Pathogenicity of fungi associated with seedling disease of cotton.
Fungi associated with cotton seedling disease were isolated directly from a vertisol at Narrabri, New South Wales, Australia, in which cotton failed to establish, and from cotton seedlings grown in the same soil, and seedlings grown in 7 Queensland cotton-growing soils (also vertisols). Fusarium spp., Penicillium spp., Trichoderma spp., Rhizoctonia solani, Pythium vexans, unidentified pythiaceous fungi and a number of miscellaneous unidentified fungi were isolated from soil or soilborne plant debris. Pythium ultimum, R. solani, Fusarium spp., and unidentified pythiaceous fungi were isolated from lesions on seedling hypocotyls. The pathogenicity of the fungi was investigated using cotton seedlings grown in steamed potting mix; 93% of seedlings survived in the absence of fungal inoculum. Of the fungi isolated from the Narrabri soil, P. ultimum and R. solani were most pathogenic, reducing seedling survival at 14 d after inoculation to 33%. Isolates of P. vexans and Fusarium spp. reduced seedling survival to 64 and 73-87%, respectively. Isolates of R. solani or P. ultimum from Queensland soils completely prevented seedling emergence in most cases, while isolates of Fusarium spp. were not pathogenic. Isolates of both R. solani and P. ultimum varied significantly in pathogenicity.