The three major diseases plaguing Mid-South soybeans in 2013 were, once again, frogeye leaf spot, Cercospora leaf blight, and soybean rust.
While it’s not uncommon for frogeye leaf spot and Cercospora blight to be found in Delta soybean fields, the overwintering situation in south Louisiana and other states where soybean rust can survive the winter on kudzu, can result in soybean rust becoming increasingly problematic and adding to the region’s future disease mix.
Over the past two seasons, frogeye leaf spot has become more of a concern, says Tom Allen, plant pathologist at the Delta Research and Extension Center at Stone-ville, Miss.
“Farmers with a past history of the disease should consider planting a resistant variety or rotating to a non-host crop such as corn,” he says. “Frogeye leaf spot can be particularly problematic since the fungus that causes the disease has developed resistance to the main class of fungicides farmers use, strobilurin fungicides, for preventive disease management.”
Crop consultant Justin George, Merigold, Miss., says frogeye leaf spot also was the main soybean disease he encountered in 2013, followed by Septoria and Cercospora leaf blight.
Incidences of Septoria and Cercospora could be found in Delta soybean fields, but neither was of great concern to growers, he says. “If we measured the importance of these diseases in 2013, then frogeye was a 9 on a scale of 1 to 10, and the other diseases rated a 3.”
Among the Delta farms he scouts, George says, the majority of the acreage was planted to one of three soybean varieties. Each is high yielding, but two of the varieties are extremely susceptible to frogeye. “However,” he says, “we still made high yields where frogeye leaf spot pressure was high, thanks to timely fungicide applications.
“Two popular soybean varieties — Armor DK 4744 and Asgrow 4531 — are both susceptible to frogeye leaf spot. Despite this, they are still popular and will continue to be planted because of their high yielding potential. But both will likely require a fungicide treatment for disease prevention.
“We didn’t know how badly it would affect yield until the combines ran. In the end, we couldn’t tell that it greatly affected yield. We saw an insignificant difference in disease development where preventive fungicide treatments were made.”
Bill Ryan Tabb, a soybean producer in Bolivar County, Miss., says he, too, had some trouble with frogeye leaf spot in a couple of different soybeans varieties this past growing season. All of the affected acreage was treated with a fungicide, and some of the fields were treated twice.
“This was the third or fourth year that we planted Armor DK 4744, and we have never really had any trouble,” he says. “Frogeye leaf spot was so prevalent in that variety that you’d think something in the weather conditions caused it to explode.”
According to weather data from the Delta Research and Extension Center at Stone-ville, summer 2013 was wetter than the previous year, but the average temperature and percent humidity were about the same as in 2012.
Between July 1 and Sept. 16, 2013, the average maximum temperature at Stone-ville was 91 degrees, with an average minimum temperature of 68 degrees. The maximum average humidity was 97 percent, while the average minimum humidity level was 43 percent. A total of 5.27 inches of rain fell during the summer growing season.
Tabb says, “Armor DK 4744 is still one of our better-yielding varieties, but we now know we have to spend more money for fungicides with that variety. I’m going to plant that variety again next year. The frogeye infestations this year are not going to scare me away from this soybean, which yields well.”
In addition to increased frogeye leaf spot pressure, Tabb says he experienced some difficulty with soybeans not defoliating properly.
“I don’t know what causes that, but we had a couple of fields where plants just stayed green and the leaves remained attached to the plants. I’m not sure if that was a disease effect, because it didn’t seem to matter if we sprayed fungicide on it or not.”
George attributes the relatively low disease pressure overall in 2013 to the prevalent use of fungicides.
“With $15 soybeans, a lot of growers are seeking to make high yields,” he says. “Soybeans are no longer a stepchild crop — they’re now a cash crop, and the preventive use of fungicides will alleviate some of the disease pressure, year-in and year-out.”