Bacterial wilt is the most serious of the soil borne diseases of tobacco in South Carolina. It is very difficult to manage. The disease is concentrated in the eastern-most counties in the Pee Dee Region, but is present and increasing in severity in other important tobacco-producing counties.
Symptoms of bacterial wilt appear first as a wilt of leaves on one side of the plant. Eventually, the entire plant wilts, and infected plants usually die. Stalks appear dark brown or black at the ground level and look very much like black shank. However, bacterial wilt-infected plants have black streaks in the tissue just under the outer bark. Portions of lower stalk tissue will ooze milky strands of bacteria when placed in a clear container of water.
Bacterial wilt is a disease that is caused by a bacterium (Ralstonia solanacearum), which lives in the soil. These bacteria cause disease when they infect the roots through wounds. Any type of root wounding provides an entry point for infection. Therefore, shallow cultivation will help to avoid wounding roots, which provide points for infection. Natural wounds occur in the root system as a result of root growth through the soil; therefore, a certain amount of natural infection can take place, if the bacterial population is high enough in the soil around the root system.
The bacterium that causes bacterial wilt also infects a number of other crop plants, such as tomatoes, potatoes, peppers, eggplant and peanuts. Ragweed is a very common weed that is a host for the bacterium. Therefore, it is very important to recognize and control this weed thoroughly in areas planned for tobacco. The bacteria are very persistent in soil, and long rotations (three years or longer) may be necessary in some fields to assist in managing the disease. Rotation is imperative for management. Multipurpose chemicals (Telone C-17 and Chlor-O-Pic) also assist in control.
Several new varieties with high resistance are available, which also assist in control. New and older varieties with fairly high resistance include CC 13, CC 27, CC 37, CC 700, GL 939, K 149, K 346, NC 196, NC 299, NC 810, PVH 2110, SP 168, SP 210, SP 220, SP 225, SP 227, SP 234, SP 236, and SP NF 3 (tobacco variety test). Bacterial wilt MUST be managed by a combination of rotation, variety selection, and possible use of multipurpose chemicals. Other helpful practices include root and stalk destruction, enhanced soil drainage (utilize a high wide bed) and early shallow cultivation to avoid root wounding. It is also VERY IMPORTANT to avoid spread of bacterial wilt by movement of infested soil on farm equipment or by other means. The following tables show results of on-farm tests utilizing rotation, varieties and multipurpose chemicals for control of bacterial wilt.
It is generally believed that infection of tobacco in the field occurs through the root system. The rapid spread of bacterial wilt within South Carolina suggests that the organism is being spread in a more rapid and efficient manner that would be expected solely by the movement of soil on equipment. County agents in South Carolina have observed an increase of hollow stalk. Hollow stalk is a disease normally caused by an Erwinia soft rot bacterium. The use of new diagnostic procedures at Clemson University have allowed use to identify bacteria to species and many of the cases of hollow stalk have been identified as Ralstonia solanacearum, the causal organism of bacterial wilt of tobacco. If inoculated onto a cut tobacco stalk, Ralstonia solanacearum will invade the plant and produce symptoms very similar to hollow stalk disease.
Field trials conducted at the Pee Dee REC and on farm have shown that the bacterium can be spread very easily during mechanical topping and harvesting. If the topper was driven through infected tobacco the mechanical topper transmitted the pathogen easily to health tobacco. A 3-4 week delay was observed before symptoms appeared. The only effective method of removing the bacterium from the cutter blades was steam or a 50% Clorox solution. Further work is underway to define the role of mechanical topping, harvesting and stalk destruction on disease spread and to develop sanitation procedures to limit the spread of this devastating pathogen on machinery (see tables).
The following points should be considered to help control mechanical
transmission of bacterial wilt: