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PR-432

2000 Agronomy Research Report

Tobacco Research

Tobacco Breeding and Genetics

R.D. Miller, B.S. Kennedy, and E.L. Ritchey

The University of Kentucky and the University of Tennessee have merged their tobacco breeding programs to form the Kentucky-Tennessee Tobacco Improvement Initiative. The main objective of both breeding programs has been the development of tobacco varieties that have significantly higher levels of black shank resistance, combined with improved yield potential. Recent burley releases include KY 8959, KY 907, KY 908, and KY 910 from Kentucky and TN 86, TN 90, and TN 97 from Tennessee; dark variety releases include KY 190, TN D94, and TN D950. The merged program combines the strengths of the existing genetic engineering program at the University of Kentucky with the traditional tobacco breeding program at the University of Tennessee. The joint venture will allow enhanced efforts in developing new resistant burley and dark tobacco varieties.

Black Shank Resistance

Development of varieties with increased resistance to black shank, the most devastating disease of burley tobacco, will continue to be a primary objective of the new initiative. Several breeding lines currently being evaluated have displayed medium to high black shank resistance. These lines may be released as new varieties or used to produce improved tobacco hybrids. Lines containing the L8 black shank gene, previously developed at the University of Kentucky, will be crossed with Tennessee lines having resistance genes from the flue-cured variety Coker 371 Gold. This will produce hybrids that have resistance genes derived from Florida 301, L8, and Coker 301. These new hybrid varieties should do a much better job of controlling black shank than existing varieties, particularly in fields that contain a mixed population of race 0 and race 1 black shank.

Blue Mold Resistance

A new emphasis on incorporating blue mold resistance into existing varieties will also be an integral part of the new initiative. Because of the lack of a phytotron (which is necessary to safely contain the pathogen) at either the University of Tennessee or the University of Kentucky, breeding for blue mold resistance has been impractical due to the inherent risk of causing a widespread epidemic throughout the tobacco-producing region. The successful development of blue mold resistant tobacco varieties will require offshore breeding nurseries, which have been established in Guatemala and Mexico. Since blue mold generally occurs throughout Central America each year, establishment of nurseries in these countries will increase the probability of having significant blue mold pressure to screen for resistance in breeding materials.

Fusarium Wilt Resistance

The incidence of Fusarium wilt in Kentucky tobacco crops is increasing each year. The disease, caused by a soilborne fungus, has been only a minor problem for burley growers for decades due to the widespread planting of disease-resistant varieties. However, the majority of varieties planted today have not been thoroughly evaluated for resistance to this disease; many appear to have little or no resistance to Fusarium wilt. Commercial burley varieties will be screened during 2000-2001 to give growers more information about cultivar susceptibility. Germplasm and breeding materials are also being screened for resistance to the disease. Resistant germplasm will be used to incorporate fusarium wilt resistance into existing varieties. However, it will be several years before new resistant varieties will be available.

Release of KT 200

KT 200, a new black shank resistant burley hybrid, was released by the Kentucky-Tennessee Tobacco Improvement Initiative in April 2000. Black shank is prevalent throughout the burley-producing region of the United States and is responsible for millions of dollars in lost profits for tobacco farmers. The disease is becoming more widespread throughout Kentucky each year. Burley varieties are rated for black shank resistance on a scale of 0 to 10, with higher values indicating better resistance. At the present time, only three varieties have a resistance rating of 6 for both races of black shank. All three varieties have a yield potential rating of 3 or less, on a 10-point scale and are susceptible to tobacco etch (TEV) and tobacco vein mottling virus (TVMV). TN 90, TN 86, and KY 910 have a race 1 black shank resistance rating of 4, while TN 97 has a rating of 5. KT 200 has a resistance rating of 6, and data collected in numerous trials suggest that a yield rating of 6 is appropriate. In addition to black shank resistance, KT 200 has high resistance to black root rot, wildfire, tobacco mosaic virus, tobacco vein mottling virus, and tobacco etch virus. The release of KT 200 should provide a burley variety with good resistance to black shank, combined with good yield potential and resistance to TVMV and TEV.

KT 200 is a late-maturing variety, flowering approximately five days later than TN 90 and 14 days later than MS KY 14 X L8. KT 200 is not significantly different from TN 90 for plant height, leaf number, or leaf size. The growth habit of KT 200 is semi-upright, similar to TN 90. Although the plant is very compact during early growth, it grows rapidly as it approaches flowering and is large at harvest. Leaf color is darker green than standard burley varieties. KT 200 produces cured leaf that is medium-bodied and is tan to reddish-tan under normal curing conditions. Because KT 200 has produced variegated tips under poor curing conditions and has shown a tendency to sunburn more quickly than check varieties in some test plots, it is not recommended for production in fields where black shank is not a potential problem.

