Black root rot is caused by the soil borne fungus Thielaviopsis basicola, which

causes disease in over 137 species of plants (Honess at a1. , 1994). T basicola

survives for long periods in the soil as resistant resting spores. The wide host

range and resistant resting spores make T. basicola almost impossible to

eradicate from soil. Infection of cotton is favoured by soil temperatures below

20'C. Research in the USA has shown that severe disease symptoms result

when the population of the black root rot fungus reaches 100 spores per grain

of soil. Populations of 600 to 700 spores per grain of soil are commonly

observed in Australian cotton fields.

Black root rot contributes to seedling loss caused by the seedling disease

complex. Stand losses of 30% or more have been recorded. Seedlings affected

by black root rot are stunted and slow growing and crop maturity is delayed. In

California black root rot was regarded as a minor disease 30 years ago but is

now considered to be more serious than Verticillium wilt (Note: the defoliating

strains of Verticillium in the USA are more pathogenic than the strains currently

in Australia). Yield reductions of 25 to 509", were attributed to black root rot in

California (Garber at a1. , 1985; Hake at a1. , I 985) but the potential for yield loss

in Australia had not been quantified.

Black root rot was first observed in Australia in 1989. Since then its severity and

distribution increased annually. At the commencement of this project, black root

rot occurred throughout the Macquarie valley, the Namoi valley and the Darling

Downs, and was common in the Macintyre and Gwydir valleys. In some fields

100% of plants were affected. Anecdotal evidence suggested that black root rot

may increase the severity of Fusarium wilt (J. Kochman, Pers. Coinm. ).

Permanent bed systems may have increased the severity of black root rot by

increasing the population of the pathogen along the planting line.