Biology and Epidemiology

- Survival and sources of inoculum

It appears that Peronospora hyoscyami f. sp. tabacina does not overwinter from one year to another in countries with temperate climate. This seems to be the case in France and in many other European countries. Overwintering of the fungus probably occurs in one or more countries of the Mediterranean basin, where the disease is endemic. In some of these countries, tobacco stalks are not always destroyed at the end of the season. Peronospora hyoscyami f. sp. tabacina can survive as mycelium in the aerial parts of plants or in their roots. Some wild species of Nicotiana (Nicotiana glauca ...) can also contribute to the survival of the fungus during the winter. The role of oospores (figure 1) is not well known. Oospores may be the cause of primary infections in some nurseries. In France, these structures are no longer observed in tobacco leaf tissues at the end of the season.

The possibility that the fungus survives  and disseminates on cured tobacco has led to some controversy. Recent studies conducted in the USA have shown that Peronospora hyoscyami f. sp. tabacina was not alive after "curing" tobacco leaves infected with the pathogen

- Penetration into the plant and invasion (figure 2)

Sporangia ensure infection within hours, especially on the upper leaf surface. They germinate on the leaf surface and directly penetrate the cuticle and epidermal cells. Infection via the stomata is possible. Following infection the mycelium progressively invades the mesophyll cells. Its progress can be inter- and intra-cellular. It later invades the veins, petioles and vessels of the stem. Depending on weather conditions, the first yellow spots appear 3-7 days after the initial infection.


- Sporulation and dissemination of the fungus

The appearance of sporangiophores and sporangia is slightly later than the appearance of the visual symptoms. Sporulation is most abundant at temperatures between 15 and 23° C when the relative humidity above 95 % (figures 3 and 4). Sporulation is limited outside this temperature range, but may occur to some degree between 2°C and 30° C. Sporulation also requires a minimum period of darkness. Very hot and dry periods inhibit sporulation. Once produced, the numerous sporangia are released by the combined action of rising temperature and lowering humidity such as frequently occurs during the morning hours. The spores, once released, are picked up immediately by wind currents and can be transported over long distances (tens of kilometers).

The disease can also spread through movement of infected plants. In some countries farmers are specialised in the growing of seedlings which they produce in large quantities and ship to different tobacco production areas. Seedling infection can go unnoticed and infected seedlings can be shipped to distant tobacco growers, thus contributing to the early development of the disease and its subsequent dissemination in the field.

- Favourable conditions for the fungus development This fungus is an obligate parasite and highly sensitive to weather conditions. It is particularly favoured by long periods of cool, wet and cloudy weather (figure 5). Conversely, when the weather becomes warmer, to hot and dry, the mildew epidemic will gradually diminish.
Young tobacco plants are particularly susceptible. Leaves approaching maturity are much less so.

U.S. situation

In the USA blue mold epidemics usually spread from south to north. The disease is spread by airborne spores blowing from infected fields and plant beds. Blue mold is not known to survive through the winter in north of Florida. Initial outbreaks in the United States originate from airborne spores from winter tobacco crops in Cuba, Mexico, or Latin America. Wild tobacco plants (Nicotiana spp.) growing as weeds in the southwestern United States can also serve as a source of airborne inoculum.

(Mina Mila - North Carolina State University)