ANNUAL REPORT
Defining the spatial distribution and movement of inoculum of Rhizoctonia solani AG1 for the
precision management of aerial blight on soybean
Early Soybean Production System
INVESTIGATORS: W. Scott Monfort and Craig Rothrock
In 2010, 2 producers’ fields near Hazen were used for the research. In addition, a field near
Stuttgart that was used in 2009 was assessed for distribution of populations of Rhizoctonia solani
prior to planting rice. For each field 96 GPS locations were assigned to represent the fields in
2010. For the field near Stuttgart 48 GPS locations have been used for sampling. In each field,
soils were assayed at each location for populations of R. solani twice for the Hazen fields and
once for the Stuttgart field. In addition, plants were sampled from each GPS location for the
Hazen fields two times during the vegetative stage and disease was assessed late in the season.
This data will be used to develop spatial maps of distribution of the pathogen, disease, and yield.
Various methodologies in 2010 were explored to determine the most efficient means to
quantify inoculum of Rhizoctonia species in the soybean / rice fields. The four methods tested
were: the multiple pellet soil sampler, elutriation of soil organic matter, suspension of surface
organic matter in selective media, and the toothpick baiting method. Of the four methods, it was
determined that the toothpick baiting method offered the most efficient means of recovery of
isolates of interest. This methodology was favored due to the ability to rapidly assay a larger
amount of soil than the other methods with little expense. Through testing, it has been
determined that this method does require controlling soil water to allow saprophytic growth of
the fungus in each sample to colonize toothpicks and assess inoculum potential for a site.
Because of the need to control and have a uniform soil environment for each sample, future soil
samples will be taken by GPS position in each field and brought back to the lab to be assayed as
opposed to the field assay previously conducted. From this research a standardized protocol for
population assessment has been developed to map populations in fields for subsequent work.
Aggregation of inoculum of R. solani AG1 in fields was documented. However, the dry
conditions that occurred during the growing season in these field in 2010 likely contributed to a
high degree of sampling error brought about by a lack of saprophytic growth of the pathogen. In
effect, the amount of recovery was not indicative of the amount of inoculum present as the
fungus was more dormant than active due to dry conditions.
The pathogen was recovered from plants in the vegetative stage, indicating the progress
of plant colonization by R. solani. However, the dry conditions showed reduced colonization by
the pathogen compared to 2009. In one field at one sampling time, recovery of R. solani AG1
was high. This area was in the process of being furrow irrigated and was close to saturation for
the duration of the 48 hr sampling period. Additionally, Rhizoctonia solani AG11 was found at
high levels throughout the sampling areas of both fields. This population of R. solani has been
shown to be weakly pathogenic to soybeans and rice but its importance, possible influence, or
involvement in the sheath blight / aerial blight pathosystems in unclear. Aerial blight did not
develop past the initial colonization of plants early in the season in the fields assessed in 2010.