Early Season Foliar Diseases on Soybean & Corn

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• Fungi and bacteria can infect the foliage of young soybeans and corn
• In most cases, early season damage by foliar diseases is cosmetic and does not require intervention
• Proper disease identification is essential to ensure appropriate remedial actions are followed, if required
   

Foliage produced by young corn and soybean seedlings is susceptible to several fungal and bacterial diseases. Although these issues do not often require intervention and should not be a cause for concern, they occasionally may signal issues at later points in the season or be confused with other abiotic disorders. This article will briefly touch on some of the diseases that you may encounter in your fields and their characteristics to help you in your early season scouting efforts.

 

Corn

Anthracnose
Anthracnose is caused by the fungus Colletotrichum graminicola. This fungus survives in and on residue, where it produces fruiting bodies that contain spores. Spores are rain-splashed onto the leaves of developing plants early in the season. Lesions are generally less than ½ inch long, brown, and irregularly shaped. Lesions often have a yellow to red/pink margin, and frequently have a target like appearance to them (Figure 1). Black hairs may be observed within lesions with the aid of a hand lens. Severely infected leaves may turn yellow, often at the margins first, giving the appearance of fertilizer or herbicide burn. Corn is susceptible to the foliar phase of the disease when conditions are warm and wet through V5/6. After this point in time there is a physiochemical shift that occurs in the plants, and new foliage is more resistant to infection. This fungus can also cause a top dieback and stalk rot in corn. Although early season anthracnose does not indicate that a top dieback or stalk rot issue will develop, fields with heavy amounts of foliar anthracnose early in the season may be more likely to develop issues later on and should be marked to check for stalk rot and top dieback around the R4/5 growth stage. Direct yield losses due to early season infections are rare, but if it is determined that chemical intervention is required due to predicted weather forecasts, products containing a QoI (FRAC group 11), DMI (FRAC group 3), or premixes of group 11, 3, and/or 7 fungicides will provide approximately three weeks of protection to foliage. 

Holcus leaf spot
This disease is caused by the bacterium Pseudomonas syringae pv. syringae, which is rain-splashed from residue onto young foliage. Lesions are small, circular, and white to brown in color with a brown to reddish margin and a light green / yellow halo (Figure 2). Holcus spot closely resembles gramoxone or paraquat drift injury. Spots caused by these herbicides should also be found on nearby weeds. This is not a yield-limiting disease, and because it is caused by a bacterium, fungicides do not impact it. 

Holcus leaf spot in corn. Note the pale green/yellow halo. Image Tom Allen, MSU

 

A Note on Early Season Tar Spot

There have been several questions this year on tar spot and forecasting applications calling for severe risk (and hence a need to spray) in our vegetative corn that has been rolling due to a lack of moisture. We simply do not understand the biology of the tar spot pathogen at this time, so predictions made by weather-based applications need to be viewed cautiously. These applications take weather data and previously collected data on symptom development to predict when tar spot may become an issue. The problem is that we do not know what triggers the pathogen spores to be released (and therefore leads to infection) nor do we understand how differences in hybrid, inoculum pressure, or pathogen diversity may impact disease onset and development. In order to see foliar diseases develop, prolonged leaf wetness and humidity is required.  This is one reason that in a typical year we do not observe much, if any, significant disease in vegetative corn.  Typically foliar diseases reliably build (and therefore a fungicide is more likely to result in a higher probibility of a ROI) after canopies close and wet weather occurs. 

View forecasting applications for corn diseases as mere guides to help you know when to start looking for this disease. The best way to know if tar spot is starting and if a fungicide is needed is to stay on top of your field scouting and your local conditions. As a general guideline, if you start to see stroma on >50% of plants and at least 3% severity before VT, a VT application of a premix fungicide should be applied. Remember that this is not a rust and tar spot does not blow into the region from warmer areas. Any suspect fields or images should be confirmed by a local plant disease expert.

 

Soybeans

Brown spot
Septoria brown spot is caused by the fungus Septoria glycines. This is a very common, residue-borne foliar disease that can be found in most soybean fields under periods of cool, wet weather. Symptoms typically start as purple spots or lesions on the unifoliate leaves. Lesions are irregularly shaped and dark brown in color. Blotches can coalesce and leaves may turn yellow and senesce after infection (Figure 3). Spores of the fungus are produced within lesions and can be rain splashed up the canopy by rain. The fungus requires cooler temperatures and high humidity to continue to grow and develop, and therefore it is often only observed in the lower canopies later in the season. Control of early season brown spot is not recommended, and the disease can be controlled with foliar fungicides applied between the R1-R3 growth stages. 

Figure 3. Early season brown spot of soybeans can be frequently encountered in soybean fields and should not be a cause of concern. Image S. Geisler 

Frogeye leaf spot
Frogeye leaf spot is another predominantly residue-borne disease requiring high relative humidity, caused by the fungus Cercospora sojina. Although it is not frequently observed in young, developing plants, it can occur on occasion, especially in no till, continuous soybean production, and prolonged warm, wet weather. Lesions start as small specks that expand to form round to irregular lesions with a tan to cream colored center. Leaf margins often have a red to purple margin. On the underside of the leaf surface, long silvery spores can be observed with the aid of a hand lens (Figure 3). Young, developing leaves are more susceptible to infection than fully developed leaves. There is excellent resistance to frogeye leaf spot in many soybean cultivars, and resistant varieties should be used in areas with history of frogeye leaf spot issues. Fungicides applied at R3 are the most efficacious for frogeye leaf spot suppression. Fungicides containing a solo strobilurin active ingredient (e.g. azoxystrobin, picoxystrobin, pyraclostrobin) should be avoided due to widespread resistance to strobilurin fungicides reported throughout the United States. Instead, use a premix fungicide containing multiple modes of action. 

Bacterial blight
Bacterial blight is equally as common as brown spot in soybean. This disease is caused by the bacterium Pseudomonas syringae pv. glycinea. It is residue borne and rain splashed onto young, suceptible foliage. Symptoms include small yellow/black spots on leaves with yellow halos. Spots eventually dry out and fall from the leaf, giving it a tattered appearance. Spots are pin sized and much smaller than those of brown spot and frogeye leaf spot (Figure 4). Disease is favored by cool wet conditions and halts under hot conditions. Bacterial blight often is most frequently observed in continuous soybeans. As mentioned with Holcus spot, fungicides do not impact this disease and management is not required. 



 

Figure 4. Top- early season bacterial blight on unifoliate leaves. Bottom, tattered appearance of leaves affected by bacterial blight later in the growing season. Image D. Mueller