Soybean Disease Update and Frogeye Leaf Spot Fungicide Suggestions: July 17, 2013

Several diseases to discuss this week.  In addition, stay tuned for some additional information regarding frogeye leaf spot fungicide decisions.

Frogeye leaf spot

Frogeye leaf spot

Over the past week to 10 days the number of reports of frogeye leaf spot have increased dramatically.  In most cases the reports have been from susceptible MG IV soybean varieties in the Delta.  In two instances a strobilurin fungicide (4 fl oz/A) was applied to the soybean crop at R3 and frogeye leaf spot incidence has increased dramatically.  Both instances occurred in fields of susceptible soybean varieties.

Managing frogeye with fungicides since strobilurin-resistant isolates have been detected in multiple states as well as MS may become a difficult situation in the future.  In the past, a strobilurin fungicide was suggested since applying that product would likely limit the spread of the disease to leaf material that had not previously been infected by the fungus.  However, with the increased likelihood of resistance within the fungal population developing to the strobilurin fungicides as well as the number of acres planted to susceptible soybean varieties that suggestion is no longer made.

As a general comment, this year in particular frogeye appears to produce more coalescing lesions on some soybean leaves.  To determine if the lesion is either a result of frogeye leaf spot or some other malady, use hand lens (a 20x will work fine) and look for prolifinc sporulation on the underside of the leaf.  Even though sporulation can occur on the upper leaf surface, this year in particular I’ve observed most of the sporulation on the underside of the leaf.

With that in mind, there are some simple guidelines to follow when making a fungicide decision at the R3/R4 growth stages, or when simply attempting to manage frogeye leaf spot:

1)     Know your variety and whether or not it is susceptible/tolerant to frogeye.

2)     Apply a straight strobilurin product at the R3/R4 growth stage if you planted a frogeye resistant/tolerant variety.

3)     Consider using a pre-mix or tank-mix option (strobilurin + triazole, or strobilurin + carboximide) in the event of a susceptible variety.

4)     Don’t cut fungicide rates.  Personally, I prefer full labeled rates, for example 6 oz of azoxystrobin or pyraclostrobin as opposed to 4 ozs of the products.  Reduced fungicide rates, over time could increase the potential of fungicide resistance.

5)     If you applied a strobilurin at the R3/R4 growth stage timing, find frogeye levels to be increasing (say in a susceptible variety) make an application with a triazole fungicide to rotate chemistries.

For additional information on this particular topic see:

https://www.mississippi-crops.com/2013/06/01/soybean-fungicide-management-considerations-for-2013/

If you have a situation where frogeye has increased following a fungicide application please call as I would like to get samples for a graduate student on campus that is conducting a research project into the specific fungi that have fungicide resistance in MS.

Aerial blight

Limited reports of aerial blight have occurred over the past two seasons.  However, one field of heavy aerial blight was detected south of I-20.  Remember, when scouting for aerial blight it is best to be in the field early in the morning to observe the general symptoms of fungal growth between leaves.  Once the plants are no longer wet with dew it can become increasingly difficult to determine if aerial blight   Fungicides that contain a strobilurin active ingredient (whether as a stand-alone product or as a pre-mix) are excellent options and will provide an economic benefit if aerial blight is an issue in your soybean field.

Septoria brown spot

Rarely does brown spot cause yield loss.  Extremely specific conditions are necessary for brown spot to result in leaf shed in the upper plant canopy.  In the six years that I’ve been with MSU I have only seen that situation develop in a single soybean field.

Target spot of soybean.

Target leaf spot

In some fields target spot is becoming apparent.  Target spot is a normal disease occurrence in soybean fields and typically resides in the lower to middle canopy.  Lesions tend to start small and can grow in size to be the size of a dime.  Large lesions generally have concentric rings, hence the name target spot.  To my knowledge a fungicide application has not been suggested in the past to prevent yield loss as a result of target spot.

Southern blight

I continue to receive telephone calls regarding southern blight in vegetative stage soybean fields.  Southern blight prefers soils with more organic matter and this year in particular we have observed the disease in some situations whereby the farmer tilled under his wheat crop or the soybean crop followed corn.  Once the disease is detected no management alternatives exist.  Waiting for the soybean stem to harden generally results in a reduction of observable symptoms.

For additional information regarding southern blight: https://www.mississippi-crops.com/2012/05/18/southern-blight-of-soybean-observed-in-a-few-isolated-fields/

Soybean rust symptoms on the top side of the leaf.

Soybean rust

Low levels of soybean rust were detected in two additional counties this week.  Rust was detected in the soybean sentinel plot in Amite (Tuesday, 7/16/2013) and Jackson counties (Monday, 7/15/2013) at the R5 and R4 growth stages, respectively.  Historically speaking, this is the earliest we have ever detected soybean rust in three counties.  However, by the end of July 2012 we had soybean rust in five counties so we are tracking in a fashion similar to the disease progression that occurred in 2012.  With a late-planted soybean crop in some parts of the state farmers need to be aware of the developing situation.  But, with that in mind fungicide applications made to vegetative stage soybean plants have not proven to be economical in the past.  Therefore, we will continue to scout for the presence of soybean rust as well as other diseases of economic importance prior to suggesting the use of a fungicide.

To stay up to date on soybean rust throughout our region log onto www.sbrusa.net or follow monitoring progress here on the blog at https://www.mississippi-crops.com/disease-monitoring/ .