The interaction of Thielaviopsis basicola (Berk. and Br.) Ferr. and Rhizoctonia solani Kuhn as pathogens of seedling cotton.
AuthorChapman, Margaret Ann
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PublisherThe University of Arizona.
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AbstractSoil inoculum density and the incidence of black root rot of cotton caused by Thielaviopsis basicola were monitored in two adjacent fields planted mid-April to Gossypium hirsutum 'Acala 1517' at Duncan, AZ (1160 m elev.). Forty soil cores (3.5 x 15 cm) were taken from the root zone and 80 plants were collected biweekly in the two fields from 5/7/87 to 9/28/87. The inoculum density (cfu/g air-dried soil) was determined by plating soil dilutions onto a selective medium. Disease severity was rated on a scale of 1 (slight cortical decay) to 4 (severe cortical decay). Mean inoculum density in Field 1 soil was 65 cfu/g soil and 20% of the seedlings were infected with a severity rating averaging 1.6. In Field 2 the inoculum density, percentage of infected plants, and disease rating were 225 cfu/g soil, 93, and 3.2, respectively. No cortical decay was noted after June 6 in either field. Yields were similar in both fields. Field and laboratory studies indicate that high disease incidence of black root rot caused by T. basicola results in reduced incidence of seedling disease caused by Rhizoctonia solani. For example, 1 mo after untreated seeds of G. barbadense 'Pima S-6' were planted into a cotton field in Coolidge, AZ with natural inoculum levels of 12 cfu/100 g soil and 225 cfu/g soil of R. solani and T. basicola, respectively, 98% of the plants were infected by T. basicola while only 2% were infected by R. solani. Eleven out of the 12 R. solani propagules were pathogenic to cotton. The same levels of inoculum, were added to autoclaved field soil in growth chambers at 18 C and 5200 lux light in four treatments. The first treatment, T. basicola alone, caused a 98% incidence of black root rot. Rhizoctonia solani alone resulted in a 38% incidence of disease, whereas inoculum of T. basicola and R. solani, together caused a 98% and 17% incidence of disease caused by T. basicola and R. solani, respectively. Simultaneous inoculations of 4-day-old G. hirsutum 'DP 90' seedlings on water agar plates with inoculum of T. basicola and R. solani resulted 5 days later in a significant decrease in infection by R. solani as compared to that in the seedlings inoculated with R. solani only.
Degree ProgramPlant Pathology