Tobacco blue mold was reported last week in two southernPennsylvania greenhouses.  North Carolina’s Tobacco Blue Mold Forecast Centre is not issuing blue mold forecasts for the 2012 growing season and there will be no predictions on the potential for spores from this outbreak to spread towards Ontario.  As of today, there have been no reports of the disease from Ontario, however growers should be monitoring their fields, and any plants still in the greenhouse, regularly for this disease. 

 The risk of blue mold is related both 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. 

 Tobacco leaves infected with blue mold show circular yellow spots on the upper surface of the leaf (Figure 1). 

Figure 1 – Yellow blue mold lesion on upper surface of tobacco leaf

 Greyish blue fungal growth is produced on the underside of these spots (Figure 2) when the fungus is active, with the spots becoming light brown as the leaf tissue dies. 

Fungal growth on tobacco leaf underside corresponds to circular lesion on upper surface.

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 (Figure 3). 

Tobacco field infected with blue mold.

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”.  

 Aliette WDG, QuadrisFLand 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.  The recommended rate of Aliette varies with plant growth stage – consult OMAFRA Publication 298 for more information.