There are specific situations in which fungicide application on soybeans will be profitable. Scenarios for economically sound fungicide application include instances when there’s no rotation, says Connie Strunk, plant pathologist for South Dakota State University.
“If you’re corn on corn, soybean on soybean, you see some benefit to fungicide,” said Strunk. “If you’re in minimum tillage with crop residue that’s still on the surface, we see some benefit to [application]. If there’s disease susceptibility to your hybrid cultivar variety through your selection process, if there’s some disease susceptibility there, that is when a fungicide could be profitable.”
It’s also wise, according to Strunk, to consider fungicide application when there’s a history of moderate disease pressure. “But the biggest one that connects all of it together is that prolonged wet, humid weather,” she said. “Disease needs some moisture, whether it’s rainfall or humidity or dew, to have that sporulation process take hold within our plants.”
Strunk, speaking during a recent SDSU Crop Hour webinar, said there are various points on the soybean plant where diseases can start.
“If we’re seeing it on the roots, or like those lower stems, inner nodes, generally it’s going to be seed-borne or soil-borne, that’s where we’re seeing that disease come from,” said Strunk. “If we’re finding it on the lower leaves of the plants, for example within the roots, it could be SCN (soybean cyst nematode) attacking the roots, it could phytophthora, SDS (sudden death syndrome), any of the other root rot diseases attacking those roots.”
Higher up from the soil surface, said Strunk, indicates other originations of the problems. “If we’re seeing on the lower leaves of the plant, it could be brown spot, like a residue borne disease, because we tend to see those residue borne diseases attack those lower leaves first because of that dew, humidity, rain causing the drops to come down; having that go up into the canopy where that sporulation will occur within, resulting in disease pressure.”
She also points out factors and conditions that make the formation of disease more likely. “The level of disease that we’re seeing out there results in how susceptible our variety is. How much inoculum is present or in the area? How conducive [is our weather]? Is it warm? Is it wet? Is it cool or is it hot and dry? And then the big kicker is really the growth stage of the crop,” she said, “when that disease starts to develop whether or not we’ll have a yield gain or yield benefit by utilizing that fungicide.”