A newspaper from the southern United States reported today that blue mold was found in a burley tobacco field near Abingdon, Virginia earlier this week. There are no predictions on the potential for spores from this outbreak to spread towards Ontario, because North Carolina’s Tobacco Blue Mold Forecast Centre is not issuing blue mold forecasts for the 2013 growing season.
The risk of blue mold is related to proximity to inoculum and weather conditions. The blue mold pathogen can be spread as windblown spores from southern growing regions. Once blue mold is present in a growing region, disease development depends on weather conditions. Blue mold spores require moisture on leaf surfaces to infect plants, and spores are killed by sunlight. Consequently, blue mold development occurs most rapidly during cloudy, wet periods, and the disease is slowed or stopped when conditions turn dry. Prolonged periods of leaf wetness are most favourable to the development of this disease.
As of today, there have been no reports of blue mold from Ontario tobacco, however growers should be monitoring their fields regularly for this disease. Tobacco leaves infected with blue mold show circular yellow spots on the upper surface of the leaf.
Greyish blue fungal growth is produced on the underside of these spots when the fungus is active, with the spots becoming light brown as the leaf tissue dies.
Curling and puckering of leaves may also occur. The disease first develops on lower leaves, but it rapidly spreads to upper leaves during wet, cool weather.
Growers are asked to report any suspected cases to the OMAFRA office in Simcoe at 519-426-4434 or 519-426-3823. More information on this disease can be found in OMAFRA Publication 298 – Flue-Cured Tobacco Production Recommendations or in the CTRF publication “Flue-Cured Tobacco Best Management Practices – Blue Mold” (September, 2004).
Aliette WDG, Quadris FL and Actigard 50 WG fungicides can be used proactively for the preventative control of Blue Mold in the field. Fungicides should be rotated to prevent the development of resistance, however growers should note that the restricted entry intervals, preharvest intervals and maximum number of applications per year differ among these three products. For example, Aliette cannot be applied within 5 days of harvest, while Quadris and Acitgard have a preharvest interval of 21 days. Refer to the product label for detailed instructions on how to apply these products.