Bean pod mottle virus (BPMV) continues to pose a threat to soybean production by reducing soybean yield and affecting soybean quality. 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 percent (2006) of the approximately 1,200 soybean fields sampled and tested each growing season. By mapping BPMV incidence at the county scale, BPMV incidence (risk) was found to occur at varying intensities within and among Iowa counties. Generally, risk of BPMV tended to decrease in a gradient from south to north within county tiers (Figure 1).

Three potential sources of BPMV inoculum in Iowa are
•  overwintering infested bean leaf beetles
•  BPMV-infected seed
•  alternative weed hosts infected with the virus (primarily legume weeds such as Desmodium spp)

High levels of BPMV can develop if large populations of bean leaf beetles can successfully survive the previous Iowa winter.

Risk factors for high BPMV incidence
The survey data showed the latitude coordinate for Iowa county tiers greatly influenced the risk for BPMV, with southern most latitudes having the highest risk for BPMV incidence. Therefore, the further north an Iowa county is located, the lower the risk for BPMV (Figure 1). The fact that BPMV incidence differed among counties and years indicates the presence of risk factors associated with high BPMV incidence. One of the factors that we found to explain much of the variation in county BPMV incidence (risk) was the number of days that mean daily temperatures were below 32 F between October 1 and April 30. Time of planting was also associated with BPMV incidence. Early planted soybean fields had higher risk for BPMV incidence, especially following warm winters that favor bean leaf beetle survival.  

Figure 1. Relationship between BPMV incidence with county tier latitude

We have developed a preliminary, pre-plant BPMV prediction model that uses the number of consecutive days with mean temperature below freezing to predict the likely risk for BPMV at the county scale. The more winter days with mean daily temperatures below freezing, the lower the survival of bean leaf beetle populations, hence, the lower the risk for BPMV.  This model differs from Lam and Pedigo model, who used accumulated subfreezing air temperatures to predict winter mortality of bean leaf beetles. Our new model predicts the risk of BPMV, based on the three-year BPMV survey among Iowa counties.

Management of bean leaf beetles and BPMV
The overwintering and first summer bean leaf beetle generations are the most important in the spread of BPMV within soybean fields. A number of management practices are recommended for managing bean leaf beetles and/or BPMV. One of the recommended BPMV management practices is to minimize BPMV 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. 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.

This new model also can help growers in planning mid season foliar insecticide application, depending on the likely risk for BPMV in a particular season. Mid season foliar insecticide application should follow bean leaf beetle population density thresholds. Our field studies have indicated that early season source of BPMV inoculum leads to high end-of-season BPMV incidence and higher yield losses.

Predicted risk for BPMV in 2010
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, using our new BPMV pre-plant prediction model. Near the end of 2009 growing season, we surveyed nine counties (two to three fields per county), and our predicted risk for BPMV was very similar to the actual risk we 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 (79 counties (Figure 2). 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 (orange-colored counties). The Iowa 2009-2010 winter season was colder, compared to 2008-2009 winter. This means fewer bean leaf beetles would have survived the 2009-2010 winter.

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 in the green-colored counties. However, counties in the southeast 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. Erin Hodgson and Nick Schmidt recently published the predicted bean leaf beetle winter mortality for the southeast Iowa to be 82 percent.
 
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.

Figure 2. 2010 predicted BPMV incidence at the county scale

Emmanuel Byamukama is a post-doctoral research associate, Alison Robertson is an assistant professor, and Forrest W. Nutter is a professor, all based in the Department of Plant Pathology, Iowa State University.