Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/12/1999
Publication Date: N/A
Interpretive Summary: Bronze wilt is a new disease of cotton that caused substantial yield losses across the cotton belt during the unusually hot summers of 1996 and 1998. The only organism consistently associated with the disease is a bacterium called Agrobacterium biovar 1, which infects the roots of the cotton plant. Sixteen cultivars were inoculated with Agrobacterium and incubated at different temperatures. At soil temperatures of 33 and 36 degrees C (91 and 97 degrees F), the bacterium developed much higher concentrations in roots than at lower temperatures. At 36 degrees C cultivars that show bronze wilt in the filed also developed bronze wilt in the controlled environment chambers. This provides additional evidence that Agrobacterium plus a suitable stress, such as hot temperature, is the cause of bronze wilt. An Agrobacterium-36 degrees C screen is proposed as a quick screen for bronze wilt susceptibility, and this screen was applied to 120 cultivars and breeding lines.
Technical Abstract: Sixteen cultivars of cotton were inoculated with a mixture of three Agrobacterium biovar 1 strains and grown with complete nutrition in controlled environment chambers at constant temperatures of 24, 27, 30, 33, and 36 degrees C. After six weeks, leaf, stem, shoot, root, and whole- plant weights and Agrobacterium concentrations in roots were determined. Bacterial concentrations per gram of whole plant tissue increased progressively as temperatures were increased, with a major increase occurring between 30 and 33 degrees C. Concentrations of Agrobacterium at 33 and 36 degrees C were two- to ten-fold higher than at 27 and 30 degrees C in each of the sixteen cultivars. Optimal cotton growth occurred at 30 degrees C, and a major decrease in growth occurred between 33 and 36 degrees C. Certain cultivars developed bronze wilt symptoms at 36 degrees C and symptom severity was correlated with reduction of growth. The coefficient of variability for root or shoot weight among cultivars was much greater at 36 degrees C than at 33 degrees C or lower temperatures. These observations indicated that root, shoot, or plant weight of Agrobacterium-inoculated plants after six weeks of growth at 36 degrees C can be used as a quantitative measure of bronze wilt susceptibility in cultivars.