Protection Methods

- During cultivation

There is no effective method to control this disease once it has occured in a tobacco field. The diseased plots should be worked last. Root injuries should be avoided. The equipment and tools used must be disinfected with formalin water or bleach. After sucker or top removal by hand, workers should wash their hands. It is also strongly advised to remove and destroy the root systems and stalks at the end of the crop. This measure will protect the soil from re-infestation by bacteria residing in these organs.

- Next crop

In the countries and regions where Ralstonia solanacearum is a major threat to tobacco growing, disease control requires the combined implementation of several complementary control methods.

Nurseries must be grown in plots that have not had susceptible crops. The water quality will be monitored. Water from canals or rivers may be contaminated. This does not seem to be the case for well water.

When planning crop rotation the presence of Ralstonia solanacearum should be taken into account. Management of fields should be a concern for each grower who wants to preserve the soils and delay as much as possible the development of soil-borne diseases. Rotation is not easily applicable in the case of Ralstonia solanacearum, given its host range (figure 1). The longer a rotation can be maintained, the lower the soil inoculum rate will become. Rotation will be most effective if resistant varieties of tobacco or other host crops are grown. Fescue and soybean are frequently recommended as rotation crops.

Several resistant "flue-cured" tobacco varieties are currently available. In order to create them, different sources of resistance were used:
- high resistance, polygenic and recessive, obtained from 448A IT (United States);
- moderate resistance, monogenic and partially dominant, obtained from Japanese cultivars.

Other sources of known resistance are being studied. The combination of these different genes in one variety seems to be the strategy adopted by breeders trying to obtain a stable high-level resistance. In many situations, resistance is not complete, and some plants may show symptoms of varying severity.

Several different agricultural practices will ensure maximum benefit to the plants. Host crops should be grown in fields with adequate drainage. Avoid excess irrigation.  A balanced fertiliser, without excess nitrogen should be applied. If nematode damage is possible, measures for control should be taken (see the fact sheets of  Globodera tabacum and Pratylenchus spp.).

Immunisation tests with a hypovirulent strain of Ralstonia solanacearum have been reported in Japan. The effect of this bacterium applied in the days before or after planting tobacco leads to a significant reduction of the disease.

U.S. situation

In southeast U.S. an intergrated approach is used to manage bacterial wilt including the following practices:

Crop Rotation -  This practice is perhaps the most essential thing that growers can do to minimize losses due to Granville wilt. As is true with any other soil-borne pathogen, the longer the rotation, the more efficient the control. Multi-year studies have shown that best crops to rotate with are fescue, soybeans and small grains; however if there is a problem with root-knot nematodes in the same field where Bacterial wilt occurs soybeans are not a good rotational crop.

Stalk and Root Destruction - Roots and stalks from the previous crop should be destroyed as soon as possible after harvest. The decay of old plant residue through stalk and root destruction soon after harvest decreases the number of bacteria present in the soil. 

Resistant Varieties - Varieties which carry varying levels of resistance to Granville wilt are available. None of these varieties is immune to this disease and some losses might be expected in severely infested areas with use of any variety. In most cases, good protection is provided when resistant varieties are used in combination with other disease control practices such as stalk and root destruction and crop rotation.

Chemical Control - The fumigants Chlor-O-Pic 100, Telone C-17, Telone C-35, and Pic+ may help control Granville wilt if used in combination with other cultural control practices. Although not mentioned in the fumigant label, in U.S. it is recommended to have a 3-week waiting period between time of application and transplanting as phytotoxicity may be observed otherwise. The following rates are used in tobacco fields in U.S. Actual control may vary due to other control practices and environmental conditions.

(Mina Mila - North Carolina State University)