No abstract

Fusarium wilt of cotton, caused by the fungus Fusarium oxysporum Schlechtend. f. sp. vasinfectum (Atk.) Snyd. & Hans, was first identified in 1892 in cotton growing in sandy acid soils in Alabama (8). Although the disease was soon discovered in other major cotton-producing areas, it did not become global until the end of the next century. After its original discovery, Fusarium wilt of cotton was reported in Egypt (1902) (30), India (1908) (60), Tanzania (1954) (110), California (1959) (33), Sudan (1960) (44), Israel (1970) (27), Brazil (1978) (5), China (1981) (17), and Australia (1993) (56). In addition to a worldwide distribution, Fusarium wilt occurs in all four of the domesticated cottons, Gossypium arboretum L., G. barbadense L., G. herbaceum L., and G. hirsutum L. (4,30). Disease losses in cotton are highly variable within a country or region. In severely infested fields planted with susceptible cultivars, yield losses can be high. In California, complete crop losses in individual fields have been observed (R. M. Davis, unpublished). Disease loss estimates prepared by the National Cotton Disease Council indicate losses of over 109,000 bales (227 kg or 500 lb) in the United States in 2004 (12).

Sixty-one isolates of Fusarium oxysporum f. sp. vasinfectum were collected from cotton plants (Gossypium spp.) with symptoms of Fusarium wilt to determine the composition of races present in the southeastern United States. Analysis of partial sequences of the translation elongation factor gene revealed four novel genotypes, as well as the presence of races 3 and 8 for the first time in the United States outside of California. The majority of isolates (16 of 27) sampled from Arkansas were novel genotypes. A subset of isolates representing the novel genotypes was compared with previously described races using sequences from translation elongation factor, phosphate permase, and β-tubulin genes and their pathogenicity on a total of six Upland (Gossypium hirsutum) and Pima (G. barbadense) cotton cultivars. Two of the novel genotypes belonged to a clade containing races 1, 2, 4, 6, and 8 and two shared ancestry with race 3. All new genotypes were pathogenic to at least some of the cotton cultivars tested. The Pima cv. Phytogen 800 was relatively resistant to all genotypes of the pathogen. These results indicate that the population of F. oxysporum f. sp. vasinfectum in the southeastern United States is more diverse than previously recognized.

Rothrock, C. S., Winters, S. A., Miller, P K., Gbur, E., Verhalen, L. M.,

Intensive cereal management can increase the yield and performance of winter wheat (Triticum aestivum L. emend. Thell.), but wet soils often delay planting along the Gulf Coast. Field experiments were conducted at four locations in 1991 and two locations in 1992 to evaluate the effects of two seeding rates (84 or 168 kg ha−1) and two. topdress N rates (90 or 90 + 45 kg ha−1) for an early and late‐maturing cultivar planted at a recommended date or 35 d later. Main effects were generally more significant than interactions except for those involving environment (year‐location). Late planting resulted in significant reduction in grain yield even though more spikes per square meter were produced. Wheat kernels from the recommended planting date were heavier and each spike contained more kernels, which resulted in higher grain yield. Additional N significantly increased yield when the crop was planted at the recommended date by increasing kernels per spike and spikes per square meter, but did not increase yield of the late‐planted crop. Grain yield was not affected by seeding rate for the early planted wheat. Seeding rate effects were influenced by the environment for late planting; however, there was evidence that the higher seeding rate increased yield of the late‐planted crop. Delayed planting reduced yields due to a decrease in kernel weight and kernel number per spike, but the magnitude of yield loss was reduced by using an early‐maturing cultivar and a higher seeding rate.