Soybean Disease Update: August 8, 2015

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist August 8, 2015 11:48

Soybean Disease Update: August 8, 2015

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Over the past week, additional disease has been noted in the MS soybean production area. Increased calls regarding phytotoxicity associated with fungicide applications have also been received. The continued hot and dry conditions throughout much of the state for the better part of the last month have increased the incidence of Cercospora blight as well as fungicide phytotoxicity.

Frogeye leaf spot as observed in a moderately-susceptible soybean variety following a stand-alone strobilurin application.

Frogeye leaf spot as observed in a moderately-susceptible soybean variety following a stand-alone strobilurin application.

Frogeye leaf spot

Even though temperatures in excess of 95F are not considered to be conducive for frogeye leaf spot, the presence of the disease has increased over the past several weeks. I observed plots in Stoneville on Thursday afternoon and was shocked at how much the frogeye had increased over the past 7 days. In susceptible varieties, regardless of the environment, it appears that frogeye leaf spot will increase over time. Keep in mind that even following a fungicide application the symptoms associated with frogeye leaf spot will increase. Fungicide applications work by protecting yield. Even in the fungicide trial plots in Stoneville, frogeye leaf spot has greatly increased since the fungicides were applied approximately 21 days ago. Be mindful that given the widespread nature of fungicide resistance that fungicides that contain a stand-alone strobilurin (QoI) active ingredient are no longer effective for managing frogeye leaf spot. On Thursday (8/6), I observed a field that was sprayed with 6 fl oz/a of azoxystrobin approximately 14 days before I observed the field. Frogeye leaf spot had substantially increased as a result of the susceptible variety planted as well as a fungicide that would no longer reduce the symptoms associated with the disease. Fungicides to manage frogeye when the disease is present should either include a pre-mix (or tank mix) of more than one mode of action, or a stand-alone triazole (DMI) product. However, once soybean plants have reached the R6 growth stage a fungicide is no longer as beneficial.

Severe phytotoxicity following a foliar fungicide application with a product previously known to produce phytotoxicity.

Severe phytotoxicity following a foliar fungicide application with a product previously known to produce phytotoxicity.

Fungicide phytotoxicity

Stressful situations can greatly increase the symptoms associated with fungicide phytotoxicity (or burn or injury). Over the past two weeks I have received countless phone calls where the injury appeared “worse” after a given period of time. Over the past three years I have observed the injury to present more symptoms between 21 and 28 days post-application. Yesterday (August 7) I observed fields in Sunflower County that had received a fungicide application with a product that produced phytotoxicity. In furrow-irrigated fields, where stress was likely reduced due to plentiful irrigation, symptoms were not near as visible as across the road in pivot-irrigated soybean. A band of affected plants near the road likely indicated where the ag pilot had turned off the applicator and more product was applied simply due to deposition. The symptoms associated with phytotoxicity will oftentimes be confused with sudden death syndrome or another root-based disease issue. The severe interveinal chlorosis that accompanies some fungicide applications can be observed across entire fields, rather than an isolated clump of plants.

If you have questions regarding the specific response of the soybean plant to a specific fungicide refer to: http://www.mississippi-crops.com/wp-content/uploads/2015/07/Fungicide-phytotoxicity-products.pdf . But, keep in mind that additional products are likely available that will lead to phytotoxicity. In addition, based on observational reports, soybean variety has a lot to do with whether or not a fungicide produces phytotoxicity on soybean plants.

Galling associated with the root-knot nematode.  Foliar symptoms can be difficult to tell apart from several other root-based issues as well as fungicide phytotoxicity.

Galling associated with the root-knot nematode. Foliar symptoms can be difficult to tell apart from several other root-based issues as well as fungicide phytotoxicity.

Root-knot nematode

Over the past three years I have observed root-knot nematode infested fields throughout the state. In the Delta, I have observed more root-knot infested fields each year since I arrived. Root-knot nematode infested fields produce symptoms on foliage that will be similar to sudden death syndrome as well as fungicide phytotoxicity. Remove the root system from the ground to observe whether or not galling is present on the roots. Remove more than one plant from the soil and stay around the edges of severely infested areas since dead plants will likely have severely rotted root systems and observing the galls may be more difficult. Galls will appear similar to nodules; however, the gall will encompass the entire root instead of simply being attached to the root. A field I observed on Friday (8/7) had been sprayed with a fungicide that produced phytotoxicity. The injury associated with the fungicide was assumed to be an overspray from the aerial application rather than a root-knot nematode issue. Compounding stress in a field situation can make the phytotoxicity associated with some fungicides appear more severe (see below). In addition, keep in mind that some fungal diseases can be a secondary issue on plants affected by the root-knot nematode. Southern blight is one of those diseases and will typically produce white mycelia on the outside of the main stem. Dead plants will more often than not produce white to brown sclerotia that appear at the soil line.

 

Present (as of 8/8/15) soybean rust situation in the southern U.S.

Present (as of 8/8/15) soybean rust situation in the southern U.S.

Soybean rust

On Wednesday, the first soybean rust was observed in a soybean sentinel plot in Amite County. The plot was planted in an extremely protected location. Shade from the pecan tress immediately above the sentinel plot increased the potential for the disease to develop. Remember that high temperatures and lack of rainfall are not considered to be conducive conditions for soybean rust. Additional scouting conducted on Tuesday, Wednesday and Friday revealed that no additional soybean rust was present in MS. At this time, and due to the continued hot and dry conditions throughout much of MS, we continue to be in a wait and watch scenario. To date, soybean rust has only been identified in 10 counties in the southern U.S. The find on Wednesday in MS was only the second county where the disease had been identified on soybean.

For additional information on soybean rust in the U.S. refer to: http://sbrusa.net/cgi-bin/sbr/public.cgi

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist August 8, 2015 11:48
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