Abstract
Pleospora herbarum (anamorph Stemphylium vesicarium) is the causal agent of purple spot of asparagus. Airborne concentrations of P. herbarum ascospores and conidia and Alternaria sp. conidia were monitored using a Burkard volumetric spore sampler for three growing seasons in two Michigan no-till asparagus fields. While P. herbarum ascospores and conidia and Alternaria sp. conidia were all detected early in the sampling season, most P. herbarum (greater than 78%) conidia were sampled following the last spear harvest in each field each year. This suggests that conidia were primarily responsible for disease on the fern. Diurnal periodicities were observed for atmospheric concentrations of P. herbarum and Alternaria sp. conidia with peak concentrations from 700 to 1300 h and 900 to 1700 h, respectively. P. herbarum ascosporal concentrations did not follow a diurnal pattern. The concentration of airborne Alternaria sp. conidia was positively correlated with airborne P. herbarum conidia, but not with P. herbarum ascospores. The results of this study suggest that an Alternaria disease forecaster such as FAST or TOM-CAST could be useful to predict disease on asparagus ferns since P. herbarum conidial concentrations were positively correlated with Alternaria sp. conidial concentrations. TOM-CAST has been adopted by most growers in Michigan and has been validated as a useful tool to prompt chlorothalonil applications to 'Jersey' cultivars of asparagus. As the asparagus industry transitions from 'Jersey' to 'Milennium' cultivars, new fungicides become available, and weather parameters not included in TOM-CAST are identified as key for disease development, modification of TOM-CAST may be necessary for satisfactory disease control.
Published Version
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