Protection Methods

- During cultivation

Once the first spots are observed fungicides should be applied immediately. Several active ingredients are used to control Peronospora hyoscyami f. sp. tabacina: for further information on them, please, use the e-phy link.

As for metalaxyl, widely used in the past, there are now documented cases of mildew resistance to metalaxyl as well as mefenoxam, a more purified form of metalaxyl.

In addition to fungicide applications, a number of other measures should be followed. In nurseries, maximum ventilation is recommended in order to reduce the moisture content. It is wise to avoid sprinkler irrigation. If this is not possible, irrigation should take place when the weather is warm and early enough during the day so that the plants have time to dry. One may try to control the first outbreak(s) by treating nurseries with a high dose of a fungicide and by destroying infected seedling(s). Marketing of seedlings produced in such conditions is possible only if the epidemic of mildew is totally under control and farmers (buyers) are warned of the risks. Shelters should be thoroughly cleaned and crop residues destroyed.

In the field, the same recommendations with respect to irrigation should be followed. At the end of the crop, the stalks should be cut and plots quickly tilled to bury and destroy all plant residues along with "pathogen strains" that could enable the survival of the mildew from one year to another.

 
- Next crop

The seedling nursery for the next crop should be placed in a sunny location, taking particular care to avoid a damp and shady place. If it is established in the same location as the previous year, one is advised to apply the sanitation and disinfection measures recommended in the section Nursery and planting diseases.

Resistant or tolerant varieties are available in several countries, and now also in France. Different resistance sources from various wild species of Nicotiana (N. debneyi, N. goodspeedii, N. excelsior, N. velutina, N. rustica) have been used. Unfortunately, the resistance levels transferred to N. tabacum are not comparable to those expressed in wild species. Resistance is often incomplete and gradually becomes active as the plant grows. In fact, although representing a real asset, resistant varieties should be used in conjunction with a complementary chemical control program.

Given the rapid progress of blue mould and the risks it poses to the tobacco crop, preventive fungicide applications are necessary in the nursery and field. The choice of fungicides and application rates should be determined by a technical advisor according to local farming practices.

One should avoid establishing tobacco crops in poorly drained fields which show high potential for water retention and in soils excessive in organic matter. The fertiliser must be well balanced and definitely not excessive. When possible, planting densities should be reduced in order to have more air circulation among plants in the field. Other recommended measures during production could also be applied.

Lastly, several warning services exist in Europe (and Mediterranean basin) and the U.S. (now including some Central American countries and Canada). These services rely on observers' networks. They report the first outbreak(s) of blue mould observed and inform interested individuals on the potential for continental development of mildew epidemics. In the U.S., information is broadcast through various media that will inform where blue mould is a threat and when chemical control should start. In that country special models have been developed to predict the path of spores in the atmosphere and thus accomplish better control of mildew outbreaks.

In Europe, with the help of CORESTA (Cooperation Centre for Scientific Research Relative to Tobacco) warnings, one can reasonably argue that 1) each year, the blue mould epidemic is continental and it follows the same path. It first appears in North Africa, including Morocco and the Mediterranean basin (Syria, Iran). Then the epidemic spreads to northern Europe via Turkey, Greece, etc.; and 2) the blue mould pathogen no longer seems to survive from one year to another in some countries of northern Europe.

The CORESTA Blue Mould Early Warning Service was replaced by the  Euro-Mediterranean Blue Mould Information Service in 2005.  The new service functions along similar lines to the former CORESTA service under the auspices of the European Association for Tobacco Research and Experimentation (AERET), which is a group of tobacco field extension specialists from the EU.

These observations should allow us to significantly change the strategy for control of blue mould in northern Europe. Knowing the location of the disease throughout the country, we should at least be able to delay the first fungicide applications and thus reduce their number, and to use products other than metalaxyl or mefenoxam, especially when Peronospora hyoscyami f. sp. tabacina is reported in countries far away from the country of interest.

U.S. situation

The North American Blue Mold Forecast Center at NC State operated for 15 years issuing forecasts two or three times per week, and more often if necessary, from March through August. The forecasts were based upon daily occurrence reports from blue mold cooperators in tobacco-producing states in the United States, Mexico, and Canada. Meteorological surface wind models were used to generate reports of favorable weather conditions and of regional weather, as well as the outlook for new outbreaks (high, medium, or low risk). The forecasts were suspended at the end of 2011; however, additional information on the disease and control recommendations are available on the Blue Mold Forecast website. 

Because air currents disperse this fungus, crop rotation and stalk and root destruction do not affect this disease in the USA. Fungicides registered for control of blue mold in USA are:

Greenhouses are recommended to be treated with Dithane Rainshield (0.5 lb/100 gal. spray) every week after plants are the size of a quarter. 

Field applications are recommended until topping time. Research has demonstrated that after topping the physiology of the leaf changes and the blue mold pathogen is not very successful in penetrating the tobacco leaf and cause new infections.

(Mina Mila - North Carolina State University)

*The number of pesticides available for a specific use is constantly evolving but the author has nonetheless chosen to include the names of some registered active ingredients at the time of writing these pages. He will try to update this list as and when withdrawals and new registrations are made. When choosing your crop protection method, please refer to legislation prevailing in your country. This remark is also valid for all organic products based on micro-organisms or natural substances.