In a field study evaluating the diversity and density of the soilborne mycobiota in a cotton (Gossypium hirsutum L.) production system, Macrophomina phaseolina (Tassi) Goidanich was isolated on potato dextrose agar from dark brown to black lesions on feeder and secondary roots. Multiple proliferations of feeder and secondary roots were also observed. Isolate RB 656 obtained from these lesions was tested for pathogenicity in the greenhouse by mixing 25 ml of 2-week-old cornmeal sand inoculum (3 g of cornmeal, 100 g of sand, and 20 ml of distilled water) with 5 liters of autoclaved soil (Leefield loamy sand, pH 6.2) per pot (40 × 200 cm). Control pots containing autoclaved soil alone were included for comparison. On 17 September, 10 cotton seeds of DPL 90 were sown per pot. Each treatment had five replications. Forty days after planting, plant heights from pots containing M. phaseolina were lower (14.8 cm) than those in the control pots (19.6 cm), but stand counts were similiar. On this date, four plants were left in each pot to allow the remaining seedlings to reach full maturity. On 9 December, the four plants per replicate pot were removed and roots evaluated for damage. Lesions similiar to those seen originally were observed on the secondary and feeder roots of the infested pots, and the characteristic proliferation of feeder and secondary roots was noted. Tap roots in pots containing M. phaseolina were smaller (11.6 cm) than in the control pots (18.6 cm). Isolate RB 656 was reisolated from the damaged roots in the treated pots. This is the first report of M. phaseolina on cotton in Georgia.
