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7/14/2008 - 7/20/2008

Unusual Foliar Diseases Showing up in Iowa Corn

By Alison Robertson, Department of Plant Pathology
Over the past week I have received emails, phone calls and samples of two leaf spots that occur in Iowa from time to time, Holcus leaf spot and Physoderma brown spot.

HOLCUS LEAF SPOT
Holcus leaf spot is caused by the bacterium Pseudomonas syringae. Symptoms are light tan (sometimes almost white), round to oval spots, which may appear water soaked at the margins or have a light brown border occur on the lower leaves (Figure 1 and 2).

The spots are initially about one-fourth inch in diameter, but often grow larger and coalesce into irregular spots and streaks of dead tissue. Later the lesions dry out, turn light brown, and have a papery texture.

holcus leaf spot 2   Figure 1. Light tan, round to oval spots characteristic of Holcus leaf spot. Alison Robertson.

close-up of Holecus leaf spot  Figure 2.  Close-up of Holcus leaf spot showing the light brown border. Alison Robertson.

Similar diseases. Holcus spot is often confused with eyespot, a fungal disease caused by Aureobasidium zeae.  Eyespot lesions are much smaller (approximately one-eighth to one-fourth inch in diameter), very round spots with a yellow halo, distinct brown border and light colored center that appears translucent when the leaf is held up to the light (Figure 3). Holcus spot symptoms also can resemble chemical injury to leaves.

eyespot leaf disease on corn

Figure 3.  Characteristic symptoms of eyespot. Alison Robertson.

Favorable conditions. Warm (75-85 degrees Fahrenheit), wet, windy conditions early in the season favor infection and the development of Holcus leaf spot. Symptoms often appear suddenly after a heavy rain, but then do not spread to new leaves. Corn is most susceptible prior to tasseling. 

Management . Holcus leaf spot is mostly cosmetic and does not result in yield loss. Fungicides are not effective against this bacterial disease.

 

PHYSODERMA BROWN SPOT
Physoderma brown spot is caused by Physoderma maydis, an organism which is closely related to the oomycetes (such as Pythium and the crazy top pathogen). 

Numerous very small (approximately one-fourth inch in diameter) round to oval spots that are yellowish to brown in color usually occur in broad bands across the leaf (Figure 4). Dark purplish to black oval spots also occur on the midrib of the leaf. Symptoms may also occur on the stalk, leaf sheath and husks.

Physoderma brown spot on corn

Figure 4.  Bands of numerous small yellow to orange-brown spots across the leaf laminar and dark purplish spots on the midrib are characteristic symptoms of Physoderma brown spot. Alison Robertson.

Similar Diseases.  Physoderma brown spot is often misdiagnosed as eyespot (Figure 3) or southern rust (Figure 5). The spots of Physoderma do not have the light colored center that is associated with eyespot.  As for southern rust, it is probably too early in the growing season to be seeing this disease.  Southern rust usually occurs mid- to late- August in Iowa when temperatures are much warmer. Furthermore, the pustules of southern rust produce thousands of orange spores than can be wiped off the upper leaf surface with your finger. 

pustules of southern rust on corn

Figure 5.  Pustules of southern rust occur on the upper leaf surface and produce numerous orange-red spores that can be easily wiped off. Alison Robertson.

Favorable conditions. The zoospores of P. maydis infect leaf tissue when free water collects in the whorl and temperatures are between 70-85 degrees Fahrenheit, thus resulting in the bands of infected and non-infected leaf tissue.

Management.  The pathogen survives in infested crop residue and soil for up to 3 years thus the disease is more common in corn following corn fields particularly if a lot of crop residue remains on the soil surface. Corn plants are most susceptible 50-60 days after germination and become more resistant to infection with age. Only Headline® lists Physoderma leaf spot on the label; however infections in Iowa are usually not severe enough to warrant a fungicide application.

 

 

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

Soybean Cyst Nematode Females now Apparent on Soybean Roots

by Greg Tylka, Department of Plant Pathology

Many Iowa soybean fields may be infested with the soybean cyst nematode (SCN) but the infestations may not be known because SCN does not always cause obvious, above-ground symptoms. In fact, yield loss of up to 40 percent has been documented without above-ground symptoms occurring.

An easy, free and reliable way to check fields for the presence of SCN is to dig roots of susceptible soybean varieties. Carefully crumble away much of the soil from the roots, and look for the adult SCN females on the roots. The females appear as small, round, white objects on the roots and are about the size of a period at the end of a sentence.

We recently observed adult SCN females on soybean roots in research plots in central Iowa. The soybeans were planted in late May. SCN females will be apparent on young roots of susceptible soybean plants in Iowa through July and August, and probably early September this year.  Roots should be checked for SCN females no earlier than four or five weeks after planting.

Additional information about the biology, sampling, and management of the soybean cyst nematode Web site.

female soybean cyst nematode SCN

Adult SCN females (yellow arrows) on soybean root.  The two larger round objects (blue arrows) are nitrogen-fixing nodules.

 

 

Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.

Correction to the European Corn Borer Economic Threshold Spreadsheet

By Jon Tollefson, Department of Entomology

A user of the revised Economic Threshold that we recently published in the Integrated Crop Management News noticed that there was an incorrect function.  I didn’t see it because it didn’t make a difference with the numbers that I used worked through it.  We have replaced the previous threshold spreadsheet with a corrected one.

I apologize for the inconvenience and ask that you make sure that you are using the correct Excel spreadsheet. The correct spreadsheet is an additional resource to this article and is now available from the original article published that was published July 9, 2008.

