Blue mould (Downey mildew) is a disease of tobacco that can decrease yield substantially if infestation occurs early in the growing season. Infected seedlings will have yellow spots on upper leaf surfaces, with blue-grey mould on the leaf undersides. Seedlings can be killed by the fungus quickly. Lower leaves of larger plants in the field are infected first.
Similar symptoms occur – yellow lesions on upper leaf surface with a greyish mould on leaf undersides. Fungus travels through plant vascular system, causing wilting and plant lodging. Under ideal conditions there is potential for severe infestation as spore re-generation can occur every 10 days.