Crop & Pest Report


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What Disease is on my Soybean Leaves? (08/17/17)

In the last few weeks, we have had many questions about leaf diseases impacting soybeans.

What Disease is on my Soybean Leaves?

In the last few weeks, we have had many questions about leaf diseases impacting soybeans. To help everyone identify (or rule out) diseases, we are highlighting several diseases/ailments we have had questions about, whether or not they occur in North Dakota. These are arranged by how common we usually see these diseases in North Dakota.

Bacterial Blight.

Importance: Common in North Dakota but rarely economically important

Symptoms: Begins as small angular lesions often with a yellow halo (Figure 1). Lesions enlarge slightly, turn black and centers will fall out (Figure 2). Occurs most frequently on upper leaves.

Favorable conditions: Short/no crop rotation, storms or winds that wound leaves, rainfall, cool-moderate temperatures.

Management: Typically, not associated with yield loss. Fungicides are not effective.

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Downy Mildew.

Importance: Increasingly common in North Dakota. Traditionally not thought to be economically important nor widespread. However, as the disease has been occurring in greater frequency and severity in North Dakota it is possible that yield loss could occur under high severity.

Symptoms: Lesions begin on upper leaf surfaces as small discrete light-yellow to pale-green spots (Figure 3) that may turn brown as they age. Fluffy gray tufts of mold appear on the underside of the leaves immediately opposite the light-yellow to pale-green lesions (Figure 4).

Favorable conditions: Short/no crop rotation, high humidity, moderate temperatures.

Management: It is unclear if management options are warranted. Limited fungicide efficacy data on downy mildew exists.

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Bacterial Pustule.

Importance: Very rare and not economically important in North Dakota.

Symptoms: Small pale-green to brown specks (Figure 5) often with yellow halos. Pustules may form on undersides of leaves within brown lesions (Figure 6). Of note, this disease is commonly confused with soybean rust, which has never occurred in North Dakota.

Favorable Conditions: Hot and wet weather.

Management: Not necessary in North Dakota.

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Unknown leaf stress.

Importance: Commonly we see leaf purpling and bronzing that we have trouble identifying the cause(s). We think sunscald, ozone, or other abiotic stresses may be contributing to these symptoms. It appears that there are differences between varieties in their response to stress conditions.

Symptoms: purpling and or bronzing of leaves (Figure 7-8)

Favorable conditions: Unknown, seems to occur more frequently under high temperatures and moisture stress.

Management: Unknown.

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Cercospora leaf blight.

Importance: Not thought to be common in North Dakota. However, symptoms closely resemble abiotic stress commonly seen in the region.

Symptoms: Brick red lesions on the petiole. Purpling of leaves leading to defoliation (Figure 9). Pink-purple discolored seed.

Favorable Conditions: Hot and wet conditions. Much more common disease in southern states.

Management: Not thought to be necessary in North Dakota.


Soybean Rust.

Importance: Does not occur in North Dakota. Very significant economic problem occasionally in Southern States.

Symptoms: Small discrete lesions that form pustules with every lesion (unlike bacterial pustule where only some lesions produce pustules) (Figure 10). Pustules can be observed through a hand lens by rolling the leaf around a finger (Figure 11). Must be confirmed in a laboratory.

Favorable conditions: Soybean rust must spread from the South, and as of August 15th, the furthest north soybean rust has occurred this year is in Mississippi and Alabama (Figure 12).

 Management: Not necessary in North Dakota.


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Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

Hans Kandel

Extension Agronomist Broadleaf Crops

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