A Noxubee County soybean field severely infected with soybean rust will represent Mississippi's first yield losses to the disease that has been present in the state since November 2004.
Rust was evaluated in the field Sept. 4, and it is the most severe case of soybean rust found in Mississippi to date. The 100-acre field near Brooksville was not treated with a fungicide.
Tom Allen, plant pathologist with the Mississippi State University Extension Service, estimated this field will lose 5 percent to 10 percent of its yield to rust, but emphasized that this is just a guess.
“It is likely that rust has been present in this location for eight to 10 weeks,” Allen said. “Additionally, with July's weather conditions that were overly conducive for disease development, rust had an excellent opportunity to develop and cause damage in this particular field.”
Allen said rust is the primary disease in the field and has defoliated numerous plants in large areas. A secondary root disease continues to injure plants that initially lost leaves to rust.
Dennis Reginelli, Extension agronomic crops agent in Noxubee County, said the field has at least half a dozen acre-or-less-sized circles of dead plants where the disease first occurred. The rust took hold on the plants, weakened them and made them susceptible to other diseases that defoliated the plants.
“The entire field has rust spores in it,” Reginelli said. “In some places, you can actually see spores flying if you shake the plant foliage.”
Reginelli said the crop had a great yield potential early in the season, but the decision was made not to give the field a preventive fungicide application.
“When you spray a fungicide application, you're trying to prevent diseases from robbing energy from the plant,” Reginelli said. “Diseases appear at different times, and if the plant has a layer of fungicide that can prevent those diseases from getting established, then they normally don't rob the plant of yield.”
While it is too late to apply fungicide to this field, Allen said, yields at neighboring fields are not threatened by this disease outbreak. Producers applied fungicides to nearby fields on schedule, and these are past the growth stage that puts them in danger of losses.
“For this season, this particular field doesn't change any of the fungicide suggestions,” Allen said. “However, this field situation will allow us to gain important information regarding the impact of soybean rust on Mississippi soybean production for the future.”
Most of the soybeans in Noxubee County were planted in mid-April and are past the vulnerable stages for soybean rust infection. Much of the soybean acreage planted farther north was planted later, after wheat was harvested, and many of those fields are still at vulnerable stages.
“It is very important from a disease management standpoint to put out fungicide on schedule as a preventive measure,” Reginelli said.