ITEMS FROM THE UNITED STATES
GEORGIA / FLORIDA
UNIVERSITY OF GEORGIA
Department of Agronomy, Griffin, GA 30212, USA.
J.W. Johnson, R.D. Barnett, B.M. Cunfer, J.J. Roberts, and G.D. Buntin.
The 1997 Georgia winter wheat crop was grown on about 450,000 harvested
acres and produced a state average yield of 44 bu/A compared to last year's
average of 48 bu/A. Weather conditions were very favorable during the autumn,
winter, and spring. Some cultivars had some vernalization problems because
of the warm autumn and winter. Cool and dry conditions prevailed through
the grain-filling stage. Powdery mildew and leaf rust also were major factors
in low yields. Producers had a tremendous problem in harvesting because
of extremely wet conditions.
Breeding.
Cultivar. The SRWW line GA 871339, was released as Roberts. Roberts was selected from the cross 'GA Gore / Coker' and has the T1A·1R translocation. This cultivar possesses moderate resistance to powdery mildew and leaf rust, good resistance to Hessian fly, and good test weight and straw-strength and is late in maturity.
GA 87467, a SRWW, was coöperatively developed and released by the Georgia and Florida Agricultural Experiment Stations in 1997. The cultvar was derived from a three way cross in 1987 (pedigree: GA 80573 // GA Gore / Balkan). GA 80573 is an experimental line from the cross 'Sus Tus V / Blueboy'. Balkan is a cultivar with the T1B·1R translocation. GA 84467 is an medium-maturing, awnless, white-chaffed short-statured at maturing cultivar. The cultivar has excellent resistance to current races of leaf rust, moderate resistance powdery mildew, and is susceptible to some biotypes of Hessian fly.
Germplasm. The Small Grain Breeding Program developed and released 12 new and effective sources of germplasm (Ceruga 7 to 18) with excellent leaf rust resistance that are agronomically adapted to the Southeast.
Research.
Waterlogging. Understanding the physiological mechanisms of water
stress (hypoxia and drought tolerances) is important for developing hypoxia-tolerant
genotypes. Hypoxia limited root elongation for both Bayles and Jackson but
enhanced production of crown roots for tolerant Jackson. Ethylene effects
on root growth and aerenchyma formation were similar to those observed for
hypoxia treatment. All concentrations of ethylene hastened the production
of crown roots for Jackson but inhibited root elongation at the highest
ethylene concentration. Ethylene also promoted formation of aerenchyma in
crown roots by increasing cellulase activity. The results showed that the
effects of ethylene on roots varied with cultivar and ethylene concentration.
Marker-assisted selection. A RAPD marker that cosegregates with the
rye-derived H21 gene resistance in wheat to biotype L of Hessian
fly was identified. RAPD analysis was used in conjunction with bulked segregant
analysis and near-isogenic lines.
Plant pathology.
Suspected Karnal bunt in the Southeast. Wheat samples from Alabama,
Tennessee, northwest Florida, and Georgia were found with suspected Karnal
bunt teliospores in 1996. Many of the 50 g samples had only 1-5 spores.
No bunted kernels were found in any of the samples. These were the only
'positive' samples found in the U.S. outside of the Southwest. In late 1996,
the teliospores were determined to probably have come from a Karnal bunt-like
smut on ryegrass. Many wheat fields in the Southeast are infested with ryegrass,
and all samples from the Southeast with suspected Karnal bunt had ryegrass
seed in them.
Most wheat in the region had been planted by late November 1996. When APHIS proposed a possible quarantine for the Southeast, farmers faced the real possibility that all wheat would be plowed down or be regulated like wheat in Arizona. By 11 February, 1997, U.S. Secretary of Agriculture Dan Glickman stated that growers and agribusinesses in the Southeast would be compensated if a quarantine was issued. This permitted growers to proceed with spring fertilizer applications and other crop management. On 17 March, 1997, the USDA announced that the teliospores found on wheat in the Southeast were not Karnal bunt but were the type associated with ryegrass, and no quarantine would be implemented. The exact genetic relationship between T. indica and the ryegrass smut has not been determined yet.
