Bean pod mottle virus (BPMV) continues to pose a threat to soybean production by reducing soybean yield and affecting soybean quality. However, Iowa State University entomologists say their prediction model shows the risk may be even lower than a year ago.
In 2009, pre-plant BPMV risk was predicted to be very low with only nine counties — all in southeast Iowa — predicted to have moderate risk disease. Near the end of 2009 growing season, the predicted risk for BPMV was very similar to the actual risk entomologists predicted for seven of the nine counties.
"Using the same model, the predicted pre-plant risk for BPMV incidence in 2010 is likely to be zero-to-low for most of the state," says entomologist Alison Robertson. "Soybean growers in southeastern-most counties, however, are likely to encounter low-to-moderate risk for BPMV incidence in 2010. This is because these counties had fewer days with temperatures below freezing during the 2009-2010 winter."
Overall, the 2009-2010 winter season was colder compared to 2008-2009, meaning fewer bean leaf beetles would have survived the winter, she adds.
"Given the low predicted risk for BPMV in 2010, overwintering and first summer generation of bean leaf beetles are likely to be too low to warrant the need for seed or foliar insecticides. However, for counties in the southeastern part of the state, scouting will be needed to detect spotty areas where bean leaf beetle populations may be high enough to warrant control measures," entomologist Emmanuel Byamukama says.
This growing season, we will be collecting soybean leaf samples and bean leaf beetles across the entire state to validate our prediction model as well as bean leaf beetle winter mortality.
In a recent 3-year survey study conducted during the 2005 through 2007 soybean growing seasons, BPMV was found to be one of the most prevalent soybean diseases in Iowa, with BPMV being detected in 10% (2005) to 40% (2006) of the approximately 1,200 soybean fields sampled and tested each growing season.
Potential sources of BPMV inoculum include overwintering infested bean leaf beetle; BPMV-infected seed; and alternative weed hosts infected with the virus. High levels of BPMV can develop if large populations of bean leaf beetles can successfully survive the previous Iowa winter.
The overwintering and first summer bean leaf beetle generations are the most important in the spread of BPMV within soybean fields, entomologists say. One of the recommended BPMV management practices is to minimize transmission early in the growing season by applying an insecticide seed treatment.
"Until now, insecticide seed treatment decisions had to be made without knowing the likely risk of BPMV," says entomologist Forrest Nutter Jr. "With the pre-plant BPMV risk prediction model, growers can now make bean leaf beetle and BPMV management decisions with a higher level of certainty."
The new model also can help growers in planning midseason foliar insecticide application, depending on the likely risk for BPMV in a particular season. Midseason foliar insecticide application should follow bean leaf beetle population density thresholds.
Iowa State's field studies have indicated that early season source of BPMV inoculum leads to high end-of-season BPMV incidence and higher yield losses.