Spatial Analysis of Aspergillus flavus S and L Strains
T. V. Orum, Plant Pathology Department
D. M. Bigelow, Plant Pathology Department
P. J. Cotty, ARS SRRC, New Orleans, LA
M. R. Nelson, Plant Pathology Department
The distribution of S and L strains of Aspergillus flavus is more stable
than previously realized. Analysis with GIS/geostatistics shows that
patches of similar S strain incidence persist over years. This information
will be exceptionally useful to programs involved with or planning large-scale
treatments to reduce aflatoxin contamination because it can be used to spatially
The Aspergillus flavus community at the soil surface is genetically diverse. The community
can be sub-divided into two strains (S and L) based on the morphology and growth rate of the
sclerotia (Cotty 1989). Because S strains consistently produce large amounts of aflatoxin
(>98 %), strain composition at the soil surface will influence the level of aflatoxin in the
crop. We have found areas in Yuma County that consistently average above 80 % S strain
incidence. A reduction in the S strain incidence in those areas has the potential to reduce
aflatoxin contamination in cotton. Some L strain isolates produce no aflatoxin at all
(Cotty et al. 1994). These isolates are of particular interest in aflatoxin management.
An experimental use permit for commercial field trials of atoxigenic strains to modify the A.
flavus strain composition was granted to P. J. Cotty in 1996 for treatments in 1996, 1997,
and 1998 (Cotty et al. 1997). Understanding spatial patterns of strain composition will be
useful in possible future treatments with atoxigenic strains.
The pattern and scale of the strain composition of A. flavus is determined by collecting
soil samples from the top .5 cm of soil and plating in the laboratory. Most biological
variables are spatially autocorrelated at some scale. Autocorrelation means that a variable
(such as the percentage of isolates that are S strain) is correlated with itself (auto) as
opposed to another variable (cross). We use the term "patch" to describe an area in which a
variable has similar values - i. e. an area in which there is spatial autocorrelation. The
range of distances over which there is spatial autocorrelation is an estimate of the patch
size. Since 1994, repeated sampling of soil in Yuma County, AZ cotton growing areas has
shown that S strain incidence is patchy and that patches persist overtime (Orum et al. 1997).
There is spatial autocorrelation over two ranges: 2.5 km and 25 km. Sampling of fields
adjacent to the originally sampled fields in July 1996, October 1996 and March 1997 have
demonstrated that patches of low S strain incidence extend beyond field boundaries and do not
correspond in an obvious way with the crop in the field or with field crop sequence. Many
locations have shown a very stable strain composition over time. For example, samples from
the corner of one field in the Texas Hill area of eastern Yuma County, has had an S strain
incidence between 80 and 100 % for eight consecutive sampling dates
(Figure 1). In another field in the area, strain S incidence
has been under 35 % seven of eight times (Figure 1).
Mapping strain incidence
Because S strain incidence is relatively stable over time, strain composition maps of
regions will be significant. In July 1997, we began sampling from other locations broadly
distributed in the Texas Hill and North Gila areas of Yuma County so that we can better map
S strain incidence. The point data can be interpolated using geostatistics to produce either
grid cell maps or contour maps. Variogram analysis shows a strong spatial autocorrelation with
a range of 2.5 to 3.5 kilometers. This supports the appropriateness of the geostatistical
approach to the mapping and is a refinement on our previous estimate of important spatial
structure between 1 and 5 km using the nested ANOVA's. Using July 1997 data,
we have generated preliminary sub-regional maps of the Texas Hill area. Much of
the area averages over 60 % S strain with a significant subset averaging over 80 %
However, there are two patches that average below 60 %
Using geographic information systems (GIS) software, we overlaid S strain incidence
contours onto USDA SCS soil maps. The patches of low S strain incidence are in different soil
types and the highest S strain incidence areas include diverse soil types. Therefore, our
analysis of soil characteristics needed to go beyond the USDA SCS soil classification scheme.
In collaboration with T. L. Vinnie and Dr. D. F. Post of the Department of Soil, Water,
and Environmental Science at the University of Arizona, forty six soil samples, taken from
the same sites as samples for analysis of strain composition, have been analyzed. Preliminary
results indicate a negative correlation between S strain composition and boron concentration
and a positive correlation with sand content
(Table 1). We are just beginning analyses of the soil data
and are cautious about interpretations at this point.
Knowledge of the distribution of S strain composition and/or soil characteristics that
support high populations of S strain can be very useful in focusing the application of control
measures such as the atoxigenic biocompetitive L strain being applied experimentally on
approximately 500 acres of Arizona cotton.
- Cotty, P. J. 1989. Virulence and cultural characteristics of two Aspergillus flavus strains
pathogenic on cotton. Phytopathology 79:808-814.
- Cotty, P. J., P. Bayman, D. S. Egel, and K. S. Elias. 1994.
Agriculture, aflatoxins, and Aspergillus. Pages 1-27 in: The Genus Aspergillus:
From Taxonomy and Genetics to Industrial Applications. K. A. Powell, A.
Renwick, and J. F. Peberdy, eds. Plenum Press, New York.
- Cotty, P. J., D. R. Howell, C. Bock, and A. Tellez. 1997.
Aflatoxin contamination of Bt cottonseed. Cotton, A College of Agriculture
Report. University of Arizona. Series P-108: 435-438.
- Orum, T. V., D. M. Bigelow, M. R. Nelson, and P. J. Cotty. 1997. Spatial
and temporal patterns of Aspergillus flavus strain composition and propagule
density in Yuma County, Arizona, soils. Plant Disease 81:911-916.
This is a part of publication AZ1006:
"Cotton: A College of Agriculture Report," 1998, College of Agriculture, The University of
Arizona, Tucson, Arizona, 85721.
Any products, services, or organizations that are mentioned, shown, or indirectly
implied in this publication do not imply endorsement by The University of Arizona.
The University is an Equal Opportunity/Affirmative Action Employer.
This document located at http://ag.arizona.edu/pubs/crops/az1006/az100610d.html
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