Main Symptoms

Peronospora hyoscyami f. sp. tabacina develops on tobacco throughout the growth cycle. On seedlings, disease development is very fast. It quickly invades the leaves, covering them with bluish, fuzzy to felt-like masses of conidia. It also causes leaf distortion, occasional soft rot, and eventually death.

On mature plants, the fungus develops on leaves, causing yellow spots on the upper leaf surface which finally becomes necrotic (figures 1 and 2). Careful observation of the underside of the leaf, at the area where the spots are, reveals a bluish conidial mass that may cover the spots (figures 3 and 4).

This bluish sporulation is very characteristic of the disease, hence its English name: Blue mould. The lesions become quickly necrotic and turn light brown (figure 5). Blue mould spreads rapidly on the leaves if the weather conditions are favourable.

Some of the information on the worldwide disease distribution is obtained from surveys conducted by CORESTA (Cooperation Centre for Scientific Research Relative to Tobacco). The reliability of the information is not confirmed in all countries.

U.S.  situation

Occasionally spots from undetermined cause (called ghost spots) that are similar to lesions caused by blue mold have been observed in tobacco fields (Figure 1). These spots are different from lesions caused by blue mold because one cannot find gray mold on the underneath side of the leaf as is the case with blue mold. Thus it is recommended that agronomists always observe both sides of the leaf and look for the gray mold on the underneath side in order to confirm blue mold.

(Mina Mila - North Carolina State University)

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