Phylloplane bacteria antagonistic to the sunflower pathogen Alternaria helianthi

Thirty-eight of the 61 isolates of bacteria obtained from diseased and healthy sunflower leaves inhibited the germination of conidia and growth of germ-tubes of Alternaria helianthi in vitro. Inhibition included reduced conidial germination, germ-tube swelling causing vesicle formation, excessive germ-tube branching, lysis of germ-tubes, absence of sporulation and a reduced rate of hyphal growth. Some bacteria appeared to be endoparasitic, persisting inside the lumina of conidia and causing erosion of the conidium wall which resulted in the destruction of conidial cells. Bacteria attached themselves to conidia, hyphae and conidiophores and, in the field, are probably dispersed with the fungus. Five bacterial isolates that showed a high level of inhibition were identified as members of the genus Bacillus and comprised three species, B. subtilis, B. cereus and B. mycoides. The inhibitory substance produced by an isolate of B. subtilis was heat stable and therefore probably not an extracellular enzyme. Inhibition caused by the 38 isolates was not due to acid production by the bacteria. The size of lesions formed by A. helianthi was reduced when an isolate of B. mycoides was applied to sunflower plants with the conidial suspension. B. mycoides had no effect on infection by A. helianthi when applied to sunflower leaves 24 h prior to inoculation with conidia of A. helianthi

Components of quantitative resistance in sunflower to Alternaria helianthi

Components of quantitative resistance, spore production, incubation period, infection frequency and mean lesion size were measured in 17 sunflower accessions inoculated with conidia of Alternuria helianthi under controlled conditions. The same accessions were also rated for disease reaction in the field in 1994 and 1995 using a generated epidemic and varied in their disease reactions from highly susceptible to highly resistant. Spearman's ranking of accessions was highly correlated (r = 0.9) for both years; however, the ranking of components measured under controlled conditions with field severity was generally poor. Regression analysis of components with field severity ratings of the accessions showed that mean lesion size was highly correlated (r = O.74) and infection frequency was moderately correlated (r = 0.58) with the field severity ratings taken over the two years. Infection frequency was also well correlated (r = 0.75) with mean lesion size. Spore production and incubation period were poorly correlated with the field severity ratings for both years. An index based on infection frequency and mean lesion size gave a better correlation with the 1995 field severity ratings than either component alone, but in 1994 the index was not as well correlated with field severity as mean lesion size alone. It is suggested that mean lesion size, determined from plants 7–9 days after inoculation could be used to select for resistance to A. helianthi in the greenhouse. Infection frequency could also be used as a predictor of resistance, but to a lesser degree

Evolution of pathotypes of Puccinia helianthi on sunflower in Australia

Sunflower rust caused by Puccinia helianthi is a major problem in sunflower production in Australia. The disease can be effectively controlled through the use of resistant hybrids but the commercial life of these is often short, due to the evolution of new pathotypes of the fungus. Since 1978,23 pathotypes have been recognised, mostly from commercial crops. Possible pathways for the evolution of these pathotypes are proposed. Almost all pathotypes identified since 1986 trace to a common progenitor, Aus4. Avirulence/virulence patterns of many pathotypes suggest that sexual recombination may be acting to generate new pathotypes, but the sexual stages of the fungus are rarely seen in the field. It is assumed that bursts of evolution occur in seasons that favour completion of the sexual cycle, and that mutation contributes steadily to the development of new virulence genes in the population of P helianthi

Fusarium wilt of cotton: Population diversity and implications for management

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)

Variations in soil population of Fusarium oxysporum f. sp. vasinfectum as influenced by fertiliser application and growth of different crops

During the 1995–96 growing season, the effects of fertiliser application and growth of different crops on the population of Fzrsarium oxysporum f. sp, vasinfictum (Fov) in field soils were examined. In soil samples collected 35 days after fertiliser application, densities of Fov were significantly lower in soils receiving 180 units of anhydrous ammonia than in those receiving 40 or 180 units of urea. When compared with samples collected at sowing, densities of Fov were significantly reduced in soil samples from plots planted to either the less susceptible cotton cultivar DP90 or soybean. The proportions of Fov in the total F: oxysporum population increased significantly in soils planted to the most susceptible cotton cultivar Siokra 1–4, but decreased significantly in soils planted to the less susceptible cultivar DP90