We have sampled ryegrass seed from over 200 wheat fields in the Southeast. Teliospores typical of those found on ryegrass in Oregon were found in 12 samples from eight counties throughout central Georgia and from one site in Lincoln County, Tennessee. The 1997 APHIS survey in Georgia found 10 of 165 samples to have teliospores typical of the ryegrass smut but no bunted kernels. All samples with teliospores had ryegrass, and one smutted ryegrass floret was found in a sample. The ryegrass smut appears to occur widely and at very low levels. Two known fields where teliospores were found on wheat and ryegrass seed in 1996 and 1997 have severe ryegrass infestations. Karnal bunt has not been identified from the southeastern U.S.
Stagonospora nodorum epidemiology. Wheat seedlings were placed in the field among growing wheat or in wheat stubble over a 3-year period. Seedlings were in the field for 12 weeks, then moved to the greenhouse for observation of symptoms. Isolation of S. nodorum from leaves was made on agar media. Results show that inoculum (spores) is available for infection of plants year round. The least inoculum is available during dry periods. Inoculum remains viable in wheat stubble on the soil surface for about 2 years. Seedings rarely were infected more than a few feet from the edge of the wheat field indicating that spores rarely move beyond their source. Ascospores are disseminated longer distances than conidia (asexual spores). Although the type of spores initiating infection were not identified in this study, circumstantial evidence on the basis of inoculum movement indicates that conidia are the source of inoculum.
Sexual compatibility in Stagonospora nodorum. The sexual stage of S. nodorum is common in some parts of the world and rarely or never in other areas. Septoria nodorum has not been found in the southeast but occurs commonly in Europe. A major problem in studying the sexual stage is that it is difficult to produce in culture. There are two mating types. A reliable procedure that results in development of the sexual stage in culture in 70-100 days when compatible isolates are paired on agar plates, was developed by Edward Arseniuk and Paul Czembor in Poland. Both mating types have been identified among isolates from Poland and Georgia. Polish and U.S. isolates are compatible with one another and produce fertile pseudothecia with ascopores. Identification of both mating types in Georgia means that the sexual stage may occur here. The reason it has not been found in nature in the Southeast is unknown.
Publications.
Barnett RD, Soffes Blount AR, Mislevy P, and Johnson JW. 1997. Selection
and management of oats for forage production in the southeastern United
States. In: Proc Third South American Oats Cong (Rebuffo M and Abadie
T eds). INIA La Estanzuela, Colonia, Uruguay, 11-12 November, 1997. pp.
57-63.
Bruckne PL, Barnett RD, and Johnson JW. 1997. Registration of 'Morey' wheat.
Crop Sci 37:1008.
Buntin GD, Johnson JW, and Raymer PL. 1997. Evaluation of winter wheats for resistance to Hessian fly, 1995 and 1996. Arthropod Manage Tests 22:436.
Cunfer BM. 1997. Taxonomy and nomenclature of Septoria and Stagonospora species on small grain cereals. Plant Dis 81:427-428.
Gates RN, Hill GM, Johnson JW, and Bruckner PL. 1996. Grazing response to rye populations selected for improved yield. Proc Soil and Crop Sci Soc Florida 55:86-88.
Huang B, Johnson JW, Box JE, and Nesmith DS. 1997. Root characteristics and hormone activity of wheat in response to hypoxia and ethylene. Crop Sci 37(3):812-818.
Lee SG, Yong WS, Johnson JW, and Byeing HK. 997. Effects of water stress on leaf water potential, photosynthesis and root development in tobacco plant. Kor J Crop Sci 42(2):146-152.
Roberts JJ, Johnson JW, Long DL, Fowler Jr. HA, Cunfer BM, and Bland DE. 1997. Registration of 12 winter wheat germplasm lines resistant to leaf rust: Ceruga-7 to Ceruga-18. Crop Sci 37(2):630.
Seo YW, Johnson JW, and Jarret RL. 1997. A molecular marker associated with the H21 Hessian fly resistance gene in wheat. Mol Breed 3:177-181.