AF36 Competitively Excludes Toxin Producing Fungi
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The species Aspergillus flavus exists throughout Arizona’s environment in agricultural and desert soils, in dust, in the air, on crops, and on native plants. Aspergillus flavus communities are composed of many genetically isolated groups or strains and these strains have different abilities to produce aflatoxins. Some strains, like the S strain, produce very high levels of aflatoxins. It is not unusual for an S strain isolate to produce over 1-million ppb in laboratory tests. Some strains produce much less aflatoxin , and atoxigenic strains, such as AF36, produce no aflatoxins. Strains often interact and compete during crop infection. This interaction influences the quantity of aflatoxins produced.
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Not only does AF36 produce no aflatoxin, it interferes with aflatoxin production by other strains. AF36 reduces contamination in bolls in which it naturally occurs. However, only a small percent of the Aspergillus flavus in Arizona is AF36. Treatments seek to increase the proportion of Aspergillus flavus that is AF36. |
The composition of strains existing in soils and crops in Aspergillus flavus communities differs between fields and areas in both strain composition and average aflatoxin producing potential. Proper application of AF36 allows this atoxigenic strain to competitively exclude aflatoxin producers . This change s the mixtures of fungi in treated fields so that the fungi have a lower potential to produce aflatoxins. The end result is less aflatoxin in crops grown in those fields.
The objective of AF36 treatment is to replace aflatoxin producers with AF36. This reduces the ability of fungi within treated areas to produce aflatoxins. |
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Aflatoxin Contamination Occurs in Two Phases
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Phase One The crop is first contaminated when Aspergillus flavus infects the developing bolls through wounds or cracks. Pink bollworm damage has often been associated with contamination. However, any wound to the developing boll may be sufficient to permit Aspergillus flavus infection.
Phase Two The second phase of contamination is usually most important in Arizona. It occurs when mature seed (open bolls, seed cotton, or ginned seed) is exposed to high relative humidity (above 85%) or rewetting while temperatures are warm (above 80° F). Aflatoxin producing fungi are blown onto Arizona seed-cotton shortly after boll opening. If environmental conditions cause seed moisture to rise above 12%, these fungi will infect and contaminate the seed. |
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Aflatoxin increases during the second phase may occur both before and after harvest. Thus, while the crop is held in the field awaiting harvest or in modules waiting ginning, toxin contents may gradually increase. Increases also may occur in seed piles and in the hands of the end user. Rank cotton, dense canopies, dew, and late irrigation increase the severity of contamination in the second phase. Early harvest reduces the second phase of contamination.
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How to Use AF36
AF36 is formulated on sterilized (the germ and undesirable microbes are killed) wheat seed. The wheat seed serves as both a carrier and as a food source for the fungus. To be effective, AF36 must grow on the wheat seed and produce spores. Growth initially appears white and fuzzy and then becomes greenish as the spores form and powdery green as the spores mature. The spores then spread throughout the field and compete with aflatoxin producers. For the fungus to grow there must be adequate humidity (generally supplied by irrigation) and temperature (above 70° F). |
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When crops are grown under conditions favorable for aflatoxin contamination, the quantity of Aspergillus flavus in the environment increases. In the warm Arizona cotton seasons, irrigation and periods of high humidity stimulate rapid fungal growth. AF36 is applied either just before or during the early stages of Aspergillus flavus increases. AF36 competes with aflatoxin producers and replaces them without increasing either the overall quantity of fungus in the field or the amount of the crop infected. This results in fewer aflatoxin producers being associated with the crop and, as a result, less aflatoxins. Furthermore, when AF36 infects a boll together with a toxin producer, it inhibits aflatoxin production. This is a second mechanism by which AF36 reduces contamination.
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| In theory, application of an atoxigenic Aspergillus flavus strain (i.e., AF36) at lay-by should give the atoxigenic strain preferential exposure to debris from the rapidly developing crop. This provides an advantage in the competition for crop resources during infection and during Aspergillus flavus population growth associated with crop production. However, the best time to treat may vary depending on soil type, planting date, and irrigation practices. Dry, sandy soils may require later treatment. |
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Use of AF36 changes the types of the fungi associated with the treated crop so that the atoxigenic strain (AF36) is very common and the incidence of aflatoxin producers is greatly reduced.
However, influences of AF36 treatments extends beyond the treated crop. Treatments provide beneficial displacement even in fields adjacent to treated fields and over multiple years.
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Just as AF36 disperses to neighboring fields, the aflatoxin producers from untreated fields can move into treated fields on dust, insects, and wind. Multi-year, area-wide approaches should allow reductions over entire areas and thus reduce the incidence of aflatoxin producers carried on wind and dust.
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Fungi that make aflatoxins typically survive until the next season as spores and on crop debris. Similarly, some of the influences of AF36 applications remain between seasons. Thus, even one year after treatment, there is more AF36 and fewer aflatoxin producers in treated fields than prior to treatment. This suggests that multiple years of treatment will result in cumulative benefit and that the incidence of aflatoxin producers might be reduced to low levels over an entire production area through the use of AF36 treatments in an area-wide aflatoxin management program.
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| This Yuma Valley field was treated only in 1996. That single application had an influence that was still seen in the results of the 1999 soil analyses. |
Aspergillus flavus typically becomes associated with crops in the field during crop development, and remains associated with the crop during harvest, storage and processing. Thus, crop vulnerability to aflatoxin contamination remains until the crop is ultimately used. Even aflatoxin that may form after cottonseed is marketed (i.e. at the dairy) can be attributed to the initial cottonseed supplier during litigation. Atoxigenic strains seeded into agricultural fields prior to crop development remain associated with the crop until use and may provide long-term postharvest protection from contamination.
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Improve the Aflatoxin Picture
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- Participate in area-wide AF36 treatments
- Harvest early, harvest dry. Tarp modules.
- Reduce Irrigation after initial boll opening.
- Do not mix seed from spindle picked cotton with seed from ground gleaned cotton.
- After ginning, keep seed from different fields separated until after aflatoxin content is determined.
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