As Asian soybean rust finds in Mississippi continue at a brisk pace, plant pathologists in the state are fielding many questions about fungicide treatments. On Aug. 21, Tom Allen, assistant Extension/research professor at the Delta Research and Extension Center in Stoneville, Miss., and Billy Moore, professor emeritus with Mississippi State University, released the report below.

While some of the information applies across the Mid-South, Allen warns that it was written for Mississippi producers only. “Producers outside Mississippi need to follow the fungicide guidelines and recommendations suggested by their own” state plant pathologists.

The need for the document became apparent when frequent “misunderstandings about fungicides became apparent. It isn’t the easiest thing to keep track of — strobilurins are used here, triazoles there. Hopefully, this will help growers keep things straight.”

The Allen/Moore report reads:

“Over the past two weeks we’ve received numerous calls regarding the soybean rust situation. Most of the calls have focused on specific product choices as well as whether or not a particular field should be treated.

“We firmly believe that the decision regarding whether or not soybeans require a fungicide has to rest with the producer. Ideally, every field should be considered as a unique situation and thus a decision to spray should be made on a field-by-field basis.

“At this point in the season, do not overreact regarding the identification of soybean rust in the state. With regards to the identification of rust in Mississippi, we can only present the information regarding the presence or absence of the disease, level of severity within a given field/county, outline the pros and cons of each of the fungicide classes, and try and ease some of the anxiety about this particular disease.

“Personally, the initial yield loss projections for the Mid-South following the detection of soybean rust in the United States in 2004 were incredibly overstated. However, that statement made, we don’t think we can ever become complacent about rust. There is absolutely no way to tell what the future will hold. Moreover, there is no right or wrong answer for treatment suggestions simply for soybean rust since this is not a black or white situation. Many of the factors about this disease seem yet to be determined but we do know that the level of infection and areas where infection predominantly occur appear to be correlated to rainfall.

“To bring everyone up to date on the situation in Mississippi: 18 counties (Bolivar, Calhoun, Carroll, Coahoma, Grenada, Holmes, Humphreys, Issaquena, Leflore, Montgomery, Quitman, Sharkey, Sunflower, Tallahatchie, Warren, Washington, Yalobusha, and Yazoo) have been determined to have a limited amount of rust infection in soybeans. Except for the field situation in Montgomery County, the level of rust in almost all of these counties has been extremely low.

“We will likely identify more soybean rust in Mississippi over the next days, weeks, and months. When this happens, do not overreact. Instead, verify the information at hand to determine what the risks from the disease will likely be and the potential treatment strategies if that is deemed necessary.

“First, we need to spend a little time presenting information on the specific classes of fungicides. We would also like to mention that as plant pathologists we are not in the business of controlling diseases.

“Plant diseases should be prevented or their spread or increase should be prevented. Soybean rust is one of those diseases that if it reaches a certain level within a field (judged to be approximately 10 percent infection based on the total number of plants within a field that contain infected plant material) then fungicides are not effective against the disease due to an overwhelming level of inoculum that can be produced in a single field.

“We generally treat soybeans with two broad classes of fungicides: strobilurins or triazoles. In addition, there are products that contain both fungicide classes and are marketed as pre-mixes. The two fungicide classes can also be combined into an on-farm tank mix.

“A strobilurin will not provide ‘curative’ activity against a plant disease. This essentially means that when this product class is applied in the presence of disease it will only protect plant material that comes in contact with the fungicide and has not been previously infected.

“Conversely, fungicides that constitute the triazole chemistry have some ‘curative’ ability (we put that in quotes because plant pathologists abhor this term). However, they are still intended for use and are most beneficial when applied in the absence of disease or when disease levels are very low within a given field situation. With that said, they are effective against soybean rust in the event that a field has a level of infection less than 10 percent.

“Products that contain both a strobilurin and a triazole (pre-mix or a tank mix) are effective against a broad spectrum of diseases and have both some ‘curative’ ability as well as preventative. This is one of the most important issues to consider when selecting a product if and when rust comes into the picture. This is also a reason that spray coverage and depth of penetration into the canopy are important. Products should be applied in no less than 5 gallons of water by air and 15 gallons of water by ground.”

Before tackling specific scenarios, the authors emphasize that “soybeans at growth stage R5.6 are believed to be in the safe zone at this growth stage and do not need to have a fungicide applied specifically for the purposes of preventing soybean rust. However, if seed quality is included in your management strategy then this statement should not be taken as a suggestion to change anything you are currently doing. With that caveat, soybeans at growth stages R6 and beyond do not require a fungicide application specifically for the purposes of preventing soybean rust.”

For those outside the 18 positive counties

“If soybean rust has not been found in your area at the present time it is not perceived to be a threat to soybeans in the R3/R4 growth stages. Producers that were planning on applying a fungicide to soybeans at the R3/R4 growth stage timing for soybeans on irrigated ground with good yield potential can apply a strobilurin. But this only applies to those areas not included in the 18 positive counties.”

Dryland soybeans

“For those with dryland soybeans at R3/R4 and low yield potential, consider the price of soybeans and your yield potential when deciding whether or not to apply a fungicide. This is where making a decision on a field-by-field basis is important.

“For those with dryland soybeans at R3/R4 and good yield potential, in some cases particular fields have received many of the rain showers in July and August. In some of these cases a fungicide might be a beneficial decision. Depending on the location of the particular field either a pre-mix (strobilurin plus triazole) or a tank mix (strobilurin plus triazole) may be warranted in counties where rust has been detected. A strobilurin alone may be warranted in the Mississippi hills, northeastern counties, or southern counties where soybean rust has not been detected. Again, this is a field-by-field decision.”

Irrigated soybeans

“For those with irrigated soybeans at R3/R4 with good yield potential, the decision to spray and which product would be the most beneficial will depend upon the particular location of the soybean field in geographical regards to the 18 positive counties.

“For those with irrigated soybeans that have received an R3 fungicide about two weeks ago (or longer depending upon the particular field situation), we’ve had numerous questions in particular regarding fields in the Delta and in the vicinity of commercial fields that have had rust detected within them. At this point, there is no reason to spray another fungicide over the top of the plants. However, if a particular producer is concerned with the threat of rust and has soybeans in one of the 18 positive counties then a triazole alone can be applied.

Additional scenarios

“For those with irrigated or dryland soybeans that received an R3 fungicide and are now at approximately R5 with good yield potential (essentially all soybeans between R5 and R5.5), any decision at this point with regards to soybean rust should also include a fungicide decision that could provide some seed quality.

“Results from late applications of strobilurin fungicides to prevent severe soybean seed deterioration during extended periods of warm inclement weather have had mixed results. Typically when temperatures become cooler the threat of severe deterioration declines. To benefit by preventing the pod and seed-rotting diseases as well as getting some added rust protection, either a pre-mix or a tank mix at this growth stage range would be a good choice.

“By using a spray that contains both products on the off chance that there was some rust infection (albeit at low levels within the field) the mixed product would provide some ‘curative’ and preventative qualities.

“Soybeans currently between R5 and R5.5 in counties where rust has been detected can be treated with a triazole alone to both prevent infection and help reduce any potential infection that may have already occurred. But this decision should only be made if the producer is concerned about soybean rust and has weighed all of the options above regarding soybean selling price as well as individual soybean field yield potential.”

For more on Mid-South soybean rust, see SOYBEAN RUST.