Fusarium Head Blight Inoculum: Species Prevalence and Gibberella zeae Spore Type

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Abstract
The objectives of this study were to examine the relative abundance of Gibberella zeae ascospores and conidia and other Fusarium species on wheat spikes in a field environment, to relate inoculum counts of G. zeae to airborne spore counts, and to evaluate an inoculum bioassay technique. The inoculum levels of Fusarium species and airborne spores of G. zeae were measured in North Dakota during the 1999, 2000, and 2001 growing seasons. Spores were collected from wheat spikes in a 24-h potted-plant bioassay in a fallowed field and in a spring wheat plot bioassay. Inoculum levels of Fusarium species were assessed by placing a solution recovered from bioassays on selective medium; meanwhile, ascospores and conidia of G. zeae were enumerated microscopically. A Burkard cyclonic sampler measured airborne spore levels in the fallowed field. Wheat spikes were inoculated with known concentrations of conidia or ascospores, and rinsate was put on selective medium at different intervals to compare recovery rates. Known concentrations of both spore types were also applied directly to selective medium to compare with recovery of spore types from inoculated spikes. Fusarium graminearum was the most prevalent Fusarium species on wheat spikes, although F. moniliforme and F. poae counts were highest on some days. Approximately twice as many ascospores were recovered in both the 24-h potted-plant field bioassay and the cyclonic sampler as were conidia. Significantly more colonies were recovered from wheat spikes after conidial inoculation than after ascospore inoculation at an identical concentration regardless of time of rinsate collection. Colony numbers did not differ significantly following application of ascospores and conidia to selective medium. Results confirm the predominance of G. zeae inoculum in North America but indicate conidia play an important role in the primary disease cycle.