Is there any possibility that some advanced biowarfare program could conceivably make a contagious agent of some kind (Parasitic? Viral? Fungal? Your choice) that affects the growth of tobacco crops?
There are a number of things in nature that attack tobacco plants. Way back in 1931, these were sufficiently well known that Nature made the following claim: "In recent years investigators have recognised many virus diseases of the tobacco plant..." Even earlier, in 1908, there was a detailed scientific report on fungus attacking the roots of tobacco plants. More recent studies have explored tobacco necrosis virus, the fungus Glomus macrocarpum, bacteria, and other problems. There's an entire website, tobacco-diseases.info, that lists many of these conditions.
Your biowarfare lab certainly has a lot of options for a starting point. If I were writing the story, I'd have the lab genetically modify the fungus Phytophthora nicotianae, more commonly known as black shank. According to the American Phytopathological Society, "Black shank can be a devastating root and crown rot disease of all types of tobacco, with losses in individual fields reaching 100%. The disease was first described from Indonesia in 1896, but has since spread to most major tobacco growing areas worldwide." NC State calls black shank "among the most destructive and widespread of all tobacco diseases in North Carolina." The picture below shows the effect of black shank on tobacco plants, with wilting and yellowing leaves. Your genetically modified fungus could become even more aggressive and travel more effectively than the regular fungus. Using an existing disease means it would take longer for tobacco growers to realize they were under attack.
Model that anti-tobacco agent (fungus) on the Chestnut blight
The American Chestnut tree was nearly wiped out by a blight. (It's not the only plant to run into such problems, but this particular case is well documented and well known).
The pathogenic fungus Cryphonectria parasitica (formerly Endothia parasitica) is a member of the Ascomycota (sac fungi) taxon. It is native to East Asia and South East Asia and was introduced into Europe and North America in the 1900s.1 The fungus spread rapidly and caused significant tree loss in both regions.
The bio warfare lab would need to synthesize a similar blight that targets the tobacco plant.
Another angle to take is a bacterium based approach, like the currently challenge with the Florida Citrus Disease aka Huanglongbing
Huanglongbing (HLB; citrus greening) is thought to be caused by the bacterium, Candidatus Liberibacter asiaticus.