Spores were sampled during 2 years over wheat plots at Ottawa, Ontario. Plots were treated with corn colonized with Gibberella zeae (Schwein.) Petch (anamorph Fusarium graminearum Schwabe). In 1994, viable spores were sampled with four Burkard high-throughput jet samplers. Gibberella zeae ascospores were recovered mostly at night and showed four main release events during the 20-day sampling period, 1-3 days after rain events. Highest density of G. zeae spores (1500 spores/m3) were sampled 1.5 m away from the inoculum source, with fewer spores 5 m away. Recovery of otherFusarium species was sporadic. For all species, there were no statistical differences among the daily sampling times, although for most species, morning counts were the lowest. Other Fusarium species detected, in decreasing order of spore density, were F. crookwellense, F. sporotrichioides, F. moniliforme, F. equiseti, F. subglutinans, and F. culmorum. Most spore release events did not correlate with rainfall events. In 1995, a Burkard continuous 7-day spore sampler was used to investigate the release of ascospores and macroconidia of G. zeae. Ascospores, but not macroconidia, showed a daily periodicity. Daily average densities of macroconidia were an order of magnitude less than ascospores. Ascospore release was correlated with rainfall events and the time of day.Key words: spore sampling,Gibberella zeae, Fusarium graminearum, Fusarium sp. ascospores, macroconidia.

The spread of Fusarium head blight of wheat from a small area inoculum source was examined in wheat plots (100, 625, or 2,500 m(2)) inoculated in the center with Gibberella zeae-colonized corn kernels or macro-conidia sprayed on heads at anthesis. With the first inoculation method, disease foci were produced from ascospores released from perithecia formed on inoculated kernels. With the second inoculation method, disease foci were produced by macroconidia directly applied to the heads. Some plots were misted during anthesis. Plots were divided into grids, and disease incidence on spikelets and seeds was assessed at the grid intersections. Isopath contour maps were constructed using an interpolation procedure based on a weighted least squares method. Disease gradients were constructed from the isopath contours in the direction parallel to average nightly wind vectors using an exponential model. This study was conducted over a 3-year period at two sites: one in Quebec and one in Ontario. Both inoculation methods resulted in a discrete, primary focus of head blight in each plot, with one or two smaller secondary foci in some plots. The highest incidence of disease on spikelets or seed was commonly displaced somewhat from the inoculum source, usually downwind. The gradient slopes of seed and spikelet infection ranged from -0.10 to -0.43 m(1) in plots with ascospore inoculum and from -0.48 to -0.79 m(1) in plots inoculated with macroconidia. Seed infection declined to 10% of the maximum within 5 to 22 m from the focal center in asco-spore-inoculated plots, and within 5 m in a macroconidia-inoculated plot. Gradients were usually steeper upwind compared with downwind of the inoculum source. In misted plots, incidence of disease was higher and more diffuse than in nonirrigated plots. Based on gradients and dispersal patterns, disease foci in plots inoculated with G. zeae-colonized corn kernels probably arose from airborne ascospores rather than from splash-borne macroconidia and were the result of infection events that occurred over a short period of time. Comparison of conidial- and ascospore-derived disease gradients indicated a lack of secondary infection, confirming that Fusarium head blight is primarily a monocyclic disease.

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Corynespora cassiicola was pathogenic on the roots and hypocotyls of soybean seedlings grown in infested soil at Ottawa. Extensive superficial necrotic lesions developed on seedlings from inoculated seeds planted in the field in mid-May, but few symptoms appeared on seedlings planted in late May and in June. Infected plants were initially stunted but recovered with little effect on size or yield. In controlled temperature studies, severe root and hypocotyl rot were produced on inoculated seedlings at 15–20 °C. Symptoms were less severe at temperatures alternating from 15 or 20 °C to 25–35 °C and were negligible on seedlings kept constantly at 25–35 °C. Optimum growth of the fungus on agar media occurred at 20 °C. At 5 °C and 35 °C conidia germinated, but no appreciable mycelial growth occurred; at 30 °C growth was greatly restricted. Typical symptoms of target spot were produced on the inoculated foliage of soybeans in the greenhouse and in the field, but secondary infection was not observed in the field. Foliage infection occurred on cowpea (Vigna sinensis), sesame (Sesamum indicum), and Hartsville cotton (Gossypium hirsutum) in the greenhouse. C. cassiicola was isolated from overwintered soybean root debris, from the roots of mature field bean (Phaseolus vulgaris), and from the roots of soybean seedlings grown in soil not previously cropped with soybeans.

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