AGROBACTERIUM CONCENTRATIONS AND SEVERITY OF BRONZE WILT SYMPTOMS IN COTTON CULTIVARS TREATED WITH FUNGAL BIOCONTROL AGENTS AT PLANTING

Authors: Bell, Alois - Al

Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/3/2004
Publication Date: 2/2/2004
Citation: Bell, A.A. 2004. Agrobacterium concentrations and severity of bronze wilt symptoms in cotton cultivars treated with fungal biocontrol agents at planting. In: Proceedings of the Beltwide Cotton Conferences, January 5-9, 2004, San Antonio, Texas. 2004 CDROM.

Interpretive Summary: Agrobacterium root rot and bronze wilt of cotton are new disease problems that can cause yield losses of 15-20% of the potential crop. In this study we evaluated various fungal biocontrol agents for their ability to suppress disease symptoms and increase yields with variable levels of phosphorus fertilization. Under conditions of optimal phosphorus fertilization, some biocontrol agents actually caused yield losses and increased Agrobacterium concentrations in roots. The other biocontrol agents did not significantly affect the two diseases or increase yield. Therefore, biocontrol fungi that colonize the root surface do not appear to be desirable for controlling Agrobacterium root rot or bronze wilt under recommended cultural practices.

Technical Abstract: The fungi, Trichoderma virens (Isolates GV4 and GV6), Trichoderma koningii x T. virens fusant #12, Gliocladium catenulatum, Gliocladium roseum, Fusarium oxysporum, and Fusarium solani were trested for their ability to colonize roots, to affect Agrobacterium tumefaciens colonization and bronze wilt severity and to affect yield. The fungi were tested with both 'Paymaster 1220 BG/RR' and 'Stoneville 373' cultivars grown with low and optimal phosphorus fertilization and with and without inoculation of soil with Agrobacterium tumefaciens isolate 25A. Stable concentrations of all fungi developed in roots within a few weeks after planting, indicating the potential for season-long effects on rhizoplane populations of other microorganisms. Some of the rhizoplane inhabiting fungi, especially Fusarium species, significantly increased yields suppressed establishment of Agrobacterium from infested seed and decreased bronze wilt severity when phosphorus levels were low. The treatments, however, were not effective when A. tumefaciens isolate 25A was added to soils. Under conditions of optimal phosphorus availability Gliocladium and Fusarium species often caused yield losses and increased Agrobacterium concentrations in roots. T. virens isolates GV4 and GV6 did not significantly affect yield, Agrobacterium concentrations or bronze wilt severity. The rhizoplane inhabiting fungi do not appear to be desirable for controlling Agrobacterium root rot or bronze wilt under recommended cultivar practices.