KT 200 was compared with standard burley varieties in nine trials from 1996 through 1999 (Table 1). In the absence of black shank, it has yielded as well or better than other black shank resistant varieties but not as well as susceptible varieties such as Hybrid 403. Although the grade index (which is indicative of cured leaf quality) of KT 200 has been substantially lower than standard varieties under some curing conditions, the mean grade index over six trials was comparable to KY 17, KY 14, and Hybrid 403. KT 200 was also evaluated as GR 171 at seven locations in Kentucky, Tennessee, Virginia, and North Carolina as part of the 1997 Regional Variety Test. The average per acre yield of KT 200 was 68 pounds higher than KY 14 and 342 pounds higher than VA 509. The average grade index of KT 200 was four points lower than KY 14 and 12 points lower than VA 509. The performance of KT 200 was relatively consistent in all states participating in the Regional Test Program, indicating that it is well adapted throughout the burley region. Based on data from the Regional Test and Tennessee and Kentucky variety trials, KT 200 has been assigned a yield rating of 6; however, this rating may change as more data are obtained.

KT 200 has consistently performed better than standard black shank-resistant varieties when evaluated under disease pressure (Table 2). This is particularly true when Ridomil is not used to help control black shank. Percentage of survival of KT 200 has been equal to or better than KY 17, which is presently one of the burley varieties with the highest level of resistance. In four-year comparisons without Ridomil, KT 200 yielded 2,197 lb/A, compared to 1,301 for TN 90 and 1,671 for TN 97. In two years of comparison with KY 17, KT 200 averaged 2,418 lb/A without Ridomil, versus 1,838 for KY 17. KY 17 has a black shank resistance rating of 6, TN 97 has a rating of 5, and TN 90 has a rating of 4. Based on the available data, KT 200 has been given an initial black shank-resistance rating of 6.

Table 1. Yield and quality of KT 200 and selected burley varieties*
VarietyYield (lbs/A)Grade Index**
KT 200316858
TN 90306562
TN 86309867
TN 97314163
KY 17292758
VA 509319166
KY 14317757
Hybrid 403341360
* Average values for nine trials conducted in Tennessee and Kentucky from 1996 through 1999; all trials were free from black shank.
** A numerical indicator of quality basedon federal grades; higher values indicate better quality.

Table 2. Comparative black shank resistance of KT 200 and selected burley varieties.
VarietyPercent Black Shank SurvivalYield (lbs/A)
No RidomilRidomil*No RidomilRidomil*
1996 through 1999
KT 200809521972860
TN 90509113012605
TN 97659216712744
1997 and 1998
KT 200749619762973
TN 86528713862431
VA 509509211822688
1996 and 1999
KT 200869424182747
KY 17799318382477
*Ridomil Gold: 1 pt/A pretransplant, plus 1 pt/A at first cultivation, plus 1 pt/A at layby.


Regional Burley Tobacco Variety Test

J.R. Calvert, B.S. Kennedy, and R.D. Miller

For more than 20 years, advanced experimental breeding lines of burley tobacco have been grown at multiple sites as part of a cooperative testing program involving university-based researchers at Kentucky, Tennessee, North Carolina, and Virginia. Data and cured leaf have been collected annually from at least seven (and in some years as many as 11) field sites since the testing program began in 1978. Advanced breeding lines from public agencies and commercial companies are accepted for testing by the Regional Burley Variety Evaluation Committee. In Kentucky, test plots are grown at three locations each year, on the University of Kentucky Experiment Station Farms at Lexington, Woodford County, and Eden Shale Farm in Owen County.

Agronomic traits, including yield, plant type, leaf quality, and disease resistances, are determined for each entry. To be acceptable for release as a new variety, a test entry must compare favorably to two standard or "check" varieties. Because burley tobacco has value only as a component of cigarettes, it is essential that tobacco-product manufacturing companies evaluate and approve new breeding lines before they are made available to burley producers. Therefore, four to six major cigarette manufacturing companies participate in leaf quality evaluations and receive small quantities of leaf from all test plot entries and locations. The sample leaves are made into cigarettes and are smoked by company test panels to determine if the breeding lines exhibit normal burley tobacco characteristics. Burley breeding lines that diverge significantly from check varieties for certain chemical constituents and/or smoke flavor are automatically rejected and are not recommended for release. In the recent past, all new burley varieties being offered for sale have been tested and approved by the regional committee.

In 1999, 11 advanced burley breeding lines were tested. Eight passed minimal standards established by the committee. Twelve burley breeding lines are being evaluated in the 2000 Regional Quality test. Additionally, 16 burley breeding lines are undergoing preliminary testing.


