In such cases, long-distance transport of inoculum has been documented. It is not uncommon to experience no blue mold in a given year, or to have local areas that periodically escape the disease. The difference between continuous and discontinuous epidemic fronts is related to inoculum dispersal patterns, localized weather, density and spatial aggregation of tobacco fields within a production region, and planting schedules. The occurrence, intensity, and distribution pattern of blue mold can be greatly affected by coordinated efforts to manage the disease with chemical fungicides. Once the airborne sporangiospores arrive during the morning hours at the leaf surface, with free water, germination and infection can occur in as little as 2-4 hr.
Incubation becomes longer with less than ideal conditions and with the age of the tobacco plants.
The latent period for sporulation is generally 5-7 days. Sporulation can occur on the day symptoms first appear, but it usually occurs on the following night. Day temperatures in excess of 30 C for more than 6 hr have been reported to inhibit sporulation the following night. Spore liberation occurs with a rise in temperature, a decrease in relative humidity, and a change in turgor pressure within the leaf. Sporangiophores of P. tabacina react to dry air by desiccating and twisting counterclockwise.