North Carolina Plant Disease and Insect Clinic
Plant Disease Fact Sheets

Bacterial wilt of Tobacco

Figure 1. Tobacco plant with typical Granville wilt symptoms. Note the wilting in one side of the plant.


Figure 2. Severe symptoms of Granville wilt. Plants stunded and wilted.


Figure 3. Treating soil with a fumingant (right) is a strategy to control the disease.


Figure 4. Stem pith with typical Granville wilt symptoms.


Mina Mila 

Introduction

Granville wilt continues to be one of the most destructive diseases of tobacco in North Carolina. It causes losses from over 1 to 2% of the entire tobacco crop, costing growers from $10 to $15 million annually. Granville wilt was first observed in 1880 on a few farms in the Northern areas of North Carolina's Middle Belt. During the next 30 years losses from this disease increased on farms in Granville, Vance, Wake and Durham counties to the point where it was causing 25 to 100% field losses. During that period this disease caused banks to close, farms to be sold, and towns to decline. Today it not only plaques the Middle Belt but is the most chronic disease problem in eastern North Carolina and in the Border Belt. Its more recent extension into the Old Belt also has made Granville wilt a statewide problem. 

Symptoms

The first symptom is a wilting on one side of the plant (Figure 1). As the disease progresses, the entire plant wilts and death generally follows (Figure 2). When death does not occur, plants are usually stunted and leaves may be twisted and otherwise distorted (Figure 3). The stalk usually turns black, especially at the ground level. At this stage, Granville wilt may be easily confused with other diseases such as black shank. Dark streaks can be seen extending up the plant just beneath the outer bark (Figure 4). Infection may not be noticed immediately because wilting symptoms may not appear until the plant undergoes moisture stress. It is not unusual to observe symptoms several weeks after initial infection

Causal Organism - Pathogen biology

Granville wilt is caused by microscopic bacteria (Ralstonia solanacearum). The bacterium is spread by anything that moves infested soil or water. Major means of spread include water, infected transplants and farm vehicles moving from field to field.  The motile bacteria gain entry into the plant through natural openings or wounds. Since roots often "wound themselves" as they grow, or are wounded during transplanting, the Granville wilt bacteria have no difficulty in gaining entry into the plant. More extensive root wounding caused by nematodes or root pruning during cultivation provide more paths of entry for the bacteria. Increased disease levels in the resistant varieties have been noted where root-knot nematodes are present. High populations (greater than 250,000 bacteria per gm of soil) are usually necessary for infection to occur. The bacteria may also be spread during mechanical topping and harvesting. These bacteria are favored by relatively high soil temperatures and adequate to high moisture levels in the soil. Poor soil drainage and wet, warm growing seasons favor Granville wilt

Diagnostic method in the field 

A simple diagnostic test for Granville wilt can be done on-farm. When an infected stem segment is suspended in a glass of clear water for a few minutes, bacterial streaming occurs. The bacterial streaming appears as white ooze or a smokey stream, which originates from the cut end of the stem, where the dark streaks are observed under the bark, and slowly moves out into the water.

Management

Crop Rotation - Crop rotation must be the basis on which Granville wilt management programs are established. This practice is perhaps the most essential thing that growers can do to minimize losses due to Granville wilt. In fact, without appropriate crop rotation it is not possible to manage this disease successfully where the infestation level is moderate to high. Therefore, crop rotation must be the basis on which Granville wilt management programs are established. Crop rotation is effective because the Granville wilt bacteria live in the soil and are not well adapted to survival in the absence of susceptible plant tissue. Thus, their populations decline if a suitable plant such as tobacco is absent for even one year. As is true with any other soil-borne pathogen, the longer the rotation, the more efficient the control. However, planting a non-host crop (soybeans, fescue, corn, cotton, milo) just one year will usually significantly reduce the disease loss in the following tobacco crop. Integrating other management practices, such as improved drainage, avoiding late or deep cultivations, stalk and root destruction and the use of multi-purpose fumigants where disease occurred in past years, is better than relying on any one or two practices. 

Granville wilt is most damaging in fields where tobacco was grown the previous year, in wet areas in a field, and in years where soil temperatures are normal to above normal. Other plants the bacteria can infect include tomatoes, Irish potatoes, pepper, eggplant, peanuts, and weeds. 

Stalk and Root Destruction (R-9-P) - 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 are available which carry varying levels of resistance to Granville wilt. None of these varieties is immune to this disease and some losses might be expected in severely infested areas with the use of any variety. Nevertheless, growers have an opportunity to select those varieties which will afford them, in most cases, good protection when used in combination with other disease control practices such as stalk and root destruction and crop rotation. Consult the most recent issue of the Tobacco Production Guide for resistance ratings of currently available varieties. 

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 (Figure 3). All require a 3-week waiting period between time of application and transplanting. Always read and follow label instructions. Consult the most recent issue of the Flue-Cured Tobacco Production Guide and the North Carolina Ag Chemical Manual for more information.

Other Resources:

Tobacco Production Guide:

http://ipm.ncsu.edu/Production_Guides/Flue-Cured/flue_cured.pdf

North Carolina Ag Chem Manual: http://ipm.ncsu.edu/agchem/6-toc.pdf

 

 

Revision date: April 2010