Frogeye leaf spot Travis Faske
Frogeye leaf spot

Taproot decline, target spot emerged as new soybean disease threats in 2016

Here are some of the primary soybean diseases in the Mid-South and recommendations for management.

Finding disease in a soybean crop is one thing. Deciding how to manage it is a much more complicated challenge altogether.

With that in mind, here are some of the primary soybean diseases in the Mid-South and recommendations for management.

Taproot decline — Pathologists have confirmed a new pathogen responsible for the disease, which was previously referred to as black root rot and mystery disease, although the fungus does not have an official classification just yet.

Trey Price, plant pathologist with the LSU AgCenter-Macon Ridge Research Station at Winnsboro, La., says the disease typically is not noticed by producers until pod fill (R5).

“That’s when we start seeing foliar symptoms that closely resemble other root/stem diseases, like sudden death syndrome, stem canker, or red crown rot. However, taproot decline actually causes losses earlier in the season. It can be a seedling disease. We have observed taproot decline in vegetative stages. The disease can affect plants at any stage of development.”

Taproot decline does have characteristics that can be distinguished from the aforementioned imitators. A plant with taproot decline will typically break off at the soil line when pulled from the ground. “The taproot and lateral roots will be diseased which is evident by a blackened color,” Price says. “If you split stems and roots lengthwise, you’ll typically see white fungal growth in the center of the stem.”

Best treatment options include resistant varieties and crop rotation, he says. “Anytime you have soybeans following soybeans, you have a higher chance of developing the disease.”

Price is currently screening soybean varieties at the Macon Ridge Research Station for resistance to taproot decline.

Travis Faske

Target spot

Target spot — All foliar diseases are unpredictable and, along that line, every few years a disease catches the attention of farmers, consultants, and pathologists. That’s true of target spot, says Travis Faske, Extension plant pathologist with the University of Arkansas.

“In the 1960s, soybean breeders selected for resistance to target spot, so there has been little impact from this disease and little need for research,” he says. “Thus, for the past few years we’ve focused on frogeye leaf spot and Cercospora leaf blight, and there hasn’t been as much effort on target spot. I think that will change next year.”

Heather Kelly, assistant professor of plant pathology at the University of Tennessee Institute of Agriculture, says target spot is usually confined to the lower canopy in soybeans, “but this year in certain varieties, I saw it get into the middle canopy and cut into yields.”

Target spot shows up as brown to brick-red lesions that display concentric rings as they grow larger. The same fungal pathogen causes target spot in cotton.

Based on Mid-South trials, some of the SDHI (succinate dehydrogenase inhibitors) such as Priaxor have good activity on target spot in soybeans, according to Kelly.

Frogeye leaf spot —After three years as the top disease in Louisiana, losses to frogeye leaf spot in 2016 were minimal, says Trey Price, perhaps due to higher adoption of resistant varieties.

Even so, strobilurin-resistant frogeye leaf spot has been confirmed in several Mid-South states, and growers should be wary when planting a susceptible variety. While triazoles are effective on frogeye leaf spot, growers should refrain from overusing the chemistry, pathologists say.

Cercospora blight — Taking the No. 1 spot from frogeye leaf spot in Louisiana this year was Cercospora blight. Unfortunately, there are not a lot of good options for control.

“We’ve thrown the book at it as far as fungicides go,” Price says. “In some years, we have some fungicides that show some efficacy, but we just can’t get any consistency. It’s a very frustrating disease to work with. A part of the problem is that a large percentage of the pathogen population is resistant to strobilurin and thiophanate-methyl fungicides. Ultimately, the way to manage Cercospora blight in the future is going to be with resistant varieties.

A project for that purpose is under way, funded by the Mid-South Soybean Board and the United Soybean Board.

Other Mid-South soybean diseases include:

Soybean rust — Strobilurin fungicides should be applied prior to disease presence, whereas triazole fungicides can be effective after the disease has been observed in the field. Triazoles, however, are also most effective when applied prior to disease development.

Charcoal rot — “You usually don’t notice charcoal rot because you don’t see above-ground symptoms,” says Heather Kelly. “This year, some fields went through some hot, dry times that allowed the fungus to colonize. They laid in wait with the rain coming in late August.”

Aerial blight — The disease was present statewide in Louisiana soybean fields all summer during 2016. “Strobilurin fungicides work well,” Price says, “but in isolated areas, there is resistance to strobilurins. In that case the option would be a product containing an SDHI.”

Root-knot nematode — The southern-root knot nematode consistently robs producers of yield each year in Arkansas. The best means of control is using a resistant variety, but unfortunately, “with the Group 4s and 5s the options are pretty slim,” Travis Faske says.

Stem canker — This disease can be extremely severe on susceptible cultivars, and yield loss can be extensive. Resistant cultivars are the primary means of control.

Given that soybean varieties change every few years, it is challenging to keep up to date with the currently available varieties.

The Arkansas Soybean Promotion Board sponsors an annual screening program by the University of Arkansas in which several hundred varieties are evaluated for susceptibility to root-knot nematode, frogeye leaf spot, and stem canker. The results from this program are available at the arkansasvarietytesting.com website. Results from the 2016 testing are posted in November.

Similarly, the Tennessee Soybean Promotion Board sponsors an annual screening program by the University of Tennessee in which commercial varieties are rated for susceptibility to frogeye leaf spot, target spot, sudden death syndrome, and stem canker due to natural infections.

These results, along with fungicide efficacy data, are posted at the UTcrops.com website. Results from 2016 will be posted this winter and announced at the news.UTcrops.com blog

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