Ambrosia grayi (A. Nelson) Shinners is a pernicious perennial weed that infests over 200,000 ha of cotton in Texas and New Mexico. It reproduces primarily by rhizomes, which are found up to 3 m deep in the soil. With the exception of Roundup Ultra on Roundup-ready cotton, no herbicides can be applied in-season to control this weed without injuring cotton. A search was conducted for soilborne pathogens as potential biocontrol agents by surveying cotton fields and small lakes for A. grayi plants with root lesions. Roots were washed with running water for 2 min and segments of root lesions were placed on water agar (1.5%) with hyphae of emerging fungi transferred to potato dextrose agar (PDA). Eighty isolates of fungi were collected; most were Fusarium spp. or Rhizoctonia solani. Replicated experiments with isolates were conducted. Each isolate was grown on PDA and then 1/4 of the media in a petri plate (12.6 cm2) was mixed with 450 cm3 autoclaved soil and placed in a styrofoam cup. A piece of rhizome (3 to 5 cm in length) was placed 5 cm below the soil surface in the infested cup. Roots with necrosis were plated on PDA to recover fungi, which were grown in pure culture and used to reinfest soil. Significant disease (in repeated tests) was caused by two isolates of R. solani, compared with the noninfested check, as indicated by a reduction in number of emerging plants per pot, increase in root necrosis, and decrease in fresh and dried leaf weight. Roots growing from the infected rhizomes were discolored and rotted without distinct lesions. Both of these isolates were identified as AG4 by anastomosis with tester ATCC #46134 by the cellophane technique (1).
Reference: (1) D. E. Carling et al. Phytopathology 77:1609, 1987.