Target spot has struck many Mid-South soybean fields this growing season leading to decreased yields. Now on the back end of soybean harvest, growers are asking questions about the fungal disease. 

To get some answers, Delta Farm Press spoke with Tom Allen, Mississippi State University plant pathologist, in mid-October. Among Allen’s comments:

On potential varietal differences…

“One thing I’ll say is, at this point, we have some decent varietal observations on target spot, but not hard and fast data regarding the yield losses associated with this particular disease.

“I’d say for the last two to three years from looking at the soybean OVTs in Mississippi, I’ve been able to rate diseases that include target spot. The hard part about speaking on some of the observations is rating for target spot can be a little bit more difficult than a disease such as frogeye.

“Target spot is normally observed in the lower to middle part of canopy. In general, target spot doesn’t normally move into the upper canopy and certainly isn’t primarily observed in the upper canopy like frogeye leafspot. That presents a huge issue when studying target spot – you have to part the canopy and look to see how high the disease has moved in the canopy. Any of the ratings I’ve done provide decent ideas about where target spot will be in the canopy (bottom, middle, top) based on particular varieties.

“Now, fast-forward to this year where we’ve been able to observe specific varieties that appear to be extremely susceptible to target spot. The problem is everyone seems to have a different list of varieties and some of those have tended to be on the list of some of our better yielding varieties based on yield potential over the past several years.’

A specific example…

“One specific example of that is Asgrow 4632. That variety is resistant to frogeye and typically produces an outstanding yield. But it wasn’t the case in 2016 in some key locations based on the environment when, in certain situations, it took it on the chin from target spot.

“That isn’t the only variety in that situation. In my mind, there are probably six to eight, perhaps nine, varieties that I observed that contained a substantial amount of target spot. Based on what we have observed in the recent past with this particular disease I would consider some of the symptoms observed in 2016 to be atypical.

“In general, target spot produces lesions on leaves that tend to be 1/8th to a quarter-inch in size with concentric rings and a yellow halo. However, when a conducive environment prevails for an extended period of time, smaller lesions can develop on leaves, have less of a concentric pattern and do not contain a yellow halo. In addition, leaves with a heavy amount of infection generally defoliate as a result of the disease. Normally, symptoms as a result of target spot are observed in the lower to middle-canopy and rarely move to the uppermost canopy.”  

On symptoms observed in 2016…

“In 2016, we observed target spot on leaves, pods, petioles and main stems. Symptoms on pods, petioles, at the juncture of branches and petioles to the main stem, and on the main stem can appear purple to almost black. Lesions formed on the main stem have a general teardrop shape. Lesions on pods are generally round, slightly sunken, purple to almost black and 1/32nd of an inch in size.

“On varieties that were observed to be extremely susceptible, extreme defoliation of the mid-canopy resulted in some field situations. In rare cases, some varieties had almost complete defoliation that was catastrophic to yield in some fields because defoliation occurred during the mid-R5 growth stages.

“In Mississippi, there were some fields that would normally cut 75 bushels that cut between 50 and 55 bushels. Those fields tended to be in the geographic area that saw tremendous rainfall during a three week period in August. Symptoms of target spot were present on petioles and leaves on the ground as well as main stems all the way up to the day they cut those beans.

“The guys coming off the combines said, ‘we didn’t have to water these that much. We put out fungicides like normal and we’re cutting 50 bushels and should have made more than 70 in high-yield, irrigated situations.’”

On how target spot “works”…

“Target spot is a soil-borne fungus but can be moved by rain, wind and survive the winter on crop residue. The interesting thing is this particular fungus probably infects 500 different plant species. This year, I’ve seen target spot in pictures people have sent on okra, grape vine, sassafras, and green briar in addition to the normal cotton and soybean images. I thought ‘that’s interesting, but all those came out of the area that received lots of rainfall.’

“In a more normal year, the fungus affects just leaves. Target spot is generally observed from the time the canopy closes until R-6.5. In most cases, target spot will defoliate some of the lower canopy if a conducive environment persists. But that’s pretty common once leaves in the lower to middle canopy are shaded.

“This year, though, there was a tremendous amount of defoliation as a result of target spot. A similar thing happened during 2009, which was also a really wet year. The major defoliation event appeared to occur in a pretty small geographic area, but in some cases this stretched across multiple states. This year, the problem ranged from the south Delta of the Mississippi all the way up into northeast Arkansas and northeast Mississippi.”

Did it make any difference the way fields were irrigated?

“No. Any of the fields I looked at – dryland, furrow-irrigated, pivot-irrigated, anything – (method of) irrigation had nothing to do with the presence of severity of the disease.  The prevailing environment, more specifically the excessive rainfall received in some locations had more to do with the disease than did form of irrigation.

“Interestingly, crop rotation didn’t seem to have anything to do with the presence of target spot, either. We looked at a field of soybean following corn in northeast Mississippi and they had just as much defoliation and target spot-infected plant material as fields of continuous soybean in the area.”

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On future research efforts…

“I think there will be a bit of push to increase target spot research. However, based on the observations made during the 2016 season the environment is one of the most important components for the disease to become severe. Clearly, based the amount of rainfall and duration of rainfall events at specific growth stages has a lot to do with whether or not the disease can cause significant yield losses. Unless we receive the same environment in 2017 I don’t expect target spot to be near as problematic.

“Right now, a lot of the research effort is in trying to collect isolates and probably shift some of the research in the direction of the greenhouse and growth chambers. Then, we can likely compile some meaningful suggestions for areas with concerns about seeing yield reductions.”

In treating for target spot, is there a best method for application of a fungicide?

“We were able to rate a lot of plots for the presence of target spot this year both in variety trials as well as foliar fungicide efficacy trials. Most of the varieties we had planted for fungicide trials, though, were very susceptible for frogeye. So, we weren’t getting a really good idea of yield losses resulting from target spot simply because there was another disease in the mix.

“Even so, all the plot work had ground applications – 15 gallons per acre. I was able to do defoliation ratings as well as rating for the incidence and severity of target spot. I’ve glanced at some of those data and no fungicide stuck out and made me say ‘wow, that was the best for target spot.’

“As for field situations I looked at, any fungicide applications were made by air. The application volumes were likely less than 5 gallons per acre. None of those field situations looked that good following the foliar fungicide application. They still had tremendous defoliation, still had lots of lesions all over the plant. Product selection in most of those situations consisted of stand-alone strobilurins, stand-alone triazoles or pre-mix fungicides.”