Tobacco

Tobacco plants must develop a healthy root system.,Nematode damage results in disease entry and loss of plant productivity – and, ultimately, loss of profits. The primary tobacco diseases, such as Granville wilt, black shank, bacterial wilt and Fusarium wilt, can gain entry to previously healthy plants after nematodes provide an entry point.

To help control nematodes and manage diseases, tobacco producers rely on Telone® soil fumigants as the foundation for maximum yields and profits.

Diseases and nematodes come in a number of combinations. The major diseases that attack tobacco in the United States are Granville wilt and black shank, while the major nematodes include southern root-knot and tobacco-cyst nematodes. Nematodes often cause the initial injury to roots that provides an entry point for bacterial and fungal diseases.

  • Granville wilt, a bacterial disease, often makes its way into the root system of tobacco plants through entry points opened by nematodes. It causes the most damage in wet areas, in fields where tobacco was previously grown and in years when soil temperatures are normal or higher. Symptoms of Granville wilt include wilting that can lead to death, stunting, distorted leaves and a black stalk. These symptoms can be seen several weeks after the initial infection.
  • Black shank, a fungal disease, is one of the most prevalent and damaging tobacco diseases. It is a warm-weather disease that can be found primarily in poorly drained locations where tobacco was previously grown. Symptoms include rapid yellowing and wilting, a dark-colored lesion at the base of the stalk, and decayed roots and crowns. Black shank is spread when infested soil is moved from one area to another.
  • Southern root-knot nematodes are named for the galls they create on plant roots that look like knots in a rope. These nematodes are the world's most damaging nematode in tobacco. It causes the most economic damage of any nematode, due to its widespread distribution. They can be found in nearly any field, but are most commonly found in sandy soils. The incidence of other diseases increases significantly when these nematodes attack.
Tobacco-cyst nematodes are nematodes that continue to spread to new fields, farms and counties within Virginia, North Carolina and Maryland. The economic impact of cyst nematodes is twofold. Not only can the pests cut tobacco yields in half by severely retarding the plant growth, but cyst nematodes also weaken plant immune systems and create entry sites for secondary diseases, such as black shank and Granville wilt. 

Tobacco diseases affect tobacco yields, quality and profitability. In North Carolina alone, they accounted for an estimated loss of $109 million in 2008.

The primary tobacco diseases, such as Granville wilt, black shank, bacterial wilt and fusarium wilt, can gain entry to previously healthy plants after nematodes provide an entry point. Nematodes are microscopic roundworms that feed on roots, causing permanent damage and leaving roots susceptible to these diseases.

Tobacco is an excellent nematode host, resulting in disease-ridden crops losing up to 1,500 pounds per acre. Tobacco is highly susceptible to nematode damage, and as such, high priority must be given to nematode control and disease management each time tobacco is grown.

For more information on the economic impact of disease within tobacco production, go to the 2008 Tobacco Disease Annual Report .

Damage caused by nematodes is difficult to estimate because damage to roots may not be apparent in above-ground symptoms. Below are some descriptions of symptoms to help growers identify above-ground or below-ground nematode problems.

Above-ground symptoms

  • Severely infected plants may be stunted and may wilt under drought stress several days prior to noninfected plants.
  • Chlorosis, or yellowing, is not pronounced, but plants often express nutrient deficiencies, especially nitrogen deficiency.
  • Symptoms usually are first expressed in sandy areas of the field.

Below-ground symptoms

  • Nematode infections of roots result in swellings, or galls.
  • The galls are typically much smaller than those observed on hosts such as soybeans, okra or other vegetables.
  • Root galling results in reduced nutrient and water uptake and premature drought stress leading to yield losses.
  • The damage to the roots caused by nematodes provides an entry point for soil-borne diseases, such as black shank and Granville wilt.

In addition to monitoring visual symptoms, collecting soil samples is also an effective method to determine if you have nematodes.

Infestation Levels
To determine infestation levels of root-knot nematodes, examine the roots. First, observe the roots at random just after fall stalk and root destruction (immediately after harvest). You can estimate the infestation level by observing the galled area and using the following index:

Low infestation – 0 percent to 10 percent of root area covered with galls
Moderate infestation – 11 percent to 25 percent of root area covered with galls
High infestation – 26 percent to 50 percent of root area covered with galls
Very high infestation – 51 percent to 100 percent of root area covered with galls

Producing a good tobacco crop depends on what you do to control diseases and nematodes. To prevent and diminish losses from nematodes, experts recommend growers follow the "Big Three" management protocols. 

  • - Regular crop rotation
  • - Use of resistant varieties
  • - Use of a multipurpose fumigant, such as Telone® C-17 soil fumigant and nematicide

Rotation
Selection of rotation crops is very important, as it provides many agronomic benefits. Studies have shown that tobacco rotations are increasingly effective when used along with Telone C-17.

  • Length
    Significant disease and nematode problems are usually found in fields continuously planted with tobacco. Because crop rotation removes a suitable host for the disease-causing agent, the longer the rotation, the more beneficial it may be. Thus, a four-year rotation is more effective than a two- or three-year rotation. Where tobacco is grown continuously, growers are "feeding" populations of pests, thereby contributing to their buildup and the probability of severe disease problems in the future. 
  • Crops
    Grasses such as bahiagrass, bermudagrass, millet and sorghum are among the most effective crops in reducing soil populations of root-knot nematodes and should be grown at least one year before planting tobacco. Small grains, fescue, corn and sorghum also may be used for rotation with root-knot nematodes.

    Studies have shown that root-knot nematodes with tobacco rotations are increasingly effective when used along with Telone C-17.

    For tobacco-cyst nematodes, most crops - except for tomato, pepper and potato - are recommended for rotation.

Resistant Varieties
Resistant varieties are useful for control of some tobacco diseases, including nematodes.

Growers should not rely on the same variety for disease control year after year. Changing varieties is a good idea and should slow down the development of new problems.

Root-knot nematode resistant tobacco varieties are available for only the southern root-knot nematode. Tobacco varieties without this resistance should not be planted in fields where root-knot nematode damage is likely. Root-knot-resistant tobacco varieties should be expected to assist and not replace other root-knot nematode damage management practices.

 

Chemical Controls
Using a soil fumigant is highly recommended, combined with crop rotation and planting resistance varieties of tobacco.

Telone C-17 is a preplant soil fumigant that controls all major species of nematodes and has been proven effective for helping manage diseases such as Granville wilt and black shank.

Here are some of the secondary benefits of Telone C-17:

  • Crop grows off quicker
  • More uniform crop
  • Crop holds longer in the field
  • Healthier root system that can tolerate wet or dry conditions better

For more information on disease management, read the North Carolina Cooperative Extension Service's Flue-Cured Tobacco Guide .