 

Jon Tollefson is a professor of entomology with extension and research responsibilities.

Not a Bad Week, All in All

By Rich Pope, Department of Plant Pathology

The week ending July 13 was close to average in terms of temperature, and crops have generally made slow but steady improvement in condition over the period.

Iowa map of accumulated degree days

Normal daily accumulations of degree days vary throughout the year, with July obviously being warmer than May. An average Iowa mid-July day produces about 24 base-50 degree days in the northern third, around 25 in central counties and 26 to 27 in the southern third of the state. That means that currently, we are about four to five full developmental days behind average for the season from May 1 through July 13. Of course, a late-planted field missed the heat gained while it was in the bag, and is that much more behind in development.

Once crops shift to reproductive (the “R” stages for corn and soybean), the temperature game changes slightly. Corn development from silking through maturity is promoted by warm days (upper eighties and low nineties) with adequate moisture. Relatively cool nights during grain fill are also good for the crop, with cool meaning temperatures in the lower 60s. Cooler nights keep the corn living a few days longer to gain more dry matter. 

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

White Mold Control in a Flood Year

By XB Yang, Plant Pathology Department

Since the first outbreak of white mold in 2004, white mold has been an even year disease in Iowa.  In eastern Iowa, some producers regularly use fungicides or herbicide to control this disease, especially in even years. In a wet seson like this, many people will think that this is a year for white mold occurrence. However, this is not true. For many soybean fields in Iowa, this year is a low risk year. One should assess the risk before spraying for white mold control. This article will discuss how to assess the risk in your soybean field.

Assessing the risk of soybean white mold
The key is knowing that white mold outbreak needs dense canopy. As Dr. Grau at the University of Wisconsin puts it, white mold management is canopy management. White mold outbreaks need inoculum, favorable environments, and just the right flowering time. Missing any one of these three, the disease will not occur. In fields that historically have white mold problems, inoculum is not a limiting factor, which should be true for many soybean fields in eastern and central Iowa. 

As for the environment, white mold development needs a lot of moisture and this year is good.  However, wet weather does not necessarily mean the conditions are good for white mold if the soybean canopy is wide open during soybean flowering time. White mold fungus produces spores via its mushrooms. The mushroom growth is light sensitive. No or very few mushrooms can grow when soybean canopy is wide open. 

The last critical factor is flowering. White mold spores attach soybean plants via dead flowers.  When soybeans pass over the flowering period, no infections can occur even after canopy is closed. In Iowa, most infection occurs in the second or third weeks of July, and this year may be late due to cooler climate.   

This year, high risk fields are the fields that meet the following two conditions 1) had white mold problems in previous season, 2) have closed canopy during the soybean flowering period. Fields planted earlier or in narrow rows may meet the conditions.
 
In soybean fields with a wide open canopy during soybean flowering time, the disease should not be a concern. If the canopy is more or less closed during the flowering period, there may be some level of infection. The denser the canopy is during the flowering time, the higher the risk.

When to spray fungicide/herbicide
In eastern Iowa, some producers regularly use fungicides or herbicide to control this disease. In a year like this, unless fields are in the category of high risk, there is no need to spray for white mold control, as risk in this year is low because of poor crop growth.  We can recall that whenever a season has had a wide spread of white mold in Iowa, we have had a bumper crop.

white mold mushrooms 2

Soybean white mold mushrooms.

 

XB Yang is a professor Plant Pathology with research and extension responsibilities on soybean diseases.

Fungicide Decisions Nearing for Corn and Soybean

By Daren Mueller, Department of Plant Pathology

 

With both corn and soybean fields finally reaching the reproductive growth stages, decisions to apply foliar fungicides need to be made. On July 3, Alison Robertson wrote an article about foliar diseases on corn (see Robertson article) and XB Yang has weighed in on soybean (see Yang article), but I want to throw in my two cents-worth on this issue.

If you want to get some different angles on making this decision, there have been several excellent articles written from other states (click on the title to see the article).

• Foliar Fungicides for Corn and Soybean – Don’t Rush to Spray - University of Wisconsin

• Fungicide use in corn and soybean: to apply or not to apply? That is the question - University of Kentucky

• Some Fungicide Application Basics – how not to blow a great tool - Ohio State University

• Making Profitable Fungicide Applications in Corn - University of Illinois

• Should Hail-Damaged Corn be Treated With Foliar Fungicides? - University of Nebraska

Here are my two cents. If your mind is already made up to apply fungicides to corn or soybean, then try to follow two simple IPM approaches to select which fields get fungicides.  My first cent: Data from both university and industry sources continue to show that the greatest yield responses have come from fields with the most disease pressure.  So, target fields that are planted to a susceptible hybrid or cultivar and have disease present at the time of application.

For this year, foliar diseases of soybean have been more prevalent than years past. Brown spot and bacterial blight commonly have been reported and frogeye leaf spot and Cercospora leaf blight have been identified from scattered fields.  While fungicides will not affect bacterial blight, fungicides can manage brown spot, Cercospora leaf blight and frogeye leaf spot.

And now for cent number two. Selection of the fungicide product should be based the fungal diseases that are present.  While triazoles are a bit more effective against soybean rust (not currently a threat), strobilurins are more effective against frogeye leaf spot and Cercospora leaf blight (see article and Table 11.2).

Another important consideration, especially for this year, is yield potential.  Several of these above articles touch on this, but it doesn’t hurt to say it again.  Fungicides will not recover lost yield.  If you want to improve your chances of recouping your cost of fungicides, then target fields that have higher yield potential.

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



This article was published originally on 7/21/2008 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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