Foliar Soybean Disease Update: July 20, 2014
Aerial web blight
Active aerial web blight has been observed almost statewide this season. Keep in mind that at different times of the day aerial web blight will be observed producing several different symptoms. First thing in the morning, when dew is present on leaf material leaves can appear to contain water-soaked lesions and in some cases leaves will be matted together. White fungal growth can also be produced in the early morning hours and be observed growing between leaves in the lower and middle canopy. All soybean varieties should be considered to be susceptible to aerial web blight. In the afternoon hours, once dew is no longer present on leaves, the lesions present on leaf tissue will appear chestnut brown to maroon. In addition, in some cases, large holes will arise on infected plant material and a brown to maroon ring will appear around the margins of the holes.
Aerial web blight can be extremely damaging to soybean plants in reproductive growth stages. The fungus will knock off leaves, blooms, as well as young pods. Fungicides are effective at preventing yield loss as a result of aerial web blight. Choose a fungicide product with a strobilurin fungicide in the event of a fungicide application as the strobilurin chemistry will reduce the spread of the fungus to plant material that has not been previously infected. Applications are best made by ground using equipment that can apply a minimum of 15 gallons of water/acre using at least 60+ psi so the fungicide is blasted through the soybean canopy. Aerial applications won’t generally provide good mid to lower canopy coverage with the fungicide. The fungus responsible, Rhizoctonia solani, starts at the ground level and moves up the plant. However, when slight changes in environmental conditions occur (e.g., cooler temperatures) the disease will generally shut down and disappear. But, we’ve observe slight aerial blight for the better part of the last three weeks so be mindful when scouting soybean fields.
Aerial web blight as observed early in the morning when exhibiting the white fungal mycelium.
Aerial web blight symptoms as observed in the early afternoon once dew is no longer present on soybean foliage.
Several Plant Disease Management Reports have been published from trials conducted in MS over the years:
Downy mildew
Annually downy mildew appears when plants reach mid-reproductive growth stages. The light, yellow flecking observed in the top of the canopy on numerous leaves is generally the result of downy mildew. On the underside of the leaf, white to cream colored tufts of fungus are present and can be observed with the naked eye or a low-powered hand lens. Downy mildew is most notably an aesthetic occurrence. Fungicide applications are not necessary as downy mildew will not contribute to a yield reduction. Be mindful that some varieties will exhibit more downy mildew than others. In addition, when downy mildew lesions dry up the white tuft of fungus on the underside of the leaf can be confused for other diseases such as: brown spot, and soybean rust.
Downy mildew as observed on the upper leaf surface.
Downy mildew as observed on the underside of the leaf surface.
Frogeye leaf spot
Frogeye leaf spot continues to be observed throughout MS, especially in frogeye leaf spot susceptible varieties. Keep in mind that knowing the variety planted in a particular field should frogeye leaf spot occur will be extremely important information on several fronts. Based on a graduate student project (Mr. Jeff Standish) the vast majority of the infected leaf samples collected from 2013 and 2014 have been determined to contain fungal material resistant to the strobilurin class of fungicides. Since the fungus is resistant to the strobilurin fungicides, if a fungicide that contains a stand-alone strobilurin is applied to a soybean field don’t expect the fungicide to prevent additional disease from occurring.
Over the past several weeks I’ve received numerous telephone calls regarding the “poor performance” of some fungicide products at preventing frogeye leaf spot from occurring. Last year in particular, regardless of the fungicide applied in trial work conducted in Starkville or Stoneville the frogeye leaf spot present in the plots at the time of fungicide application observationally got worse. A fungicide application is going to work to prevent further spread or infection from the frogeye leaf spot fungus. The fungicide will not reduce the frogeye already present and will likely maintain yield.
For additional information regarding the expected reaction of soybean varieties commonly planted in MS please refer to:
Maturity Group IV:
Maturity Group V:
Or for information regarding fungicide trials conducted during 2013 in MS to prevent yield loss as a result of frogeye leaf spot refer to:
Soybean rust
At present (7/20/2014) no soybean rust has been detected in Mississippi. Sentinel plots continue to be monitored throughout the state for soybean rust as well as additional diseases of economic importance. Even though no soybean rust has been detected in MS, active soybean rust has been reported from Alabama and Louisiana on kudzu. Stay tuned to the Mississippi Crop Situation blog (Disease Monitoring tab), the public access soybean rust website (www.sbrusa.net) as well as Twitter (@baldpathologist) for up-to-date information regarding economically important soybean diseases.
Target spot of soybean. Note the yellow halo around lesions typified by concentric rings.
Target leaf spot
On Wednesday (7/16/2014) I observed excessive target spot in a soybean field in Lowndes County. The field was suffering from what appeared to be drought-type symptoms (believe it not after the wet year we’ve had) and target spot was knocking off leaves in the lower canopy. Symptoms of target spot are generally observed as round, concentric rings on affected leaves in the middle to lower canopy. Lesions can also be surrounded with a yellow halo. In severe case of target spot the disease will defoliate portions of the lower canopy. Rarely have fungicide trials in MS yielded results on target spot.
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