Several factors influence the severity of blue mold in Kentucky including condition of plant, weather and level of fungus. Generally, the burley tobacco plant becomes increasingly tolerant of blue mold with age, becoming very tolerant after flowering in most fields, unless new growth is encouraged. Conditions which favor rapid growth increase blue mold development. Older tissues may not become tolerant if the crop is over-fertilized with nitrogen, grown in the shade or receives abundant water. Apparently, previously tolerant tissues can become susceptible under such conditions.
Crops grown under stress tend to develop tolerance to blue mold at an earlier age.
The leaf must be wet for 4 to 6 hours or longer during the dark for infection or reproduction to occur. Conditions which prolong the length of the wet period generally favors blue mold development. For example, cloud cover, close spacing, shade, restricted air movement, rain, fog and heavy dew prolong leaf wetness and generally increase blue mold. The weather greatly affects blue mold because it affects both the susceptibility of the tobacco plant and development of the fungus. During such weather, leaves are wet for a long time, the fungus is protected from exposure to desiccation and ultraviolet light, and plant maturation is slowed while succulent growth is abundant.