf of 50 percent or more of the plants in the field, and the hot, humid conditions continue, a fungicide application may be warranted.


Northern leaf blight and Stewart’s disease
Northern leaf blight has also been reported in eastern Iowa. Some hybrids being grown seem particularly susceptible to the disease. Lesions of northern leaf blight are large (1 to 6 inches in length), cigar-shaped and tan in color (Photo 2).

northern leaf blight

Photo 2.  Northern leaf blight – large, tan, cigar-shaped lesions.

Northern leaf blight lesions are very similar to those of Stewart’s disease which are caused by a bacterium vectored by the corn flea beetle. Lesions of Stewart’s may also be somewhat cigar-shaped, but the margins of the lesion are wavy, and the lesion tails off down a vein (Photo 3).  Often times a flea beetle feeding scar is evident in the lesion.

stewarts disease

Photo 3.  Stewart’s disease – lesions tail off down a vein.


Southern rust
Southern rust has recently been reported in Nebraska, UNL Crop Watch, August 1, 2008. Southern rust caused considerable yield loss in Nebraska in the 2006 growing season. Growers in southwestern Iowa are advised to begin scouting their fields every few days. 

Southern rust can develop very rapidly when conditions are hot, humid and wet, and a timely fungicide application can protect up to 30 percent of yield.   Unfortunately, southern rust can be easily confused with common rust although there are a few subtle characteristics that can be used to distinguish southern from common rust as discussed in an early Integrated Crop Management newsletter article, Is that Common Rust or Southern Rust Showing up in Iowa Corn Fields.



Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases.

This article was published originally on 8/11/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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