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 In California, complete crop losses in individual fields have been observed (R. M. Davis, unpublished).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).

In vitro infection of sheep lice (Bovicola ovis Schrank) by Steinernematid and Heterorhabditid nematodes.

Control of sheep lice with conventional pesticides can be compromised by difficulty in contacting lice in the dense water repellent fleeces of sheep, particularly when sheep have not been recently shorn. Entomopathogenic nematodes (ENs) are motile and are able to actively seek out insect hosts. They have particular advantages for the control of pests in cryptic habitats, such as the fleeces of sheep and avoid many of the problems frequently associated with chemical controls. This study investigated whether ENs were able infect and kill Bovicola ovis and compared the effectiveness of different species at different temperatures and when applied to wool. Four species of nematodes, Steinernema carpocapsae, Steinernema riobrave, Steinernema feltiae and Heterorhabditis bacteriophora were tested. All were shown to infect and kill lice in Petri dish assays at 30C. At 35C, the percent infection for S. carpocapsae and S. riobrave was significantly higher than for the other two species and percent infection by S. feltiae was significantly greater than for H. bacteriophora (P<0.05). At 37C the percent mortality induced by S. riobrave was significantly greater than for S. carpocapsae (P<0.05). All species were able to locate and infect lice in wool when formulated in water with 8% Tween 80. In wool assays the percent lice infected with nematodes was significantly greater for S. riobrave than H.bacteriophora at 25C, but there were no other differences between species (P=0.05). S. carpocapsae, S. riobrave and S. feltiae caused significantly higher lice mortality than H. bacteriophora at both 25 and 35C in wool assays, but mortality induced by the three steinernematid species did not differ significantly (P>0.05). It is concluded that of the ENs studied S. riobrave is likely to be most effective against B. ovis when applied to live sheep because of its greater tolerance to high temperatures and 'cruiser' foraging strategy .