INFECTION OF COTTON BY FUSARIUM-OXYSPORUM F SP VASINFECTUM AS AFFECTED BY WATER-STRESS

Since virulence of Fusarium oxysporum f.sp. vasinfectum (FOV) on cotto n (Gossypium hirsutum) is enhanced when the fungus is cultivated in a saline environment, excessively saline water muse not be used for the irrigation of cotton. However, the limitations thus placed on the avai lable water resources may lead to conditions of enforced water stress for the plant. The present study investigated whether water stress aff ects the susceptibility of cotton to FOV. Groups of 2-month-old cotton plants of the Fusarium-susceptible Coker 304 and the moderately resis tant GSC 20 varieties were maintained without watering for varying per iods immediately before or after being inoculated with FOV (15 plants per group, two replications). Watering was suspended for 3, 6, 12 or 2 4 days before inoculation, and for 3, 6, 12 or 15 days after inoculati on. After inoculation the plants were maintained in a controlled envir onment with a 15,000 lux, 12-h photoperiod, at 28 degrees/24 degrees C D/N, 20% r.h. Xylem water potential was determined in a pressure cham ber. Percent infected leaf area and date of onset of wilt were the par ameters used to define severity of FOV infection. There was a consiste nt relation between low water potential in the xylem (-7 and -20 MPa) and severity of infection, particularly when the dry period occurred a fter inoculation. After exposure to the lowest post-inoculation water potentials, even variety GSC 20, which is normally moderately resistan t, exhibited a fairly high percent infected leaf area. This should be taken into account when the cotton grower is faced with water shortage s, especially during the period from branching to flower bud break.