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Surviving Blue Mold

December 11, 2020 |

On a cool, rainy summer day, a breeze rustles through the trees surrounding tobacco fields. Silently riding the wind are the spores of Peronospora hyoscyami f. sp. tabacina, otherwise known as tobacco blue mold. During summers like this one, when the rains just won’t stop and June feels more like early spring, conditions are just right for these microscopic trespassers to settle on tobacco plants, becoming more than just passing visitors. Settled, these blue mold spores establish a home—a place they can grow and spread to other tobacco plants.


And grow, they do. During cool, wet weather, blue mold can double every four days. In a matter of time, an infected tobacco leaf develops yellow-tinged spots with no clear borders, signs that a bluish-gray mold is taking hold on the underside of the leaf. Sometimes these lesions have a cupped appearance, and if left unchecked, they eventually fuse to kill entire leaves. The downy growth under the leaves produces spores that, once released, will hasten the spread of the mold.


In addition to leaf damage, tobacco blue mold can lead to systemic disease in tobacco plants. Once inside, the mold’s effects can reach every part of the tobacco plant. Necrosis of the stem, stunted growth and distorted new growth can result. Left unchecked, blue mold can kill the plant.


With all the strategies tobacco growers have learned over the years, the threat of blue mold infection has decreased, and large-scale tobacco blue mold outbreaks have become few and far between. Read on for more information on how growers should prepare and respond.


Blue Mold’s Achilles’ Heel


Despite seemingly unavoidable conditions contributing to the spread of tobacco blue mold, blue mold does have a nemesis: freezing temperatures. The climate in areas where tobacco thrives gives tobacco farmers an advantage over blue mold—the frosts and freezes that kill tobacco plants in the late fall also kill blue mold.


So why do blue mold outbreaks still happen occasionally in areas with this natural defense? According to Lindsey Thiessen, assistant professor and Extension specialist at North Carolina State University (NCSU), the spores are transported on tobacco transplants that are grown and shipped from frost-free areas like Florida.


“Spores are thought to travel to Florida from Cuba when wind dispersal and environmental conditions are conducive,” says Thiessen. Because blue mold is not known to survive the winter in areas north of Florida, growing your own transplants or buying transplants grown locally is a powerful preventive measure.


Remove Hidden Opportunities


The first step in avoiding blue mold damage is preventing this downy mildew in the first place.


Beware of greenhouses, which can—much less frequently—provide hiding spots that allow tobacco blue mold to overwinter. Nestled in the warm shelter of a greenhouse, blue mold may find a hospitable habitat to wait out chilly winters in float beds. Effective management of blue mold includes creating a disease-free environment to eliminate these potential safe havens by carefully disinfecting float beds and greenhouses before the growing season starts.


Farmers should also avoid growing tobacco in greenhouses between October and February, according to NCSU’s 2020 Flue-Cured Tobacco Guide. The guide also recommends spraying Dithane Rainshield weekly after plants reach the size of a quarter if blue mold becomes a concern.


Use Other Cultural Practices


Another way to minimize tobacco blue mold risk is to select plant varieties with resistance to the mold. Although they still have some susceptibility, partial protection gives plants a better chance when conditions are right and blue mold spores are present.


When moving tobacco seedlings from the greenhouse to their permanent home in the field, provide adequate space between plants. This precaution promotes air circulation between plants and allows plants to dry more quickly after rain, which helps reduce the opportunity for blue mold to take hold.


It is important to note that although crop rotation and destruction of roots of diseased plants can stop many other pathogens, neither practice thwarts blue mold from recurring each year because of the airborne nature of its spread.


Stay Aware and Prepared


Once transplanting is complete, keep an eye out for reports of blue mold, especially if the weather turns rainy and cool. Blue mold epidemics start suddenly, spread quickly and usually move from south to north. “When blue mold is observed in North Carolina, alerts are posted to the North Carolina Extension Tobacco Portal (,” says Thiessen. “Farmers can contact their county agents to get disease confirmation and report the disease in their area.”


Blue mold can multiply rapidly in favorable conditions, so take immediate action for best results. Many fungicides work better before blue mold is found on your crops. “Fungicides should largely be used preventatively or as disease is first detected. If plants are severely affected and the farmer chooses to remove them, completely removing the plants from the field and destroying them is most beneficial to prevent infections of healthy plants,” says Thiessen. 


“Fungicides are effective when applied appropriately,” Thiessen adds. “Recommendations are provided in the Tobacco Production Guide.”



Category: Diseases & Pests

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