Species of Agrobacterium biovar I are endophytic parasites of the cotton plant. The bacteria occur in all seed lots produced in the United States and generally are present in more than 90% of the seed. The bacteria are primarily present in the seed coat and infect both the radicle and cotyledons during initial stages of seed germination. Movement of Agrobacterium to new seeds apparently occurs through xylem vessels. Symptoms associated with Agrobacterium infection include: 1) necrosis and browning of secondary and tertiary roots, usually beginning at their site of origin from the parent root, 2) bronzing, lesions, blight, and defoliation of leaves, 3) leaf, crown, and root galls usually in association with other microbial infections, 4) distortion and abscission of bolls, 5) reduction of seed weight and fiber length and strength, and 6) wilt. Extensive damage to the root system also destroys the ability of the plant to actively take up water and minerals such as phosphorus, potassium and iron. The bacteria also may produce toxins. Four lines of evidence indicate that Agrobacterium is the cause of bronze wilt: 1) many environmental variables increase both Agrobacterium concentrations in the root and bronze wilt severity, 2) different isolates of Agrobacterium show distinct differences in ability to colonize roots, cause root necrosis, and predispose plants to soreshin caused by Rhizoctonia solani, 3) susceptibility to bronze wilt in modern cultivars is simply inherited, and 4) single bacterial blight resistance genes (B genes) show significant specific effects on bronze wilt severity. Bronze wilt has increased in occurrence and severity in recent years because of the inadvertent introduction of susceptibility genes into short-season cultivars, the progressive invasion of cotton seed stocks by highly virulent strains of Agrobacterium, and climate changes that favor disease development.