Protection Methods
 

- During cultivation
 
 Unfortunately there is no method in the nursery to "heal" the plants that have been attacked by Thielaviopsis basicola. Considering the risk of spreading the disease, we recommend removing diseased plants. This is especially important for seed producers.

Fungicides belonging to the family of benzimidazoles (benomyl *, thiophanate-methyl) or certain sterol biosynthesis-inhibiting fungicides (triadiménol. ..) can be active against this fungus. They are used preventively in some countries to soak the soil or substrate bearing tobacco plants. Their use as a remedy is possible only in specific contexts: such as the lack of opportunity to obtain or reproduce other seedlings, or early invasion resulting by poor planting conditions. The effectiveness of such treatments is not guaranteed. Moreover, their frequent use present certain risks related to the selection of resistant strains. This has already been reported in the case of benomyl.
 
Note that no fungicide is currently registrated in France for control of this disease.
 
The situation described above is similar in open field. One can still try ridging the plants to promote further rooting. At the end of the production season, it is vital that the tobacco stems and root systems are destroyed and removed from the field, in order to avoid increase in the soil of the decomposing debris and chlamydospores.
 
 
- Next crop
 
It is important to use healthy substrates in the nursery. The soil of traditional nurseries should be disinfected with a fumigant. The plants produced in mini-clods (plugs) should not be placed directly on the ground, especially if it has not been disinfected, instead a plastic film should separate them. Preventive treatments are sometimes applied in the nurseries with the fungicides mentioned above. We also highly recommend application of hygienic measures (Proper sanitation) described in the section of Nursery (seedbed) and planting diseases.
 
In the field, crop rotations should be used to prevent an increase of inoculum in the soil. In order to be effective a rotation should be long enough, at least 4 to 5 years, and should not include sensitive crops. Thielaviopsis basicola was observed on many plants: several Solanaceae plants (tomato, eggplant), field industrial crops (cotton, peanut, pea, bean, soybean, alfalfa, lupine) and various ornamentals (chrysanthemum, geranium, ...). Thielaviopsis basicola strains isolated from tobacco are clearly polyphagous, but their complete host range is not precisely known yet. The plants listed above are potential hosts of the fungus; however different plants may be hosts for only some particular strains. Cereals are a good rotational crop, as well as rice and sorghum.
 
Note that the cases of Thielaviopsis basicola "resistant soils" have been reported in several countries. Although present in these soils, the fungus causes no damage in tobacco. In USA the mechanism of "soil resistance" might be influenced by pH, the amount of exchangeable aluminium and the saturation percentage of cation exchange capacity of the soil. In other cases, the soil resistance might have a biological origin such as the antagonistic activities of Pseudomonas fluorescent.
 
Soil disinfection is sometimes achieved with varying degrees of success in field. This is a very expensive method and can be considered only occasionally and in few countries, and in any case not in "resistant" soils.
 
The tools used for tillage in contaminated fields should be carefully cleaned before used in other healthy fields. The same principle applies for the tractor wheels. A thorough rinse with water and disinfection of such equipment is often enough to remove the soil and Thielaviopsis basicola.
 
Field draining, plant fertilisation and irrigation should be perfectly controlled, and the soil pH should be maintained around 6. For this reason a moderate use of lime is advised. One should be aware of certain organic matters if they are added to the soil.
 
In France growers may have an analysis of their soil in order to approximate the rate of inoculum of Thielaviopsis basicola. When they grow a susceptible variety, they can avoid planting it to a highly contaminated field. Heavy and wet soils should be excluded from tobacco cultivation, especially if a susceptible variety is chosen. Planting should take place under good weather conditions, in any case not during a rainy and cold period.
 
The use of resistant varieties is by far the most effective means to control black root rot. Nowadays there are a number of Virginia and burley tobacco cultivars with a high level of resistance. This resistance originates from Nicotiana debneyi which hybridizes relatively well with cultivated tobacco. It is of monogenic dominant type.
 
Some varieties show a "tolerance" to black rot, which appears to be related to a greater root system . The lower susceptibility mainly reflects a higher growth of plants. Unfortunately these varieties multiply abundantly reproduction structures of Thielaviopsis basicola at their roots and thus contribute to increasing the inoculum level among tobacco fields. Their use can be catastrophic with regards to pest management of soils. On the contrary this is not true for varieties with monogenic resistance.

*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.

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