Dynamics of airborne Fusarium macroconidia in wheat fields naturally affected by head blight

Dispersal of Fusarium macroconidia was studied using a volumetric spore sampler that sampled air in winter wheat crops, with a natural Fusarium inoculum. The spore sampler was operated over a 3-week period around wheat flowering, between 1994 and 1997; in these years head blight ranged from traces to about 100% of spikes affected. The numbers of spores per m(3) air were counted and related to the meteorological conditions. An association was found between rainfall and peaks of the macroconidia sampled from the air. In particular, no or a very few conidia were sampled from the air before rainfall, but their number progressively increased during rainfall; in the presence of high humidity, conidia continued to be sampled at high densities for some hours after rain had ceased and they usually reached their peak under these conditions. Finally, density of the airborne conidia rapidly decreased when relative humidity dropped. Two regression equations were found, which accounted for the effect of meteorological conditions on the number of airborne macroconidia, in rainy (at least 0.2 mm rain) and non rainy days. These equations produced an accurate estimate of the dynamics of airborne conidia over the sampling season; they included average air temperature, amount and intensity of rainfall on the preceding day, and number of hours with high relative humidity (>80%). In the first equation, an empirical weight accounting for the pattern of airborne conidia over a sequence of rainy days was also included.