Maleic Hydrazide (MH) Residues in Burley Tobacco

G. Palmer and R. Pearce

Maleic hydrazide (MH) residue in burley tobacco has been an issue since the chemical was introduced. Considerable research and education efforts continue to attempt to find ways to improve the residue levels in cured leaf. Over the last five years a new approach to sucker control offered growers improved sucker control and higher yields while reducing residue. The program is a two-phase approach targeting application technique and chemical selection. The change in application technique is away from fine spray nozzles to a coarse spray that creates larger droplets that hit the leaf and roll to the stalk where they contact the suckers directly. The chemical change is a reduction of MH from 2 to 1.5 gal/A and the addition of either Prime+ or Butralin at 0.5 gal/A. Although producers praise the improved sucker control and have noticed increases in yield, the question remains as to the effects on MH residue.

To try to answer this question, samples were collected in 1997, 1998, and 1999 by the Burley Tobacco Growers' Cooperative Association and analyzed for MH residue. These years can be categorized in terms of difficulty of sucker control as moderate, difficult, and very difficult, respectively. Samples were collected randomly from all burley tobacco warehouses. A total of 300, 378, and 381 samples were collected over the three-year period. These samples included tobacco from 11 states in 1997, 10 in 1998, and eight in 1999. Of the total samples, Kentucky produced the following percentages over the three years: 63.3, 72.8, and 72.2. Tennessee produced 21.0, 15.3, and 14.7. These figures loosely represent the percentages grown in these states.

Although average MH residues increased over the three years, they did not reach levels seen in previous years when weather factors created difficult conditions for sucker control (Table 1).

Table 1. Average MH residue from the burley belt.
YearKYINMONCOHTNVAWVOtherAll
199771.21112.3570.105.0169.3657.3217.95098.3765.04
199879.4737.8961.7511.0059.3848.4816.5626.3369.66
199980.1395.8978.8021.8053.1345.0736.0015.0071.54

A residue level of 80 ppm is considered a target level with some countries requiring less on finished products. For this reason a distribution of MH residue levels may be more meaningful than average levels (Table 2). Although there are a few samples that indicated overapplication, especially in 1998 and 1999, the trend indicates a shift of most samples into the 0, 1-40, and 41-80 ppm ranges. Therefore, more producers must be making changes that have shifted the MH residues to levels that are more acceptable. The control difficulties associated with weather problems in both 1998 and 1999 are indicated by those outliers in the 301-700 ppm range. No samples were found to contain these levels in 1997. The high levels are an indication of poor control that producers tried to remedy by overapplication and/or high application rates. The tobacco was most likely harvested within a short time frame of the last application. A switch to coarse nozzles and a combination of MH and Prime+ or Butralin would have most likely produced good sucker control under poor weather conditions while reducing these astronomical residues to more acceptable levels.

Table 2. Frequency of distribution of MH residue levels in ppm in Kentucky.
Yearppm MH
01-4041-8081-120121-160161-200201-240240-300301-400401-500501-600601-700
19971648563920713
1998297582382071146021
199940527058141235502


Effects of Foliar Fertilization on Dark Tobacco Yield and Value

B. Maksymowicz

The use of foliar fertilizers in tobacco production has increased significantly in recent years. While there is little data to support the benefits of foliar fertilizers if soil pH and fertilizer recommendations are followed, there is a perception by many growers that benefits of foliar feeding programs far outweigh the costs.

Small plot studies were initiated at the University of Kentucky Research and Education Center in Princeton in 1999 to study the response of dark fired tobacco to a basic foliar fertilization program. The results reported do not imply that other programs or application timings would give similar results. The foliar fertilization programs currently being used are too numerous to fairly evaluate in one growing season.

Two treatments were compared: a dry fertilizer program formulated according to soil test recommendations and the same program with five supplemental foliar applications of 7-14-14 liquid fertilizer applied in 50 gallons of water per acre. The variety used was Narrow Leaf Madole. Each plot was 3,000 sq ft and treatments were replicated four times. All plots received 300 units of nitrogen, as ammonium nitrate, applied pre-transplant.

Soil Test RecommendationsDry Fertilizer Application (preplant)
32 lb/A phosphorous15 lb/A phosphorous
241 lb/A potassium250 lb/A potassium
pH: 6.5250 lb/A pelleted lime

Dry fertilizer was applied on May 15. Tobacco was transplanted on May 19, and the foliar fertilizer applications were made on June 24, July 9, July 16, July 23, and August 1.

All pesticide and sucker control recommendations were made according to recommendations.

Results

Sixty sticks per plot were harvested. Tobacco was cured and stripped into three grades.

Leaf GradeYield (lb/A)
Dry Fertilizer OnlyDry + Foliar Fertilizer
Lugs406444
Seconds866795
Leaf14101245
Total26822484

Statistical analysis showed no differences between grades or total yield.

Observationally, the plots receiving the foliar treatments appeared "greener" and "healthier" during the course of the growing season, but the above data show that visual appearance in the field does not always translate into increased returns.

These studies will be expanded as a part of a cooperative effort between the University of Kentucky and the University of Tennessee to further evaluate the responses of dark tobaccos to foliar fertilizer programs.


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